One of the key findings of paleoclimate research (Module 1) is that the temperatures we are experiencing today have not been observed for about 125,000 years. The left panel in the figure above shows the recent temperature record reconstructed from proxies and observations. What you see is the sharp rate of warming Earth is experiencing today has not been recorded in the past. What is also very clear is that the rate (very fast) and amount (about 1.1 degrees C) of warming we have observed over the last 120 years cannot have been caused by natural processes, for example sunspots and volcanism. Climate models can simulate the temperature records pretty well based on the physics of the atmosphere and the amount of carbon input from various sources including fossil fuels. In the right panel of the figure above, the simulated natural inputs are shown in green, the simulated natural and current fossil fuel inputs are shown in brown, and the observed temperature curve in black (the Hockey Stick we briefly referred to in Module 1). Ranges of simulations are shown in shading. What they show very clearly is that the current rate and amount of warming cannot be caused by natural processes. The only way that this rapid rate and amount can occur in the simulations is to add fossil fuels at the rate we are adding them today. This is one of the central conclusions of the 2022 Intergovernmental Panel on Climate Change (IPCC) report, and we will come back to it in Modules 4 and 5
Earth’s climate is ever-changing and this is one of the main conclusions of Module 1. Before accurate measurements of temperature existed, we have historic records in addition to proxy records. We can observe in them the fall of dynasties as a result of climate change, for example the fall of the Mayans as a result of devastating drought in Central America in 900AD. Now, fast forward just a little, and we have multiple accounts and proxy records of two significant changes in climate that impacted medieval societies— the Medieval Warm Period (AD 950 to 1250) and the Little Ice Age (AD 1450-1850) see figure below. The Medieval Warm Period is famous because of its connection to some interesting events in the European and North Atlantic regions. During this time, it appears that wine production in Great Britain was abundant, even though in today's climate, wine grapes struggle at this high northern latitude. This is also the period of time when the Vikings colonized Greenland (they originally called it "Vinland"), indicating that this region was warmer than it is today. In a recent examination of climate proxy records from around the world, Penn State’s Michael Mann and his colleagues determined that the temperatures in the North Atlantic region were indeed warmer than the 1961 - 1990 period, but globally, the climate was not as warm as today, as can be seen in the below figure.
The Little Ice Age is similarly famous for its connections to European history. During this period, the winters in Europe were cold enough that the canals in the Netherlands froze over, allowing for skaters to travel through the countryside on these frozen pathways — this activity is recorded in some of the masterpieces of Dutch painters such as Bruegel. The Little Ice Age was a time of minor advances in many of the Alpine glaciers, and it also signaled the end of the Greenland colonization experiment. This was generally a difficult period in European history, marked by plagues, famine, fighting, and political turmoil. The cause of the Little Ice Age, like the cause of the Medieval Warm Period, is not entirely settled, but it does coincide with a period of volcanic eruptions, which should cool the climate and a period of decreased solar activity known as the Maunder Minimum. It has also been suggested that a slowdown in the thermohaline circulation in the oceans (see Modules 3 and 6) may have contributed to this cooling.
In the Medieval accounts, we can see that proxies and historic information are consistent. But the recent warming in the iconic Hockey Stick most certainly stands out in terms of its magnitude and its abruptness. In this module, we take a look at the wide range of observations that give us a sense of how the climate has been changing over past centuries but especially today. We will see the dire threats from persistent droughts, more devastating fire seasons, stronger hurricanes and melting ice sheets.
On completing this module, students are expected to be able to:
After completing this module, students should be able to answer the following questions:
Below is an overview of your assignments for this module. The list is intended to prepare you for the module and help you to plan your time.
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On June 29, 2021, the mercury at Portland, Oregon reached 116oF. December 10 and 11, 2021 saw EF4 tornadoes roar through Kentucky and other states, killing 88 and wounding more than 630. In December 2021, 202 inches of snow fell in the Sierra Nevada of California. Hurricane Sandy made landfall in New Jersey on October 29th, 2012, a very late point in the year for a storm to reach the northeastern US coast. January 8th, 2013 was the hottest day ever for Australia with an average temperature for the entire continent of 40.3oC (nearly 105oF). The AVERAGE (day and night) temperature in Phoenix in June 2017 was almost 95 degrees. And nearly 61 inches of rain fell around Houston during Hurricane Harvey in August 2017. Each of these events provoked arguments in favor or against global warming. Yet, on their own, none of them were definitive proof one way or another.
To make a case for climate change, we need to find the average global climate signal, which is often difficult to discern from the noise of short-term variations and regional differences associated with what we call weather. It is very important to remember that climate is the time-averaged weather, so when we are talking about climate change, we are not talking about the weather that you experience on a daily or seasonal basis — a single heat wave is not evidence of global climate warming, just as one cold snap does not constitute global climate cooling. But repeated, unusual heat waves will shift the average temperature of a region, and this can be taken as a manifestation of a warming climate. The climate is inherently variable over time and space, so detecting a meaningful trend is a challenge that requires great care, a lot of data and often some complex statistics.
Now, a word of warning, you might find the remainder of this page a little dull! However, it happens to be one of the most essential topics of the whole issue of climate change. We absolutely have to understand the significance of trends if we are going to interpret them!
Let’s consider a hypothetical case to help you better understand the nature of the problem. The mean annual temperature is the average temperature over the course of a year, and it varies in a way that we can simulate with a randomly generated string of numbers (geoscientists often refer to such variation as “noise”) added to a constant, long-term mean temperature. We would see something like this:
In this case, you might say that if the temperature strayed out of the green zone, you have unusually hot or cold weather, and you would expect this kind of temperature excursion to be a standard feature of the natural variability of the region's climate. But, the green zone has more or less fixed limits, so the standard for calling something a heat wave does not change over time.
Now, let’s look at another hypothetical case where the climate really is warming, but there is still the natural variability or noise on a shorter timescale.
If you think of the upper edge of the green zone on the previous figure as indicating the line for defining a heat wave, look what happens in a case where there is a steady warming of the climate, with the same kind of weather causing the rapid ups and downs. If at time 0, we said that a temperature of 21°C was a heat wave, that becomes the mean climate temperature by time 60 in the above figure — so, what previously was a rare warm spell is now just the standard. This means that, by older standards, "heat waves" become more common as time goes on.
Note that, once again, we can find areas in this above figure where a shorter time period would seem to indicate cooling or warming. This reinforces the idea that we can’t really talk about climate change by looking at just a few years, and leads to the question of how much time we do need to look at to get a good understanding of climate change. In general, the longer, the better. But just to illustrate this, consider the following, where we take the same kind of hypothetical temperature record as above and systematically find the linear trends over time, with windows of varying length that slide along through the 200-year record.
Temperature is probably the most important observation regarding the global climate, but how to measure the temperature of something as large as the Earth is complex. Climate scientists have taken a variety of approaches to answering this question, depending on the timescale of interest; we’ll have a look at the results of these different approaches in this section.
The first approach is the most obvious — you use thermometer data; this is usually called the instrumental record of climate. Let’s take a look at what some of these data look like for a place familiar to the authors — State College, PA. These data come from the US Historical Climatology Network, [4] where you can find data from stations around the US. These data are the monthly mean temperatures from 1849 to 1994, so there has already been some averaging of the data to remove the day-to-day variability.
What we are seeing in the above figure is weather, which is "noisy"; what we want is the climate record from this station, which is not obvious, but we will find it in the first lab exercise for this module. The data for this one station can give us a climate record for the immediate surroundings, but going from this record at one point to the global temperature requires a bit more work.
One approach is shown in the figure below. Say you have an array of weather stations on a map:
The general approach is to draw lines between a station and all of its nearest neighbors, then find the midpoints of these lines (circles in the figure) then make a polygon that connects the circles, giving an area (in gray). This area (in km2) represents a tiny fraction, fi, of the Earth’s surface that is associated with the temperature, Ti, of this station. If you do this for all stations, and sum all the Ti x fi values from each station, you would have a global temperature (all of the fi values would add up to 1.00).
You can see from the above example that if you have good weather stations spread uniformly across the planet (land and sea) and they have been recording continuously for a long time, then one can take the mean annual temperature of each station and calculate a simple global average for each year, and thus the history of temperature change for our planet. But, as you might imagine, the stations are not uniformly distributed — they are clustered in populated countries — and the number of stations declines as you go backward in time, so the actual process of assembling an instrumental record takes some care. A variety of groups have done this using slightly different data sets and approaches. The trick here is in how you combine the individual temperature records to come up with a global average. This is complicated by the fact that some weather stations may have problems related to things like the "urban heat island effect." Man-made materials retain heat better than open land and the lack of trees also amplifies warming in cities, which are currently warming at double the rate of the global average! Thus, if urban development encroaches on a weather station, the urban heat island effect will make the local temperature rise for reasons that are unrelated to any regional climate change. Researchers have found ways of ensuring that this effect does not skew the results, and many different groups come up with results that are nearly identical, giving us confidence that the data analysis is sound. Just in case you are wondering, the mean surface temperature of the Earth as a whole is 15o C (59o F)!
There are a number of good estimates of the recent history of global temperature change, and they are shown, plotted at the same scale, in the figure below.
The figure above shows anomalies relative to the mean for 1960-1980. GISTEMP is from NASA, CRUTEM4 is from the Climate Research Unit of the University of East Anglia in England, Berkeley is from the Berkeley Earth Surface Temperature project from the University of California, and NOAA is from NOAA (no surprise here). These different groups use essentially the same data, but they have slightly different approaches to selecting which data to use and how to convert the station data into global temperatures.
It is interesting to see how similar the curves are given that they use different strategies for averaging the data, and some of the records are based on slightly different sets of weather stations. In particular, note that none of these estimates show a general cooling trend over this length of time — they all show warming. Back in the 1800s, there were fewer weather stations, and so it is more difficult to estimate global temperature back then (see figure below), but it gets steadily better as time goes on, and for the last few decades, we have excellent data due to the satellites that now circle the globe taking temperature measurements of every spot on Earth (more on this in a bit). What we see in the figure above is a detail of the blade of the "Hockey Stick" — beginning about 1900, the temperature starts to rise, then it flattens out a bit in the 1950s and early 1960s, and then it increases again at a faster pace since that time. The total warming since 1900 is about 1.1°C as a global average.
Looking in more detail at the Berkeley temperature estimate, which is based on about 1.6 billion measurements, we can see that the uncertainty, indicated in the figure below by the green band surrounding the red line, gets progressively larger as we go back in time, but the uncertainty is practically zero for more recent decades.
This is a good point to explore a question about these records. Why does the annually averaged temperature rise and fall in such a complicated fashion? The sun does not vary in its brightness in such a dramatic fashion (the solar cycle related to sunspots can account for a global temperature variation of about a tenth of a degree), and the greenhouse gases that keep our planet warm do not vary in their concentration this much. Instead, it appears that a good deal of the variability seen in these records is related to things like volcanic eruptions and climate system oscillations like the El Niño – La Niña Southern Oscillation (ENSO), which is discussed in detail in Module 6. In short, ENSO is essentially a huge, sluggish, sloshing back and forth of warm water along the equator in the Pacific Ocean — it is like a wave that reflects back and forth between the two edges of the Pacific, and it has a global reach in terms of climate. During the El Niño phase of this oscillation, the warm water is pooled up on the eastern side of the equatorial Pacific and this has the effect of making the whole Earth warmer (the reasons for this are complex, but the effect is quite clear). Conversely, during the La Niña phase, the warm water is pooled up at the western edge of the equatorial Pacific and the whole globe tends to be cooler. The El Niño stage causes flooding rains in California, wet conditions in Florida (recommend you visit Disney during the La Niña!), but crippling drought in Australia and southern Africa.
The above figure shows the last 25 years of globally averaged instrumental surface temperature measurements. Also shown is the recent history of fluctuations in ENSO and the period of atmospheric disturbance due to the eruption of Mount Pinatubo in the Philippines in 1991, one of the largest of the 20th century; the volcano injected ash and sulfur gases into the upper atmosphere, where they blocked enough sunlight to cool the global climate for a period of about 3 years.
Satellites offer another way of studying temperature changes and they are not subject to the same problems associated with weather station data — they provide a complete coverage of surface temperature on land and at sea. But, as can be seen below, there is a very good agreement between satellite measurements and the weather station data (NOAA surface in the figure below). The only problem is that the satellite data only go back to about 1980.
The figure above shows the global instrumental temperature record in blue (NASA GISTEMP) is compared to two versions of the microwave sounder [7] satellite (MSS) data of lower atmospheric temperatures (UAH from Univ. of Alabama, Huntsville; RSS from Remote Sensing Systems, Inc.). The timing of the ups and downs in the satellite record are a near-perfect match with the instrumental record, but the magnitude of change is greater according to the satellite measurements. For comparison, we show the history of the El-Niño La-Niña oscillation and periods of volcanic eruptions that load the atmosphere with tiny particles of sulfuric gas that block sunlight and cool the planet. The eruption of Pinatubo in the Philippines had a big effect, and strong El-Niño periods lead to warming — together, these two variables (volcanoes, and El-Niño) along with small fluctuations in sunlight, account for the majority of the "noise" in these records. Another important point of this figure is that it confirms that the instrumental temperature record does a good job of representing what actually happened.
Next, we look at the spatial variations in the temperature over different spans of time.
The figure above shows the difference in instrumentally determined surface temperatures between the period January 1999 through December 2008 and "normal" temperatures at the same locations, defined to be the average over the interval January 1940 to December 1980. The average increase on this graph is 0.48 °C, and the widespread temperature increase is considered to be an aspect of global warming. The most striking feature of this map is that the temperature changes have not been uniform across the globe; the high latitudes (above about 50 degrees) in the Northern Hemisphere have warmed more than any other part of the Earth, while the tropics warmed far less. But Antarctica has been warming significantly too, and, most recently in 2022 there have been record temperatures 20 °C warmer than normal!
We now turn our attention to the spatial pattern of temperature change over a much longer range of time — back to 1884. Below is an animation of the temperature change based on the instrumental record. It is worth remembering that the quality and quantity of the data get better and better as time goes on, so the early parts of this animation have more uncertainty connected to them.
The movie below is from NASA’s reconstruction of surface temperature since 1884 and it shows how Earth has warmed over the last century plus in a very, very graphic and indisputable way. Just in case you can't see this, 2016 was the warmest year on record, and 16 of the 17 warmest years have occurred since 2000!
Click here if the video above does not play [11]
Play this movie and watch as the globe becomes dominated by the yellow, orange, and red colors signifying warmer temperatures. Note that the warming is not uniform across the globe, nor is it steady through time, but the warming trend is nevertheless clear to see.
Another way of looking at this history of warming is by taking the average temperature at each latitude for each year and then stringing those along the horizontal axis, as below:
In the figure above, as in the movie above, the temperatures are given as anomalies, or differences relative to a mean established from some arbitrary period of time (1951-1980 in this case). One thing that is clear is that the polar region of the Northern Hemisphere is the area that has warmed the most — more than 6°C during this time period, when the mean global temperature has risen by a bit less than 1°C. Also clear is the fact that starting around 1990, nearly all of the globe is warming. It's pretty hard to argue with this plot, isn't it?
But if you are still unconvinced, we have another dataset up our sleeves that is completely independent of all of the atmospheric data we have shown so far: the ground has also warmed up!
We next turn our attention to a very different means of reconstructing the temperature — studies of the temperatures measured in boreholes (i.e., holes drilled into the ground) at various locations around the Earth. The temperature profiles (how temperature changes with depth) for three representative boreholes in eastern Canada are shown in the figure below:
Note that in all three cases, the temperature curves around to higher temperatures near the surface — this reflects a response of soil and bedrock to warming from the atmosphere. In the absence of warming at the surface due to climate change, these temperature profiles would tend to follow the trends represented in the lower few hundreds of meters, and this would intersect the surface at around 3-4°C.
The basic idea here is a surprising one — that the way temperature changes down a borehole at the present time tells us something about how the surface temperature has changed in the past. This is indeed a remarkable and useful reality of some fairly basic physics of heat flow. It also provides us with an excellent way to filter out the “noise” in the climate record and focus on the main trends.
Heat is just a measure of the kinetic or vibrational energy of the atoms in some substance. If something is hot, its atoms are vibrating very fast, and because vibrating atoms affect neighboring atoms, heat can be transmitted; we often talk about this heat transmission as heat flow. Heat flows from hot regions to cold regions, and the rate of heat flow is proportional to what we call the thermal gradient — the rate of temperature change with distance. In our case, for distance, we are talking about depth in the Earth, and the center of the Earth is very hot — about 5000°C. The surface, instead, is quite cool at 15°C, so heat from the Earth tends to flow out to the surface, and this process is cooling the Earth very slowly. This situation leads to a geothermal gradient (rate of change of temperature with depth) that tends to be more or less steady at around 20 or 30°C per kilometer. The heat released to the surface is tiny compared to the energy coming from the Sun, so this geothermal heat, on a global basis, does not affect the climate.
When the surface temperature rises and becomes hotter than the temperature just below the surface, heat moves down into the ground, but it does this quite slowly. When the surface temperature becomes colder, heat flows up from the ground, cooling the ground, and this cooling is transmitted downward slowly. This general idea is illustrated schematically in the figure below:
Each of the four rectangles shows the variation of temperature with depth above and below the surface at different times. At the beginning (Time 1), the temperature below the surface increases steadily, while it is constant above the surface. Then at Time 2, the surface temperature suddenly rises and is hotter than the ground right at the surface. By Time 3, the ground temperature right near the surface warms, but that warming does not penetrate very deeply. At Time 4, the surface temperature has continued to remain high, and the heat flowing down into the ground has reached a greater depth.
Rocks have a very low thermal conductivity (conductivity is the term used to describe the way heat is transported at the molecular level) compared to many other materials, which means that it can take a long time for rocks underground to respond to changes in surface temperatures. Because of the way that the heat flows through rocks, short-term changes are smoothed out as the heat diffuses through the rocks. This means that the borehole temperature profiles provide information only about changes in the long-term average temperature.
Unlike most other methods for studying paleoclimate, borehole thermometry does not need to be calibrated against the instrumental record. Hence, borehole thermometry provides an independent record of paleoclimate against which other paleoclimate techniques can be validated. Below, we see the results of the analysis of a global data set of borehole temperatures, which give us an estimate of the global temperature change.
Clearly, the shapes of the curves, or the rates of temperature change over this time period, are in close agreement, which is important since they come from very different, independent data sources. The borehole temperature reconstruction does not match the last bit of this time period, in part because the measurements begin further down the hole, and many of the measurements were made in the 1980s and 1990s before the instrumental record ends.
The oceans have absorbed over 90% of the excess heat resulting from greenhouse gas emissions since the 1970s. So if it weren’t for the ocean, the land would be a lot hotter than it is today. Since water absorbs a lot of heat, the ocean’s temperature has not increased as fast as the land’s though, but there have been some alarming trends in the last year or so (2023-2024).
Daily Global Sea Surface Temperature records from 1981-2024 showing the significant increases in the last two years. Source: Climate Reanalyzer [12], Climate Change Institute at the University of Maine, based on data from NOAA Optimum Interpolation Sea Surface Temperature (OISST). Climate Reanalyzer content is licensed under a Creative Commons Attribution 4.0 International License [13].
Annual Global Sea Surface Temperature anomaly relative to the 1951-2000 time interval. Source: Climate Reanalyzer [12], Climate Change Institute at the University of Maine, based on data from NOAA Optimum Interpolation Sea Surface Temperature (OISST). Climate Reanalyzer content is licensed under a Creative Commons Attribution 4.0 International License [13].
The instrumental record of temperature change in the oceans goes back to about 1850 and consists of thermometer measurements made on water samples taken by merchant and navy ships as they sailed the world’s oceans. The data are understandably best for parts of the oceans along major trade routes, and they are less abundant further back in time. These measurements, just like the land-based weather station data, have to be gridded to come up with a global average sea surface temperature. As might be expected, the sea surface temperature record is similar to the global temperature records, in part because the oceans make up almost 75% of Earth’s surface. But even if we separate out the land surface temperature from the global record and compare it to the ocean surface temperature, they are quite similar, as seen in the figure below.
Although the two records are quite similar, there are some differences — the SST changes over a smaller range than the land surface temperature, and the land temperature is subject to more dramatic swings. This difference is largely due to the greater heat capacity of the oceans relative to the air — it takes a long time to heat and cool the oceans, but air temperature can change quite rapidly.
Measurements from a system of hundreds of buoys stationed throughout the oceans allow us to take the temperature of the oceans over a depth range of 2000 m. These measurements go back in time to 1955 and show that not just the surface of the oceans, but the whole upper half of the oceans are slowly warming — only about 0.1 to 0.2 °C averaged over the globe during the past 50 years — but this is a vast amount of water that has been warmed.
So, while the whole ocean has absorbed a huge amount of heat, its overall temperature has changed little. Nevertheless, the very surface of the ocean has warmed almost as much as the rest of Earth’s surface and from the middle of 2023 through to 2024 the surface warming has been quite alarming with temperatures almost a degree warmer than in 2016.
We should pause and make a point or two about these temperature reconstructions because they are very important to our understanding of how Earth's climate has been changing.
We now turn our attention to water in the atmosphere. Water is a tremendously important part of the climate system, and it has a huge influence on the weather we experience every day. Clouds are made of water droplets or tiny ice crystals, and obviously, precipitation is water; but you also can sense the hidden water vapor in the form of humidity. If you don't understand the concept of humidity, plan a trip to the Magic Kingdom in Orlando, or, worse still, New Orleans in August! As we will learn in Module 3, water is one of the most important ways of transporting energy in the climate system. When water evaporates, it takes heat energy from the surface and carries that heat with it until it condenses back into liquid water, at which point it releases that heat into the atmosphere — this is what powers energetic storms. If you watch a large fluffy cloud building up on a summer day, expanding and growing up to greater and greater heights, just remember that all of that swirling movement is driven by the energy releases from water vapor.
The evaporation of water speeds up when it gets warmer. You could confirm this by doing an experiment with two pots of water on the stove, with the burner beneath each set to a different temperature — the hotter one will always evaporate faster. The same is also true with Earth's climate system — a warmer planet means more evaporation, which means more energy added to the atmosphere. And warmer air can hold more moisture than can colder air. If we study the laws of thermodynamics, we find that for a 1°C increase in the air temperature, the atmospheric water content should increase by about 7%. Until recently, it was difficult to measure the global water content in the atmosphere, but with the advent of satellites, we can now do this.
This map above shows the relative changes in humidity (atmospheric water content) at the end of 2010 compared to the average over the period of satellite observations (1981 - 2010) — so this is a type of humidity anomaly map for the year 2011. The green areas are more humid than normal and the brown/orange areas are drier than normal. On the whole, you can see that the globe is moister than normal. You can also see that the eastern side of the equatorial Pacific Ocean is drier than normal — this is because 2010 was a La Niña year, and warm water was pooled up on the western side, cooler water on the eastern side; the cooler water evaporates less, hence the drier atmosphere above that region.
If we look at the longer record of the globe as a whole, we can see how the water content of the atmosphere has been changing over the last 40 years.
The thick green line in the figure above is the global humidity anomaly (data from NOAA), with its best fit linear trend as the blue dashed line. Over this time period, the water content has risen by about 5%. The thin blue line is the history of the El Niño - La Niña oscillation — the seesaw sloshing of warm water back and forth along the equatorial Pacific Ocean. Positive values (scale on the left) indicate an El Niño year when more of the warm water sloshes over to the eastern Pacific (the South American side); negative values mean the warm water is pooled up on the western side near Indonesia. As can be seen, some of the fluctuations in global humidity correspond to the El Niño history, with more moisture generally associated with an El Niño year. But the general trend is rising humidity, and the El Niño history does not show a similar rise — this tells us that while El Niño is important, the underlying trend is more likely related to a warmer planet.
The central point here is that a warming Earth should have a more humid atmosphere and indeed that is the case, and more water vapor means more energy in the atmosphere.
In the summer of 2021, Western Europe experienced severe and deadly flooding. The rainfall was extraordinary, possibly a 1000 year event as a storm system stalled for several days dumping 11 inches in 48 hours in eastern Belgium including 9 inches in the populated city of Liege, and up to 8 inches of rain in 9 hour period in Germany. Flooding occurred over a wide area including Belgium, Germany, Luxembourg, the Netherlands, Switzerland, and Austria. In Germany 243 people died, including 196 in Germany. In Belgium, floodwaters caused buildings to collapse and washed away cars. The floods were called the greatest natural disaster the country has ever experienced. In Germany, the Ahr river valley was particularly bad because gorges caused extensive flooding. The country’s flood warning system was cited for a monumental failure, although it appears that local authorities take some of the blame.
Fast forward to California in the winter of 2022-2023 and a series of atmospheric rivers bringing moisture from the tropics dumped huge amounts of rain on the coast and inland areas and snow on the Sierra Nevada. 78 trillion gallons of moisture fell during this time with over 30 inches and major flooding in lowland areas and up to 58 feet of snow in the mountains.
Atmospheric rivers hitting California in January 2023
Credit: https://photojournal.jpl.nasa.gov/archive/PIA25597.mp4 [18]
One near unanimous source of blame among scientists was climate change, with the flooding even exceeding current forecasts. Warmer atmosphere can hold more moisture. In addition, melting Arctic ice is causing the jet stream to weaken and this is leading the storms systems that move slower or stall. Combined these factors are leading to more extreme events including days of flooding rains.
As stated above, warm air holds more moisture than cooler air. What does a moister atmosphere mean for precipitation? It means two things — more precipitation over the globe and a higher frequency of extreme precipitation events. The spatial pattern of precipitation is complex — far more so than for temperature — and measuring the frequency of extreme events is a challenging statistical problem. But some progress has been made on these questions, at least for certain regions of the globe.
First, let's take a quick look at the precipitation pattern of the globe, which is now being measured in great detail by NASA's Aquarius satellite.
The two images above show the rainfall averages in terms of mm/day for the month of March 2019, above, and the rainfall anomaly for the same month, below.
Earlier satellites did not have the same resolution, but the record goes back to the late 1970s, allowing us to get a picture of the longer-term mean precipitation patterns, which can be seen in the video below. Watch this movie a few times through to see the annual patterns of precipitation, and focus especially on the region around India, where the summer monsoons show up beautifully.
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On a global scale, there is no clear sign that the amount of precipitation is increasing, as can be seen in this next figure, which plots the global average precipitation rate for each month.
The figure above shows the history of the monthly average precipitation rates, averaged over the globe — kind of a difficult thing to swallow at first. Think of the values plotted along the y-axis as being the precipitation rate, averaged over the whole globe for a given month. As you can perhaps see, there is a strong seasonal cycle (with peaks in the fall each year) — I've removed the seasonal variation from the raw data to give the thicker blue line, showing variability that is not related to the seasonal cycle. One thing that is clear is that there is no general upward or downward trend over this period, although there are ups and downs, some of which correspond to the El Niño oscillations.
So, global precipitation, on the whole, is not going up, as far as the data reveal. What this means is that although the atmosphere is getting moister, the new addition of moisture is not coming out as precipitation — the atmosphere is retaining this extra water vapor.
But because the trend suggests there will be extra water vapor in the atmosphere in the future, when the conditions are right for a big precipitation event, the event might be bigger than today or in the past. In addition, predictions suggest there might be more storm events that exceed a certain threshold so that they are classified as extreme precipitation events.
More regionally, the El Niño cycle produces a dramatic change in precipitation patterns in parts of the globe, with Southeastern Australia becoming dry in summer and more prone to bushfires. In the US, El Niño corresponds to heavy winter rainfall in California as the so-called "Pineapple Express" picks up and transports heat and moisture from the tropical Pacific. Folks in California always hope that El Niño events put an end to drought. More on that in Module 8. El Niño is generally not a good time to visit Florida in the winter, as the same pattern extends across the southern US.
Drought is a very familiar foe in parts of the US, significant regions of Africa, and much of Australia. Drought is often called a creeping disaster, as it decimates a region slowly. We will talk a lot more about drought in Modules 8 and 9. Here, we briefly consider the recent record of drought. As we saw previously with research on drought and the Mayans, oxygen isotopes of stalactites can be interpreted in terms of precipitation. Another indicator of drought is the width of tree rings. One of the most comprehensive tree-ring data sets is shown in a compilation of precipitation in New Mexico from 137 BC to 1992 (The New York Times, The Longest Measure of Drought: 21 Centuries of Rainfall in New Mexico [23]). The data set shows that with the exception of a wet phase in the last decade, much of the 20th century was dry in New Mexico, and, in general, the data suggest that western North America has become drier over the last 1000 years.
As it turns out, there is solid evidence that the severity of droughts has increased over the long term in many parts of the world. And, as it turns out, climate models predict that drought will become a part of everyday life in many regions in the coming century. In the US, these areas include large parts of the west and south central (Kansas, Oklahoma, Texas). Especially in California, climate models suggest that warmer years in the future are more likely to also be drier years, and this will increase the severity of future droughts.
So, how is the severity of drought quantified? The most widely used method to measure drought today and in the past is the Palmer Drought Severity Index (PDSI). This index is an accounting of the balance of supply of moisture via precipitation and demand for moisture via potential evapotranspiration (PE). The PE is the amount of water that could be evaporated given an unlimited supply of water. The National Oceanic and Atmospheric Association (NOAA) publishes a map of the PDSI over the US every month (see figure below).
The PDSI has increased (become drier) for the driest parts of the globe since 1950. For most of the US, the PDSI has decreased for this time period. However, the last century has seen severe droughts in certain regions. For example, the major drought of the 1930s Dust Bowl in the Central US caused severe hardship for farmers and others in this region. The 1950s and 1980s also saw severe droughts in the Great Plains of the US. We will find out in Module 4 how models project that the severity of droughts will increase in many parts of the globe with future climate change. Moreover, we will learn more about the impact of drought on water supply in Module 8 and on food in Module 9.
Drought is currently at a critical level out west. Lake Powell, which is the second largest reservoir in the western US is currently at its lowest level in history and approaching the level where water and hydroelectric power supplies are threatened. This event would be especially devastating for communities that rely on the lake for water, such as Las Vegas, Nevada. We will come back to this issue in Module 8.
The latest (2022) report of the Intergovernmental Panel on Climate Change (IPCC), the large international team of scientists tasked with predicting our climate future and its impact, issues stern warnings about drought in the western US, Australia, the Mediterranean region and sub-Saharan Africa. Drought will have a major impact on human health, largely because of its effect on accessibility to clean drinking water, and require major adaptation of communities. Adaptation can involve conservation measures as well as finding new sources of water, for example desalinization. Such adaptation will be easier in developed countries like the US compared to the developing world, where countries like Yemen and Madagascar are already struggling with the devastating impact of drought and do not have the resources required to adapt.
Summer 2023 will be remembered for the truly devastating and catastrophic fires that destroyed the historic tourist city of Lahaina in Maui. As of the date of writing the fire caused 115 fatalities with 66 people still unaccounted for. More than 2200 buildings were destroyed and damage expected to exceed $6 billion. The cause of the tragedy is still under investigation, but it was likely sparked by electric utility poles downed by hurricane force winds. These same winds were responsible for the exceedingly rapid spread of the fires that made evacuation extremely difficult. The failure of warning systems to alert residents is also under investigation. Almost certainly, climate change is at least partially responsible for the fires. A lush tropical island, Maui like the rest of Hawaii, has become much drier since 1990 especially during the summer wet season. Hotter temperatures result in thinner clouds which hold less rainfall. Another critical factor is the area around Lahaina, once the site of sugar cane farms, is now covered by invasive grasses that acted like a tinderbox during the fires. Sadly the Lahaina tragedy is a sign of the future of many places around the globe.
Fire ravaged the waterfront in Lahaina, Maui on August 8th, 2023
Credit. https://commons.wikimedia.org/wiki/File:Os-lahaina-town-fire.jpg [31]
The 2023 summer began with smoke-filled skies across the eastern US from fires raging in Canada. The quality of the air in New York City was the worst of any large city in the world. The last five years have brought devastating fires to California and other western states! Images from the news have shown San Francisco’s air full of smoke and glowing red. Every year brings new records and new tragedies. Deadly fires are not unique to California; in fact, Australia has a history of particularly devastating fires. And climate change is going to provide all of the ingredients for more fires in the future: more fuel (from extreme rainfall events), more effective natural ignition (often dry lightning) and often, sadly, arson, as well as the conditions to keep fires burning (drought and heat). The latest IPCC report issues some stern warnings about fire, predicting that the livelihood of millions of people will be impacted by wildfires in the future. The citizens of California, Australia and other parts of the world need to get used to apocalyptic fires. The last five years have been a wake up call.
The deadly Camp fire, the most deadly and destructive fire in California history, began November 8th, 2018 from a spark from an electrical wire. By the time it was finished two weeks later, the fire had burned more than 1500,000 acres around the town of Paradise. The fire occurred late in the season and was whipped up by strong winds and abundant fuel from the previous rainy season. A virtual firestorm quickly converged on Paradise, a ridge-top town that was used to fires but had grown fast and without good evacuation planning. On November 8th, the evacuation order came too late, and many folks did not receive it because cell towers were down. Because the call was so late, evacuation could not occur in an orderly fashion, from one side of the town to the other. Hundreds of vehicles quickly jammed the three routes out of town and the fire quickly trapped many people in their cars. Folks who did not get the evacuation order were trapped in their homes. A total of 86 people in Paradise were killed and the town was almost completely burned down.
In the wake of the fire, the large utility company PG&E filed for bankruptcy to avoid major lawsuits. Turns out the company discussed turning off the power grid in the days before the fire began but decided not to. Now in the wake of the fire, companies will be much more likely to turn off the grid if there is a chance of fire and this will impact millions of customers in California.
As it turns out the Camp fire was only one of two major fires that began on November 8th. The massive Woolsey fire west of LA also began that day, burned 97,000 acres, killed three people, and forced the evacuation of 295,000 people.
The Camp and Woolsey fires are the “new normal” for California and the state must deal with difficult issues such as vegetation management, sensible development, evacuation planning, and regulation of power companies.
2020 was a record-breaking fire year in California. More acres burned than ever before (over 4.3 million), at least in recent times, with over 9200 separate fires including the first “gigafire”, the August Complex Fire that burned more than 1 million acres (over 1500 square miles)! Total damages exceeded $2.5 billion and 32 people perished. Although arson was to blame for some of the fires, more often they were sparked by lightning that occurred in thunderstorms without rain. The largest fires in Northern California, the SZU Lightning Complex fire, the LNU Lightning Complex Fire, and the August fire were all started by lightning strikes.
The fires were caused by the continued drought in the state and across the western US, and, indeed large fires occurred over much of the region as far east as Colorado and as far north as Washington state. In all over 52,000 fires consumed almost 9 million acres.
The year was also notable for fires that encroached on Seattle and especially Portland. In fact, wildfires caused an especially scary situation in the latter metropolitan area where 40,000 people had to be evacuated and 500,000 lived in evacuation warning zones at the height of the blazes. These fires in the northern part of California, Oregon, and Washington were responsible for extremely hazardous air quality for weeks, and in all over 17 million people experienced unhealthy or hazardous air this year. In fact, the air in Marion County in Oregon was so polluted it registered off the measurement scale. The air quality in Northern California including Sacramento and San Francisco was unhealthy for weeks, and young, elderly, and people with asthma, COPD, and other preexisting conditions were advised to remain indoors. The smoke can cause burning eyes and lung ailments including bronchitis and aggravate heart and lunch diseases. The elevated levels of fine particulates including hazardous compounds may also cause longer-term health issues. This public health situation was especially difficult as it superimposed on Covid-19.
2020 will be remembered for the scale of the western fires but also for their profound impact on public health. On top of the possibility of evacuation and loss of property, many residents of the western states were awakened to the prospect that their “new normal” fire season included toxic air.
2021 has already been a devastating fire season. In Oregon the Bootleg Fire has burned over 400,000 acres and destroyed more than 400 homes. The fire was started by lightning and took a month days to contain. This fire was rapidly eclipsed in size and damage by the Dixie Fire in northern California. The fire was caused by a downed power line and to date has burned over 560,000 acres and still is not contained as of mid August. The fire destroyed Greenville, a town that dated back to the Gold Rush with a population of 1000 where people were forced to rapidly evacuate as the flames approached.
The 2020 fire season in California was the worst on record. However, these are not the most devastating fires in Earth’s recent past. Black Saturday Bushfires in February 2009 in Victoria, Australia were fueled by extraordinary heat and strong winds. At the peak of the inferno, there were some 400 blazes. Conditions leading up to the fires were extraordinary. Mercury hit 116 degrees in Melbourne in a heatwave that started the week before the fire, and in the peak of the summer drought, the dry brush was perfect fuel. On the fateful Saturday, the first fire was started by arson. Falling power lines and lightning ignited other fires. Fires consumed some 1.1 million acres, destroyed whole towns, caused some $4.4 billion in damage, and killed 173 people. Most of the damage was done in the first few days, but the blazes raged for weeks. One of the astounding aspects of the fires were observations by firefighters of sideways mini-tornadoes, technically called horizontal convective rolls. As the air at the surface warmed and rose, it was forced to move in a corkscrew pattern-oriented parallel to the ground. This created bands of alternating fast and slower surface winds. Fast winds surpassed 30 mph and ignited huge swaths of land in a catastrophic fire. There are terrifying stories of people getting swept up in the flames trying to escape the inferno on tiny mountain roads.
The late part of 2019 and the early stages of 2020 have brought more intense wildfire to Australia, especially near Sydney in the southeast. The season began very early as a result of the decade long drought and intense heatwaves (see Heatwave page this module). As of this reporting fires have burned millions of acres, destroyed thousands of buildings, and killed dozens of people and millions of animals including koala and kangaroos. The smoke has caused unhealthy air in eastern Australian cities and the plume has drifted as far as New Zealand. The fires have caused a political upheaval because Australia has a large emission of carbon per capita and produces a significant amount of energy from coal. Given how susceptible it is to climate change, many citizens feel that the country is not doing enough to curb emissions. 2019-2020 is a glimpse of the future for Australians.
So, how is increased fire activity related to climate change? Fire is very much a part of the ecosystem in places like Australia, California, South Africa, and Southern Europe. Even before people were around, fires ignited by lightning occur regularly in environments with dry seasons, as nature’s way of germinating drought-resistant species and fertilizing the soil. But people have made fire events much more common. First, many fires are started by arson or in the cases of the Lahaina and Camp fires by downed electric utility poles. Clearly, heat and drought are good for fire, and as we have seen, both of these ingredients will increase in the future as a result of anthropogenic climate change. But the other key aspect of fire is fuel and which is supplied by precipitation and active growth of vegetation. Climate change is likely to cause more variability in temperature and precipitation that will create more contrast between drought and wet years. This will lead to greater fire risk. The heavy rains in California in the winter of 2016-2017 caused a significant growth of vegetation in uncultivated hills and canyons surrounding residential areas and this dried out in the hot and dry 2017 summer. Then, once the warm fall Santa Ana winds arrived, the recipe for disaster was all ready.
The main culprit for the increase in fires in the western US and Hawaii is the long-term drought. However, development in forested and brushy areas that are prone to burn. Clearing of brush in populated areas has led to the establishment of invasive plant species that grow very rapidly and provide fuel when they die back in the dry season. There has been constant friction between responsible forest management with regulation and development. The potential for gigafires will force a reckoning between planning and forestry departments in the future so that forest and brush can be managed through controlled burns. Many communities are also replanting native plant species and using animals to clear out areas overwhelmed by invasives. At the same time, growth needs to be managed so that safe evacuation can occur in the case of a large fire. Sadly, highly destructive wildfires are part of California’s future.
As are mudslides. The huge Thomas fire destroyed so much vegetation that held hill-slopes in place. They were followed in early January by major rains that led to torrents of mud from hills into valleys. The catastrophic mudslides killed at least 20 people and caused massive property damage. Fire and mud are intricately related.
One of the main messages from the 2022 IPCC report is the need to adapt to a future in which fire is more common and more deadly. In fact California is already making progress adapting its communities to live with the threat. In some areas, development is being banned in risky locations near forests for example, and best practices are being applied for clearing brush and maintaining fire breaks. Australia has been applying these best practices for a number of years also.
What is also clear from the IPCC report is that wildfires like drought will have a major impact on human health especially for children, the elderly and the poor. The impact will also be more severe in the developing world where fires set for the purpose of deforestation are having a major effect on human health as well as wiping out large numbers of species, a topic we will return to in Module 11.
Heat waves are days to week-long events with extremely high temperatures. These events are becoming more common with a changing climate, are forecasted to become frequent in many parts of the world in the future and occur earlier in the year (June 2022 has been a dress rehearsal for that!). Heatwaves are often part of extended droughts and associated with wildfires. They are major public health risks, especially for very young and older populations, as well as the poor who do not have access to air conditioning or basic hydration. In the developing world, heat waves can be very deadly, in India in 2015 more than 2200 people died due to excessive heat.
Some of the most drastic heatwaves occur in Australia, a continent which is also characterized by devastating drought and perilous wildfires. The last few years have seen major heatwaves across the continent, shattering local and continental temperature records. In 2013 Australia got so hot that they had to add new colors to the temperature map. There were days that year when the center of the continent topped 125 degrees F (52 degrees C).
In 2019 Australia broke records again in a summer of wildfires, drought, and heat. On December 19, the average high for the whole continent was 107.4 degrees F (41.9 degrees C) shattering the record set the day before by over a degree C.
In the US, heatwaves in the desert southwest including Phoenix and Las Vegas are part of a normal summer. Phoenix regularly reaches 112 degrees F (44.4 degrees C) and it has been known to exceed 120 degrees F (48.9 degrees C). These are shade temperatures, corresponding temperatures can reach 168 degrees F (76 degrees C) in the sun right above the ground level. Because concrete traps heat, cities like Phoenix get particularly hot and are prone to heatwaves. In fact, this “heat island” effect makes cities as much as 7 degrees F warmer during the day. Nighttime often does not provide much relief, with temperatures above 80 deg F for many nights in a row.
June 2021 was a sign of things to come in the northwest US and western Canada, with temperatures topping out at 116 degrees F in Portland and 108 degrees F in Seattle. Lytton in British Columbia reached 121 degrees F, the hottest temperature ever recorded in Canada. These temperatures smashed records for these cities that are normally cool in the early summer.
The heat caused a minimum of 500 fatalities in the US and Canada among vulnerable populations, including the elderly. These cities are not adapted to extreme heat, with a low percentage (about 40%) of homes having air conditioning. The heat was so extreme that up to a billion of clams and other shellfish were cooked inside their shells in an ecologic catastrophe. The heat caused major wildfires to rage throughout the area, including the town of Lytton which was virtually destroyed. Several weeks later and further south, temperatures reached 130 deg G in Death Valley, matching the world record the hottest temperature ever recorded.
July 2022 saw record breaking temperatures in Europe and notably in London where it reached 40.2 deg C or 104.4 deg F, an all-time record. That city is not adapted to temperatures that high and will have to invest significantly in areas like air-conditioning public transportation in the future.
Heatwaves will become more common and more extreme in most places in the future as the planet warms. Europe and Australia are going to experience more and more of them, as are places in India. The southern US will seem more like the Middle East in the future, with cities like Austin and El Paso becoming as hot as Dubai is today and Phoenix approaching Baghdad, Iraq temperatures. Washington DC is going to seem more like Austin in the summer, Boston will seem more like Philly and Billings, Montana more like El Paso.
More dramatically, there may be places in the Middle East and Northern India where humans may not be able to live in the future because it is impossible for the human body to cope with the searing heat. To be more precise, a wet bulb temperature that factors heat and humidity of 95 degrees F or 35 degrees C is where it is thought that a combination of kidney, heart, or even brain failure may commence, especially for vulnerable populations.
Like drought and fire, the 2022 IPCC report stresses the need for adaptation to heat. This is already taking place in the developed world where cities are reducing the heat island effect by, for example, planting more trees, making roofs green by covering with plants, and using materials for pavement that reflects heat as opposed to absorbing it. Unfortunately, these strategies are more difficult to apply in developing nations where necessities like air conditioning are also less available. Thus, it is likely that heatwaves will become increasingly deadly in coming decades.
Antarctica and Greenland, the two large ice sheets, represent some of the most bleak and hostile places on Earth. Not many geoscientists have the mettle to explore these remote places, but they remain one of the essential frontiers for research. These large and thick ice sheets look relatively homogeneous compared to other parts of the planet, but in fact, their behavior is not completely understood. The fact remains that if the ice sheets on Antarctica and Greenland were to melt, a feat that cannot happen over decades or even centennial time scales, don't worry, global sea level would rise by about 70 meters which would drown most coastal cities such as New York, Shanghai, Mumbai, and Jakarta. Just this threat should cause global leaders to stay up at night, though!
Ice is the frozen segment of the atmospheric moisture cycle. As you will remember from the discussions of Snowball Earth and the Pleistocene ice ages in Module 1, it is a very important component of Earth’s climate system — ice is the most reflective material on the surface, and as such it can exert an important control on how much sunlight the Earth absorbs, which directly affects the Earth’s temperature.
Ice is also an important and highly sensitive indicator of climate change in the polar regions and in areas of high altitude where mountain glaciers occur. Glaciers will grow or shrink in response to changes in temperature and precipitation. The temperature response is pretty obvious — glaciers melt as the temperature rises. The precipitation response is perhaps less obvious, but glaciers can expand if winter precipitation increases, and they shrink if the winter precipitation decreases. Just as the ground temperatures respond somewhat sluggishly to surface temperature changes, glaciers respond sluggishly to climate changes, which is good in the sense that they give us a better sense of the important trends in climate change that might otherwise be obscured by short-term variations. In the following pages we discuss the key different types of ice and how it is changing: glaciers in mountainous regions, sea ice in the Arctic, and the large Antarctic and Greenland ice sheets.
We’ll start with some striking images taken by glaciologists around the world — these are photos of mountain glaciers taken from the same spot at different times, and they provide us with some fairly shocking observations on just how much glaciers can change and have done so recently.
For another Alaskan example, we turn to satellite imagery of Columbia Glacier, which obviously does not reach back into time as far, but nevertheless, there are some dramatic changes evident here:
The above scene is from 1986, and at this point in time, the end of the glacier (terminus) is located down near the bottom of the image. Below, we jump forward in time to 2011:
As you can see, the terminus here has retreated by about 15 km in just 25 years — a very impressive rate.
Grinnell Glacier, in Glacier National Park, Montana, as seen from the same vantage point over a 67 year period. The glaciers in this famous national park are all in such rapid retreat that the park may need a new name in a few decades.
Glaciers in the Alps are shrinking too. Check out SwissEduc Glaciers online [49] to see one good example — at the bottom of the page is a comparison that flips back and forth from the past to the present as you move your mouse over the image.
The same story as seen in Alaska, Montana, and the Alps holds for glaciers in more tropical settings, as can be seen from the images of Qori Kalas glacier in the Andes Mountains of Peru, below.
Studies of glaciers around the world show that an overwhelming majority are losing mass over time. In most cases, this loss of mass is reflected in the glaciers' retreating, so the length of the glacier becomes smaller. The figure below shows a selection of data from glaciers around the world, documenting this pattern of retreat.
The vast majority of glaciers on Earth are melting, and this melting began about the time that the temperature records indicate the beginning of warming, around the beginning of the 1900s.
If you combine the records of glacier length changes from around the world into one graph, we can get a pretty clear idea of what is happening.
On the graph above, the y-axis plots the length of the glaciers relative to their length in 1950 — so this is a kind of length anomaly. A positive number means that on average, glaciers were longer than they were in 1950; negative numbers mean they were shorter. Here, we can see that beginning around 1850, glaciers around the world begin to shrink, and this trend continues to the present. The average glacier has retreated almost 2 kilometers in this time.
It is possible to estimate the magnitude and history of temperature change needed to produce this history of glacial retreat, and Oerlemans (2005) did this using a simple model; the results are shown below.
The thick blue line here is the temperature history needed to produce the timing and magnitude of the glacial retreat history shown in the previous figure. For comparison, we also see the instrumental temperature record in red (Jones and Moburg, 2003) and the temperature reconstruction based on multiple climate proxies (Mann et al., 1999). Note the excellent match with the instrumental record in the last century.
Just like their smaller counterparts, the huge ice sheets of Greenland and Antarctica are also shrinking. This is critical as melting of these ice sheets impacts climate through albedo feedback (Module 3) as well as global sea level (Module 10). Melting of all of the ice in these ice sheets would raise global sea level by about 70 meters (actually, Greenland would produce 6 meters and Antarctica about 60 meters). Don't worry, this isn’t going to happen any time soon, but the concern is that large glaciers on the edge of the continents are becoming increasingly unstable and will fail in coming years. The Thwaites glacier also known as the doomsday glacier is the one scientists are most concerned about. Collapse of this glacier could happen very rapidly and raise global sea level by 65 cm. We discuss this glacier in more detail in Module 10.
Given the remoteness and difficulty associated with studying these ice sheets, we only have good data on their size for the last decade, thanks to the advent of satellite systems that can monitor these glaciers. In particular, the GRACE satellite system has provided some very important data on the changes occurring in Greenland and Antarctica. This ingenious satellite system consists of a pair of satellites that are chasing each other in the same orbit around the Earth. The distance between the satellites changes according to subtle changes in gravity on the surface of Earth. As the lead satellite approaches a region of stronger gravity (due to more mass near the surface), it pulls away from the trailing satellite and then slows down as it passes the region of excess mass. In this way, the satellites can measure the subtle changes in gravity, and since the satellites pass over the same area every few days, they can detect changes in the gravity of a certain spot over time. If a big ice sheet loses mass due to melting, its gravitational effect on the satellites diminishes, and in this way, the satellites can detect the changes in the mass of these ice sheets — they are effectively “weighing” these glaciers, which is an extraordinary achievement. The results can be seen in the videos below. Note: videos do not have audio.
Download simulation [51]
https://svs.gsfc.nasa.gov/31156 [52] REPLACE WITH https://svs.gsfc.nasa.gov/31156 [52]
Download simulation [53]
https://svs.gsfc.nasa.gov/31158 [54] REPLACE WITH https://svs.gsfc.nasa.gov/31158 [55]
These simulations above show the time series of Greenland and Antarctic ice mass changes from GRACE satellite data. You can see that melting is concentrated near the edges of the ice sheets and occurs in fits and starts (i.e., it is not gradual). The edges of the ice sheets, the ice shelves that float on the ocean, are holding the ice sheets back in a process known as buttressing. So once the ice shelves melt fast, this speeds up the melting of the edges of the ice sheet.
Other satellites passing over these ice sheets can measure the areas where surface melting produces small ponds of melt during the melting season. Much of the meltwater freezes back into the ice, but in some places near the edges of the ice sheets, the water sinks down into crevasses and travels all the way to the bottom of the ice, where it can lubricate the base and help accelerate the flow of the ice.
The above image shows the Greenland melt anomaly, measured as the difference between the number of days on which melting occurred in 2007 compared to the average annual melting days from 1988-2006. The areas with the highest amounts of additional melt days appear in red, and areas with below-average melt days appear in blue. Although faint streaks of blue appear along the coastlines, namely in northwestern and southeastern Greenland, red and orange predominate, especially in the south.
The video below shows the dramatic loss of ice on Antarctica between 2002 and 2016. The loss is mostly around the edges of the continent and focused in a few areas.
One of the most striking examples of climate change is related to the Arctic sea ice; the video below shows the drastic changes in the extent of Arctic sea ice over the last 40 years. The ocean is now permanently open to shipping in the summer.
This visualization shows the age of the Arctic sea ice between 1979 and 2023. Younger sea ice, or first-year ice, is shown in a dark shade of blue, while the ice that is four years old or older is shown as white. The graph displayed quantifies the area covered sea ice 4 or more years old in millions of square kilometers.
The Arctic sea ice undergoes large fluctuations over the course of a year, and, like all aspects of the climate system, there is a good deal of natural variability. So, while one or two extreme years do not necessarily make a trend, they may be part of a trend. Visit NASA's Earth Observatory [59] to see a nice set of maps looking down on the North Pole showing the sea ice extent over the last 12 years; each map shows the long-term mean ice extent in a yellow line. One thing that becomes apparent is that there is much more variability in the end-of-summer minimum ice extent than the end-of-winter maximum; another thing that is apparent is that the reduction in Arctic sea ice is now a long term trend.
In the figure below, we see a summary of data on the ice extent, reported as an anomaly (departure from the mean), and it now becomes apparent that there has been a more or less steady decline since about 1970.
In addition to the reduced area of coverage, the Arctic ice is also becoming thinner. The thickness of the ice in the Arctic has been monitored for a long time by the US Navy, using submarines. Now, satellites can measure ice thickness. The comparison below shows a 40% to 50% decrease in the thickness of ice from the average over 1958-1976 compared to the present.
This reduction in the coverage of Arctic sea ice is significant since it means that during the summer months when the sun is at its brightest in the polar region, and there are 24 hours of daylight, the reflective ice cover is being reduced, allowing for the absorption of a much greater quantity of solar energy that can then warm the whole polar region. This change in sea ice coverage is not just an Arctic phenomenon — in the Antarctic, the sea ice is also decreasing in area as the map below illustrates.
Finally, a word on sea level. The melting of ice sheets and mountain glaciers is partially responsible for a significant rise in sea level---20 cm in the last 100 years. However, thermal expansion of seawater as a result of warming is equally, if not more, important. Also, since sea ice is already at ocean level, its melting does not contribute to sea level rise.
Like many Americans, I used to dream of owning coastal property. I have experienced hurricanes - I went to help friends recover from Andrew in Miami in 1992, and I lived through Hurricane Fran in NC in 1996, so I’ve seen the destruction they can cause. But I still maintained my dream. Every time I go to the beach, I pick up real estate brochures; on a cold, snowy weekend, I would look at coastal properties on Zillow. But my dream is being replaced by practicality. Coastal property is a risky investment in the 21st century! The research shows that fueled by increased heat from global warming, hurricanes are becoming stronger, slower moving and wetter, all recipes for increased devastation of coastal communities.
We are going to focus on storms from 2017, 2019, and 2020. We start by looking at the year 2017. Harvey, Irma, Maria, three massive hurricanes occurred in three weeks! The big question is whether 2017 was just an unusually active year, or if these monster storms are a new normal, the grand result of a warming planet.
Hurricane Harvey, August 2017
Each of these storms had some incredible elements. The eye of Harvey roared ashore in south Texas more than a hundred miles south of the booming metropolis of Houston, the fourth-largest city in the US with a population of close to 6 million people. The storm moved north towards the city and literally parked there for 4 or 5 days drawing moisture in from the warm Gulf of Mexico. By the time the storm moved on to the north, parts of Houston had received close to 52 inches of rain. Harvey dumped a grand total of 33 trillion gallons of water on Houston and points north, causing catastrophic flooding. Just for context, 33 trillion gallons would fill a cube with sides of 3 miles, it’s a massive amount of water! Several small creeks north of the city were over 19 feet over their banks. The storm was the single largest rain event in the lower 48 states of the US ever! The total damage, still rising as I write this, is estimated to be about $150 billion, again one of the largest ever catastrophes. Harvey caused 82 deaths in the US.
At the outset, Houston is a very flood-prone city. Houston lies on a flat plain near the ocean. The natural landscape is grassland that is drained by creeks called bayou. The city lies very close to the Gulf of Mexico, which gets very warm in the late summer, close to 87 degrees F! This warmth is transferred to the air keeping coastal areas warm and humid. The Gulf is an enormous heat and water factory. A key fact to understand is that for every one degree of temperature increase, an air mass can hold 3 % more moisture, so as the Gulf has warmed over recent years it contributes more energy and rain to hurricanes making them most intense and much wetter.
Houston might have been able to absorb this change in its natural state, but the city has grown very rapidly over the last two decades as industry has surged and jobs have been plentiful. The city prides itself on being business-friendly and as a result, it has no zoning, meaning that there are few limits on construction. A shopping mall or factory can be located right next to a housing development or a city park. As a result, the city has a massive amount of concrete, roads, parking lots, and rooftops with very little consideration paid to drainage. This contributed in a major way to flooding during Harvey as the photographs below testify. Harvey was a 1000-year event, meaning the levels of rainfall are only expected that rarely. But as it turns out, the storm was the third 500-year event in the last three years. So, it is clear that it is part of a new normal. After Harvey the city has started to make minor changes to the way it is developing requiring new homes in the floodplain to be built higher, but it still is resisting zoning measures that would keep homes out of flood-prone areas.
Hurricane Irma, September 2017
Irma arose rapidly in the tropical Atlantic and at one point it had sustained wind speeds of 185 mph. This storm intensified rapidly, which is a characteristic of a large hurricane, increasing by 45 mph in one day. hit the islands of Barbuda, Antigua, St Martin and St. Barthelemy and caused catastrophic damage in these locations. The island of Barbuda, in particular, was literally flattened. Irma next set her eyes on Cuba where it came ashore as a magnitude 5 storm with sustained winds of 160 mph and a massive storm surge. The storm led to collapsed buildings and flooding of coastal areas including the historic Malecón in Havana. Fortunately, Cuban authorities had evacuated close to a million people from low-lying areas.
A few kilometers makes a major difference in the history of a hurricane and the landfall in Cuba, which was not initially predicted, weakened the storm significantly. Initial forecasts were for Irma to come ashore near Miami with wind speeds near 155 mph, but the storm tracked a little further to the west and made landfall in the Florida Keys with maximum winds of 130 mph, then again in Marco Island on the west coast of Florida with winds of 115 mph. As it turns out, there have been far larger storms in Florida including Hurricane Andrew in 1992, which flattened the Miami suburbs. But Irma was yet another reminder of how vulnerable the state is to storms. Much of the southern half of Florida is a natural swamp or marshland that originally looked like the Everglades National Park. But as in Houston, commercial interests, and in the case of Florida, the desire of citizens to own a small piece of paradise, have led to massive construction in the last decade, runaway development with insufficient environmental regulation. So, instead of swampland that served to absorb moisture and drain it back towards the ocean, large expanses of concrete funnel it into walls of water in cities and suburbs. Mangroves forest that previously protected the coast has been flattened. In fact, the Florida environment was destroyed a long time before this as the Army Corps of Engineers modified the natural drainage to provide water for the sugar industry.
As it turns out, Irma was less of a wind event than a storm surge event. As a hurricane moves towards land, it pushes water ahead of it, literally a wall of water. The result is storm surge. Hurricane Katrina in 2005 was also a storm surge event, with a surge of 28 feet measured at Pass Christian, Mississippi, just outside of New Orleans, the largest surge ever measured in a US hurricane. New Orleans is at or even below sea level and its levee system, upgraded after Katrina, is designed to deal with surge, but still, it remains highly vulnerable. The surge from Irma was about 10 feet in the Keys and Marco Island, enough to cause significant damage. Nevertheless, the storm was generally viewed by experts as another wake-up call to what will likely happen in the future if a major storm with winds over 160 miles an hour and a 20-foot storm surge hits Miami or Tampa which is extremely flood-prone. In all, Irma caused over 100 fatalities, most of which were in the Caribbean.
Hurricane Maria, September 2017
Barely a week later, monster storm Maria developed as Irma had…this storm also developed by rapid intensification with an increase in wind speed of over 60 mph in one day! By the time it hit Dominica, Maria had sustained winds of 160 mph and it caused utter devastation and killed 15 people, then it took aim at Puerto Rico. The storm hit the east coast of the island with sustained winds of 155 mph and dumped up to 3 feet of rain in mountainous areas. The impact on Puerto Rico is like the combined effect of Harvey in Houston and Irma in Barbuda. Maria caused massive destruction on the island. The power grid was destroyed leaving all 3.4 million residents without electricity. Many people had no running water for days, and sewers and cell phone networks were also out. Dams were in danger of breaching. 60,000 homes lost most or all of their roofs and only 392 out of 5000 miles of roads remained open. The storm defoliated a large number of trees on the island and led to the loss of 80 percent of the agriculture. The total damage is estimated at $90 billion, but that does not include the misery the storm caused humans. Diseases spread due to the lack of clean drinking water. The water-borne bacterial infection leptospirosis was widespread. Overall, the storm directly or indirectly led to as many as 3000 deaths but the true number may never be known, and, years later, the island is still recovering from the storm.
Hurricane Dorian, September 2019
None of these storms compared to Dorian that hammered the Bahamas in 2019. Dorian made landfall on September 1, 2019, on Grand Abaco Island with sustained winds of 185 mph and gusts over 220 mph making it one of the strongest storms on record in the Atlantic and Pacific.
Dorian was an unusual storm in several ways. The storm was enormous. Dorian was particularly deadly because the devastating winds were combined with an extremely slow forward motion of about 5 mph so that the storm ravaged the Bahamas for days. Devastating storms like Andrew and Katrina had much faster motion but the slow speed of Dorian made the damage much, much worse. After pummeling the Abacos, a group of islands in the northeast Bahamas, the storm went back over open water and made landfall without weakening on September 2 on Grand Bahama, the largest Bahama island, where it literally stalled for a day before weakening a little and moving back over open water. The damage to the Bahamas was truly catastrophic.
At landfall on the Abacos and again on Grand Bahama, Dorian’s intense winds were accompanied by a massive storm surge of about 6-9 meters (20-25 feet) and heavy rain. In total about a meter (3 feet) of rain fell over most of the northern Bahamas. There are harrowing tales of people clinging on to trees and other harrowing survival stories, but sadly many were not so fortunate. The official death toll from Dorian is 70 but is almost certainly much, much higher because there were many undocumented citizens living in shantytowns. Initially, there were over 1000 people missing, and now that number is around 300, so the death toll is likely to be 500-600. The true number may never be known.
Hurricane Ian, late September, 2022
Now to Hurricane Ian that hit southwest Florida in late September 2022. The storm was notable because of how rapidly it intensified, with windspeeds increasing from 75 mph to a 155 mph in just two days. The very large storm came ashore at Cayo Costa island just to the north of Fort Myers on September 28th 2022. Sustained windspeeds at landfall were 150 mph likely with higher gusts. It was the fifth strongest storm ever to hit the 50 contiguous states.
But the damage in southwest Florida was not just inflicted by the wind. Since the path of the storm closely paralleled the coast as it approached land, and because the highest surge is in the right front quadrant of the storm due to its counter-clockwise circulation, the storm surge over large areas was devastating.
The storm surge was between up to 15 feet above normal sea level along the barrier islands of Captiva, Sanibel and Fort Myers Beach. This wall of water caused massive devastation in these areas as observed in the photographs below.
Aerial imagery from NOAA's National Geodetic Survey of damage in the Times Square district of Fort Myers Beach, Fla., after Category 4 Hurricane Ian struck the area.
Credit: www.noaa.gov [90]
Ian moved slowly to the northeast direction across the Florida peninsula and this slow path caused heavy rainfall over a wide area, with precipitation totals up to 17 inches over a 12-24 hour period. This rainfall caused widespread flooding well inland in places such as Orlando.
One of the main stories of the storm was prediction. The different forecast models agreed closely as the storm approached southwest Florida, but because the path was so close to parallel to the coast a small change led to a major difference in the landfall location. Two days out the path was more northerly with the eye forecasted to make landfall near Tampa, but then a day out a minor jog in the forecast to the east shifted the eye well south. This change led to some delays in evacuation in the Fort Myers area.
The storm caused massive damage over a widespread area with catastrophic damage to housing along the coast especially in Fort Myers, Sanibel Captiva and Port Charlotte. More than 2.7 million people lost power at the height of the storm and a large number without clean water. Overall the storm led to 136 fatalities in Florida and a total of $50 billion in damage.
Hurricanes and climate change
So, finally, we come back to the question of whether climate change is responsible for the surge in powerful hurricanes in 2017-2022. At the outset, we must stress that this question cannot be answered unequivocally. However, there are several factors that make it safe to say that large storms will be more common in the future and that they will cause increasing amounts of damage. To develop, storms need warm temperatures (over 80 degrees), abundant moisture, and circulation as you can see in the video below.
As we will see in the lab at the end of this module, the ocean has warmed by 1 to 2 degrees C (3-4 degrees F) over the last century, and this leads to a 12-16 percent increase in moisture (3% per degree). Thus there is a lot more fuel for hurricane development. The formation of hurricanes is also helped by weather disturbances often off West Africa, but there is not yet a relationship between these events and climate change.
In the case of Harvey, the volume of water is clearly a result of an extra warm ocean; for Irma and Maria, the ferocity of the winds and the rapid intensification is also related to water temperature. For Dorian, size and slow movement is a result of a warmer atmosphere. So, climate change is adding fuel to the fire for large hurricane development and 2017 is a harbinger of things to come. There is one other factor to consider, perhaps one that will prove the most devastating in decades to come. Sea level rise. The ocean is now about a foot higher than it was in 1900. Projections are for a possible 6-foot rise in sea level by the end of this century if we don’t cut greenhouse gas levels significantly. We will discuss the issue of sea-level rise in great detail in Module 10. Such a rise would mean that even a storm such as Irma with a moderate storm surge would be catastrophic.
As with other impacts of climate change, the latest IPCC report stresses the need for adaptation to the threat of stronger hurricanes. Coastal communities will need to adapt to this threat by building homes higher and stronger, building sea walls and surge barriers, and gradually pulling back from the coast, an initiative called managed retreat. This is already happening near New York City and elsewhere in the US but again will be much more difficult to achieve in the developing world.
The night of December 10 and early morning of December 11, 2021 saw devastating tornadoes tear across the central US from Arkansas to Kentucky. Up to 71 different tornadoes have been confirmed rated up to EF-4 with wind speeds up to 190 miles per hour. The tornadoes caused massive damage totaling $3.9 billion, destroying whole communities, causing 88 fatalities and over 600 injured. 71 tornadoes have been confirmed. The worst damage was in the town of Mayfield, Kentucky, which was leveled by a strong EF-4 tornado. Much of the center of the town was irreparably damaged with houses and stores receiving devastating damage, 22 people died in that town alone. The tornado forecasts were generally highly accurate that night, but sadly, warnings weren’t always acted upon. What was particularly significant about the outbreak is how late in the season they occurred. Tornadoes in the central US are frequent in spring, when warm and cold air-masses collide over the region. It is very unusual for these strong storms to occur in the winter.
By studying the network of weather stations across the US, researchers have found a couple of interesting results — the biggest storms recorded in a given year not including tornadoes are increasing in strength, and the frequency of extreme storms is also increasing. In addition the area known as "tornado alley" where touch-downs are common is shifting eastward from Texas, Oklahoma and Kansas to Arkansas, Louisiana, Mississippi, Alabama and Tennessee.
In addition, the frequency of extreme storms is also increasing. An extreme storm is one where the rate of precipitation exceeds by a certain amount the long-term mean rate of precipitation for a given site (so an extreme storm in a wet region like the northeast US has a much higher precipitation rate than an extreme storm for a drier region). So, you might be asking yourselves whether these results translate into more frequent, massive tornadoes. This is a somewhat controversial topic. The consensus answer is that with a warmer atmosphere, tornadoes will definitely become more powerful (just like hurricanes), but the word is still out whether they will become more frequent. What is obvious from the December 2021 tornadoes is that these powerful weather systems will occur throughout the year in the future.
Download this lab as a Word document: Lab 2: Hurricanes [99] (Please download required files below.)
In this lab, we will observe the tracks of the largest storms of the last century, and learn about the impacts of those storms on land.
The goals of the lab are:
There are two Google Earth maps to load, the first, Hurricane Tracks [100]kmz [100] file [100], shows tracks of storms from 1900 to 2017. The second, Temperature Anomalies [101]kmz [101] file [101], shows average August temperatures for each year calculated relative to the average temperature between 1900 and 1910. You can switch back and forth between maps. Both maps have sliders at the top left of the screen that allow you to look at storms as well as temperature over time. The storm tracks have points that show the wind speed and pressure at different stages in its development. We definitely recommend that you don’t try to look at the storms all at once or you will see a maze of lines. Please make sure that the slider at the top left has the relevant range of dates on it, otherwise, you will not be able to view tracks for the desired storm. Also, there are a few storms including Betsy whose names do not show unless you zoom in close. Note also, we break up the 2000-2010 and 2010-2017 decades.
As in the lab for Module 1, we begin with some practice questions that you can take in the Lab 2 practice submission, where you will receive the answers to the questions. Once you feel good about these questions, move on to the graded assignment. If you have any questions about the practice questions, please let us know. Remember, you only get one attempt at the graded assignment.
The video below will help you with operations in Google Earth. We HIGHLY recommend you watch it to learn exactly how to manipulate the files and use the historical imagery.
Video: Controls for Module 2 Lab (06:35)
Part A. In the first part of the lab, we look at the tracks of hurricanes. You will need to look for the storm names. Load the name of the storm once you have found it and click on a point to find the wind speed and pressure. For certain storms, we will include the storm surge as well as the precipitation in areas near the landfall. You will also need to observe the elevation of areas close to the coast and look at historical imagery to determine the impact of the storm on coastal communities.
A. About half of the town would be flooded
B. None of the town would be flooded
C. All of the town would be flooded
Part B. In the second part of the lab, we will observe the change in temperatures of the Atlantic Ocean over the last century that is related to the generation of more powerful hurricanes. Load and turn on the temperature anomaly kmz. By pressing the year buttons on the left, you can observe the temperature anomalies in August every five years (from 1910 to 2000) and annually from (2000 to 2017) relative to the average temperature from 1900 to 1910 file.
Center the map over the Atlantic Ocean so you can see Africa as well as North America including the Gulf of Mexico. As we have learned, the warmer the temperature the more energy to fuel hurricanes as well as the ability to hold more moisture.
A. 1965
B. 1975
C. 1985
D. 1995
E. 2005
Which year would temperatures in the Atlantic have been favorable for hurricane development?
A. 1960
B. 2017
C. 1990
D. 2000
E. 2007
What is the general trend for temperature change between 1900 and 2017?
A. Warming
B. Cooling
C. Stayed consistent
Which decade would have been slow for hurricane generation in the Atlantic based on temperatures?
A. 2000-2010
B. 1990-2000
C. 1970-1980
In this module, we have covered a broad range of observations that are pertinent to recent climate change. Here is a quick recap:
This goes back to 1880; multiple analyses yield similar results, indicating a warming of about 1-1.5°C averaged over the globe in the past 150 years. The majority of the warming has occurred in the last 50 years. The map-view pattern reveals that the polar region of the Northern Hemisphere has warmed much more than other regions of the globe.
Available for a much shorter time period, these results essentially confirm the analysis of the instrumental temperature record.
Sea surface temperature (SST) records from ships go back to 1850 and make up an important part of the data that provide global temperature estimates; these records are similar, though somewhat subdued in comparison to just land surface temperatures. The SST, though, represents just the skin of the oceans; to see deeper, we rely on measurements from a system of buoys that shows the oceans are slowly warming — only about 0.1 to 0.2 °C averaged over the globe during the past 50 years — but this is the temperature change in the whole upper 700 m of the oceans, which is a vast amount of water. So while the ocean has absorbed a huge amount of heat, its overall temperature has changed little.
Through the use of multiple proxies, the average global temperature has been reconstructed about 2000 years into the past. These results indicate a Medieval Warm Period (AD 950 – 1250) that was almost as warm as today, and a Little Ice Age (AD 1350 - 1850) that was more than a degree colder than today, followed by the modern warming trend.
The temperature versus depth measurements from boreholes preserve a smoothed record of the history of surface temperature change; global studies of these records provide a smoothed temperature history that goes back to the year 1500. This temperature history is in good agreement with the instrumental record.
Mountain glaciers from around the world are shrinking; some at astounding rates. The history and magnitude of melting indicate a warming history that closely matches the results from the instrumental surface temperature record.
Satellite data reveal the mass changes of the two large ice sheets; both are losing mass fast (2.3e15 kg of ice in 6 yr), contributing about 8 mm to sea level rise in this short period.
Submarine sonar readings and satellite measurements show that the sea ice in the Arctic Ocean is declining in thickness and in areal extent, so much so that the Northwest Passage is now open for a month or two each summer.
Based on tide gauges, this record shows that sea level has risen about 20 cm in the past 100 years (2 mm/yr). This rise is a result of the melting of ice combined with the thermal expansion of the warmer ocean. Much more on this is Module 11.
After reviewing all of the data, here is what the leading scientific academies of the US, UK, Russia, China, France, Germany, Italy, Brazil, Japan, Canada, and India (the G8 climate change roundtable first held in Davos, Switzerland in 2005) jointly concluded:
Climate change is real. There will always be uncertainty in understanding a system as complex as the world’s climate. However, there is now strong evidence that significant global warming is occurring. The evidence comes from direct measurements, of rising surface air temperatures and subsurface ocean temperatures and from phenomena such as increases in average global sea levels, retreating glaciers, and changes to many physical and biological systems. This warming has already led to changes in the Earth's climate.
This is the consensus from leading scientists around the world, not politicians, journalists, or business people who may stand to gain financially from taking a stand on climate change. The motivation of these scientists is to understand what the data mean and to help the broader public understand the implications of the data. At the end of the day, the data tell us that the climate is changing; the real challenge before us lies in finding ways to respond to this change through some combination of taking steps to minimize the change and finding ways to adapt to the change.
In this module, you should have mastered the following concepts:
You should have read the contents of this module carefully, completed and submitted any labs, the Yellowdig Entry and Reply and taken the Module Quiz. If you have not done so already, please do so before moving on to the next module. Incomplete assignments will negatively impact your final grade.
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