Natural and Anthropogenic Warming
VIdeo: SPM2 (2:20)
Natural and Anthropogenic Warming (launch image in a new window)
DR. RICHARD ALLEY: This fascinating figure is from the IPCC. There's a lot of information on here. It includes the things that are changing-- radiative forcing-- or changing the climate, how much they're doing so, including the uncertainties, whether they expect the whole globe or just part of it, and the level of scientific understanding.
If we do a lot more research-- the low it probably will reduce the size of the uncertainties-- because we can learn more. But how much we understand is included in the uncertainty already. And it includes both the things that humans have done and the things the nature has done. And this goes from the year 1750 up to the year 2005.
The Biggie is our C02, together, with the other greenhouse gases that we put up, as well as the ozone that comes from human activities from pollution. So these all have a warming influence and they are pushing very strongly towards warming. Clearly, there's a couple of other little warming influences, especially us putting soot on top of snow. But there's also these cooling influences.
We've put up a lot of particles, aerosols that block the sun, and they make clouds last longer and make clouds more reflective. And together, those have a lot of cooling. And we've cut dark forests and replaced them by more reflective grasslands.
In addition, since 1750 the sun has brightened a little bit. Over the last 30 years or so, it's actually dimmed, but there's a little bit of that. Add all of these together and there's very clearly a warming influence. And the total warming influences is very similar in size to the CO2 that we've put up.
Taken together, we are pushing the world in a lot of different ways. But because of these cooling influences, if you ask how much of the warming has been caused by our greenhouse gases, the answer is more than all of it. Because it is warm despite these cooling influences.
Credit: Dutton Institute. "EARTH 104 Module 4 SPM 2." YouTube. November 19, 2014.
Source: IPCC, 2007: Summary for Policy Makers
Greenhouse gases are not the only things that affect climate. But, climate changes have causes; there are no magical “cycles” that somehow change the climate without letting us know why. (There are cycles that affect climate, but they have causes, such as features of Earth’s orbit, that we understand; they are NOT magical!) So, we can assess what things are affecting the climate.
More than a century ago, the Earth was a little on the cold side in what is sometimes called the “Little Ice Age”, because the sun was a bit dim and volcanic eruptions were putting up dust that blocked the sun. The sun brightened early in the 20th century, contributing to warming, as shown by the little red bar extending to the right for natural solar irradiance down near the bottom of the figure. But, over the last 30 years when satellites have given us the best data, the sun seems to have dimmed just a bit. We humans have cut dark forests and replaced them with more-reflective grasslands, cooling the Earth a little, and we have put up a lot of particles to block the sun, with notable cooling influence (you can find blue bars for these, extending to the left, in the figure).
You may meet someone who agrees that the Earth is warming, but argues that much of the change is natural. This is wrong; over the last few decades, warming has occurred despite nature pushing a little toward cooling, and human particles and land-use changes pushing more strongly toward cooling. The most likely answer for how much of the warming has been caused by our greenhouse gases is “More than all of it”, because of warming despite these other cooling influences.
Video: SPM 3 (1:07)
Temperatures, Sea Level and Snow Cover (launch image in a new window)
DR. RICHARD ALLEY: This figure from the IPCC starts back in 1850 and then runs up to just pass 2000 up here on the right. And it shows indications of warming happening in the climate system. You can see on top here the thermometer record of global average temperature showing not much happening and then recent warming, very clearly.
Sea level, which is given here, rises because ocean water expands as it warms and because warming tends to melt glaciers that are holding water out of the ocean. And so we see a warming influence that shows up in the rising global sea level.
And we also look, if you go to bring time snow cover, you can see that not much was happening. And then you can see it dropping, and that's happening because of warming and the spring is melting the snow. And so these are among many indicators that are showing that yes, the climate system is warming.
Credit: Dutton Institute. "EARTH 104 Module 4 SPM 3." YouTube. November 19, 2014.
Source: IPCC, 2007: Summary for Policy Makers
The temperature is going up. The figure shows a few of the indicators, but many more are known. Consider the next figure, for example.
Video: Surface Temperatures (1:38)
Decadal Land-Surface Average Temperature (launch image in a new window)
PRESENTER: This figure is from the Berkeley Earth Project. It was run primarily by physicists who did not start out as climate scientists-- with an interesting mix of funding from public sources. But also some of it came from private sources, including those with ties to the fossil fuel industry.
It's looking at the thermometer record of temperature, and just looking at the land. Now if you go back to 1750 up through about 1850, you could see that the uncertainties are really huge. So we're mostly going to focus since 1850.
Many groups have been estimating the temperature, including NASA-- the Goddard Institute for Space Studies, NOAA-- the National Climate Data Center, the British Group, the Hadley Centre, and the Climate Research Unit. And what you can see is those, plus the Berkeley Earth estimates up here on top. And what you'll notice is that the uncertainties in the Berkeley Earth are similar to the differences between the others, which also have their own uncertainties. But you'll see very clearly that there is a strong warming going on.
The different groups have used different techniques. Although, ultimately, they're all using thermometers. Whether they use them all or not this is different for the different ones. But when you have different groups with different funding, different motivations, perhaps, and some working in different places, they all give the same answer. Which is, it's getting warmer. We have very high confidence that it is warming.
Credit: Dutton Institute. "EARTH 104 Module 4 Surface Temperatures." YouTube. November 19, 2014.
Source: Berkeley Earth
The Berkeley Earth project is an interesting attempt by a group involving a lot of physicists who were not primarily climate scientists through much of their careers, to use private as well as public funding to re-calculate the temperature record from thermometers. The Berkeley work follows efforts by NOAA and by NASA in the US, and by a British group at the Hadley Center and the University of East Anglia, and other efforts by others, to calculate global temperature changes from thermometer records. You can see clearly in the figure that over recent decades when the data are best, the different groups get the same answer despite having different funding sources and different techniques. The temperature is going up.
Furthermore, if you throw away the records from thermometers in and near the cities and just look in the country, you see warming. Thermometers in boreholes in the ground show warming. Thermometers taken aloft by balloons (radiosondes), and thermometers looking down from satellites and analyzed in different ways, show warming. So do thermometers in the ocean.
The temperature-sensitive snow and ice also show warming. You would not go searching for this effect in the coldest places; if you start off at -40 and warm by a couple of degrees, the snow and ice won’t melt yet. But, the effects of warming are seen in loss around the edges, in space and time, of seasonal snow cover, river, and lake ice, seasonally and perennially frozen ground, mountain glaciers and more. The melting of land ice and the expansion of ocean water as it warms are driving the rise in global sea level. And, the great majority of significant changes in where plants and animals live, and when they do things during the year, are in the direction of warming. So, warming is occurring, despite natural and human pushes toward cooling over recent decades.
Want to learn more?
Read the Enrichment titled Global Warming Did Not Stop Recently.
We are once again taking a look at the CO2 and the Atmosphere clip. To see a little on the melting of ice, watch 7:22 - 9:04.
Earth: The Operator's Manual
Video: CO2 and the Atmosphere (9:04)
For recent updates on temperature, see NASA’s Goddard Institute for Space Studies (GISTEMP).