Optional Enrichment #1: The So-Called Greenhouse Effect
As an aside, some of our friends over in meteorology are not happy that the effect of CO2 on climate is called the “greenhouse effect.” They fully understand that CO2 does warm the planet, and they know that the glass of a greenhouse affects radiation in much the same way that CO2 in the atmosphere does—the shortwave radiation from the sun comes through glass or CO2 more easily than the longwave radiation from the Earth goes out through glass or CO2. But, the meteorologists note that this effect of glass on radiation is not the only reason why a greenhouse is warm, nor is this the major reason. Greenhouses also are warmer than their surroundings because the glass blocks the convection currents (air rising after it is heated a little) that take much of the sun’s heat away from the ground outside of greenhouses. Some meteorologists have even suggested renaming the atmospheric phenomenon to avoid possible confusion. But, the “greenhouse effect” is catchier than “the effect that warms the Earth through modulation of radiation balance, akin to the radiative effect that contributes to but does not dominate daytime warming of greenhouses.” Notice that this little discussion about terminology in no way affects the reality that more CO2 in the atmosphere warms the planet—nature works, regardless of what words we use to describe it.
How Much CO2 to Warm?
Many different models have been constructed of the Earth’s climate system, ranging from attempts by large teams to include essentially all Earth-system processes into models that tax the world’s largest computers, to small-group or individual-scientist efforts to build fast and flexible models that allow exploration of uncertainties in many parameters. Across a range of models, the equilibrium surface warming from a doubling of CO2 is often stated to be between about 2oC and 4.5oC, with a central value near 3oC (and with the most recent results pointing to a bit above 3oC). Comparisons to the past, for both the last century and for much longer times, largely exclude the low end of that range—models that change global average temperature less than 2oC or just slightly more than that for a doubling of CO2 are not able to accurately simulate the changes of the past, whereas models with larger temperature change in response to CO2 do better in simulating past changes. Based on the paleoclimatic record, warming of near 3oC or more for a doubling of CO2 seems reasonable, and values above 4.5oC cannot be totally excluded.
Note that this distribution of warming includes a central estimate, and the possibility of somewhat less, somewhat more, or even more than that, but not even less. In the physics, this arises in part because the feedbacks (discussed more just below) act on each other. Raising atmospheric CO2 causes warming. That in turn causes more water vapor to evaporate, which causes more warming. And, it causes snow and ice to melt, causing more sunshine to be absorbed, which causes more warming. And, the warming from more water vapor causes some snow and ice to melt, and the warming from less snow and ice causes more water vapor to evaporate, causing still more warming. This does not “run away”—we are not yet close to turning the Earth into Venus, hot enough to melt the metal lead at the surface. But, we cannot avoid the warming from CO2, and the interactions mean that if our models have slightly underestimated the effects of the feedbacks, the models will have notably underestimated the total warming.
Note also that many of the damages have a similar distribution—for some specified warming, the expected sea-level rise has a central estimate, and could be a little less, a little more, or a lot more. And economics gives a similar answer—for some specified sea-level rise, there is some central estimate of costs, which could be somewhat less, somewhat more, or much more, but not much less (for example, economists often assume we will behave efficiently and follow the least-cost approach to solving the challenges of a rising sea, but observations of people actually responding to challenges indicate that we often are not following the most efficient path). With this same general set of uncertainties for the warming from given CO2 release, and damages from the warming, and costs of the damages, the possible costs if a lot of things go wrong could be really high.
More on Feedbacks
Recall from earlier that feedbacks are important in estimating how much warming will be caused by the CO2 we emit, and as we noted just above, feedbacks are behind the possibility of warming being much greater than generally expected. If you force a system to change by doing something to it, many other things may then change. Some of these will amplify what you just did, making the changes bigger than you could have accomplished by yourself; these are positive feedbacks. Others will oppose what you just did, making the changes smaller than what you initially forced; these are negative feedbacks.
You, for example, have all sorts of negative feedbacks built in. If you are placed in a warmer room (the forcing), your body will begin to warm up. But then a negative feedback kicks in—you start to sweat, and that cools you off. Your body temperature doesn’t change nearly as much as the temperature outside of you changed. But, if you have certain diseases, they may fool your body so that its negative feedbacks are reduced and may even become positive feedbacks. Fever is usually a good thing, helping the body fight invading germs more effectively, but people die of fever when the feedbacks become too positive and the body “burns itself up.” If you’re in a canoe with a really enthusiastic golden retriever, you may try to lean as the dog leaps about in such a way as to stabilize the canoe—you are providing a negative feedback on the tipping. But if the dog tips the canoe, and the ice chest falls to the low side, the ice chest is acting as a positive feedback to amplify the dog’s motion and tip the canoe further. If you lose your balance when the dog lunges to the side, you may suddenly fall toward the dog, providing another positive feedback and perhaps flipping the canoe.
The Earth certainly has positive and negative feedbacks. The easiest stabilizing or negative feedback is the very fast change in radiation—a warmer place glows more brightly almost instantaneously and sends more heat toward space, tending to cool the hotter places faster. Other than this almost-instantaneous change, most of the climate feedbacks over times that are most important to us (years through thousands of years) are positive, amplifying changes. Over still-longer times approaching or exceeding one million years, the feedbacks tend to be negative, stabilizing the climate. Climate changes over years or centuries thus can be almost as large as climate changes over millions or billions of years.
The most important very-long-term stabilizing feedback was discovered by Penn State’s famous professor Jim Kasting and coworkers. Recall that any chemistry lab has a Bunsen burner or a hotplate to speed up the chemical reactions so you can get done before the class period ends—chemistry almost always goes faster at higher temperature. Rock weathering involves CO2 and water reacting with rocks to make dissolved ions that wash away to be turned into shells or the inorganic equivalents, which over millions or billions of years are subducted, melted, and returned to the surface in volcanoes that release rocks and CO2 and water to complete the cycle. The rate of subduction and volcanic eruption does not depend very much on the climate at the Earth’s surface, but the rate at which weathering removes CO2 from the air goes faster when warmer. So, if the temperature goes up, removal of CO2 from the atmosphere goes faster, cooling the temperature back down. And, if the temperature goes down, the removal of CO2 slows but the volcanoes continue to supply CO2 at the same rate, so the CO2 builds up in the atmosphere and warms the climate. However, if something perturbs the climate, such as us releasing a lot of CO2, this takes more than 100,000 years to return the climate to its original state. Note also that this natural volcanic flux of CO2 is about 1% of the human supply primarily from burning fossil fuels; the ongoing rise in atmospheric CO2 is NOT caused by volcanoes!
There may be a second very slow stabilizing feedback. Warming reduces the amount of oxygen dissolved in water, which reduces the ability of animals, bacteria, etc. living in the deep ocean to “burn” dead organic material, favoring burial of those dead things to eventually form fossil fuels rather than “burning” of the dead material to release the CO2 back to the ocean-atmosphere system. Again, this is slow, and removing the CO2 we release will take well over 100,000 years.
Most of the other feedbacks that operate between these very long times and the nearly instantaneous changes in radiation with temperature are positive, amplifying the effect of the original forcing. If we put carbon dioxide into the air and warm the Earth a little, several of these positive feedbacks begin to function. Most importantly, warmer air can “hold” more water vapor (the saturation vapor pressure roughly doubles for a 10oC or 18oF warming), and water vapor is an important greenhouse gas, so warming causes more warming.
Some of the shortwave radiation from the sun that hits the Earth bounces right back to space without first warming the Earth, especially over snow and ice, which have very high albedo or reflectivity. But, warming the Earth removes some snow and ice, which then allows more of the shortwave radiation to be absorbed, which warms the Earth more—a positive feedback.
Clouds reflect some sunlight (so cloudy days are cool), but clouds also interfere with outgoing longwave radiation (so cloudy nights are warm). The largest uncertainties in predicting how much warming will result from a given amount of fossil-fuel burning are probably related to how clouds will change, and whether these changes will produce positive or negative feedbacks. However, these uncertainties are not nearly large enough to affect the conclusion that future warming from fossil-fuel burning is highly likely, and the evidence is now fairly strong that the cloud feedbacks are also positive, with various shifts in cloud types and locations that in total amplify the warming. Vegetation also may change, affecting how much water vapor it returns to the atmosphere and affecting albedo, but this does not seem to be an especially strong feedback.
Climate Models
We clearly wish to predict the future. The knowledge that burning of fossil fuels, combined with bovine belches and a few other greenhouse-gas sources, are nearly certain to cause large problems allows us to change our ways now to improve our future well-being. To predict the future, we need to do experiments. But, we have only one world. We cannot look at many different futures of one physical world, nor do we wish to wait many decades for the experiments to end. The solution we use is to build little worlds in computers, and run the experiments on those.
Note first that this sort of modeling is used everywhere all the time. In one old Calvin and Hobbes cartoon, Calvin asked his dad how load limits were determined for bridges, and his dad said that they drove heavier and heavier trucks over a bridge until it broke, then weighed that truck and rebuilt the bridge. This is of course nonsense; the strength of the bridge is calculated in a model. Your automobile and cell phone were designed on models, too. “It’s just a model” is the sort of thing said by people who aren’t paying attention, and who might be wise to start paying attention.
Anyway, geologists are important in many ways in this effort to model the climate, with two contributions especially important: finding out how the world works, and finding out what has really happened. The computer models always will be simpler than the real world, so careful choices must be made about what to put in, and things put in must be represented accurately; hence we need to know how the world works. And, once the models are built, we need to test them. You wouldn’t trust a model that had never been tested, but you wouldn’t want to wait a whole lifetime for a test. If the models can successfully simulate very different, warmer and colder climates of the past, then the models are probably pretty good. So we need to know about climates of the past, and geologists help supply those data.
The computer models of today actually are doing very well at “retrodicting” climate, predicting things that already happened. Modelers set up the configuration of ice sheets and ocean and continental positions and orbits and solar brightness, then model the climate and see if the computer results can match the climate that is recorded by the fossils and other climatic indicators in the rock record without “cheating” (so the scientists do not go in and tweak a lot of things to make the model match the data and then claim that the model is great—the models actually do work on past climates without such cheating). The models are also doing quite well at predicting the patterns of change we have observed with instruments over the most recent decades. Predictions made by modelers over the last decades are really occurring now.
Models predict that the world will warm about 3oC or just over 5oF for a doubling of the concentration of CO2 in the atmosphere. The full warming will be delayed a few decades behind the rise in CO2, because the air can’t warm all the way until the ocean and ground have warmed and some ice has melted, which takes a while. The global warming to date, somewhat over 1oC or 1.8 oF in the early 2020s, has only recently become obviously bigger than the typical temperature variability at most places, so it is only recently that the warming has become obvious to a lot of people. If we proceed to burn all the fossil fuels, though, roughtly an 8-fold increase in atmospheric CO2 above “natural” levels is possible over a century or centuries, or a warming of about 9oC or over 16oF, large enough that no one would have any doubt about the change. (Note that the land warms more than the ocean, and almost everyone lives on land, so people are experiencing a notably larger warming than this.) The recent drop in cost of renewable energy has caused most experts to conclude that it is unlikely we will burn that much fossil fuel, but there still is a large likelihood that we will push warming past 2oC unless strong actions are taken soon.
Why Not Water Vapor?
Water vapor is the most important greenhouse gas in terms of the amount of outgoing infrared radiation intercepted, and thus the amount of warmth provided. But, we usually start discussions of global warming with CO2, not water vapor. Why? Simply, water vapor is almost entirely a slave to CO2. Put some more CO2 up in the atmosphere, and the atmospheric concentration of CO2 remains much higher for centuries, and somewhat higher for more than 100,000 years before chemical processes remove it. Put more water vapor up, and in just over a week, on average, that water has rained out. The burning of fossil fuels makes approximately equal numbers of water-vapor and CO2 molecules, but because the water vapor stays up less than two weeks and the CO2 perhaps 2000 years on average, our effect on the atmospheric concentration is more than 100,000 times larger for CO2 than for water vapor. We can change CO2 fairly easily (and are doing so!), but we can’t put up water vapor fast enough to make much of a difference, nor can most other natural processes affect global water-vapor loading very much. However, changes in the atmosphere’s water-vapor content are easily caused by changes in temperature.
Remember from back at the Redwoods that cooling reduces the equilibrium water-vapor pressure or “water-holding capacity of the air” (by about 7% per degree Celsius of cooling). Remember that as full-of-water air came in from the Pacific and was forced up over Redwood National Park and then Yosemite and Sequoia National Parks, the air cooled by about 0.6oC/100 m, raining on the way. The temperature at the top of the Sierra was controlled by the height of the Sierra and the temperature of the air before the rise began, and the amount of water left in the air at the top was controlled by the temperature at the top. The air then goes down over Death Valley, and the water-vapor content of the air there depends on the temperature at the top of the Sierra.
So, if the temperature is increased over the Pacific by an increase in CO2, the water-vapor content and its greenhouse effect are increased over the Pacific, going up the Sierra, going back down over Death Valley, and on to the Atlantic or Gulf of Mexico. Water vapor acts as a positive feedback—warming increases the water-vapor content of the atmosphere, causing more warming.
You can find lots of climate-change skeptics or contrarians or denialists who love to point out that water vapor is the big greenhouse gas, CO2 less so, so the scientists must be wrong to focus on the small one and not the big one. Sounds sensible, right? But, it is totally stupid or deliberately misleading, or somewhere in-between. If we pulled all the water vapor out of the air, more would evaporate in a week or so. Pull all the CO2 out of the air, and the cooling would remove a lot of water vapor, with a rather high chance that the whole Earth would freeze over into a snowball. Thus, although water vapor gives us more warmth than CO2, the CO2 is more important overall.
Why All the Noise?
Environmental problems seem to follow a fairly predictable path. First, someone has a good idea. Refrigerators and air conditioners and freezers are useful, but if you use ammonia in the pipes and you’re in the way when a pipe breaks, you might die, so chlorofluorocarbons were a great idea. Then, scientists discover an unintended consequence—the chlorofluorocarbons might break down ozone and allow harmful ultraviolet rays to give living things “super-sunburns,” causing cancer and other problems. There follows a period when the scientists work to improve their understanding.
But, there also follows a lot of yelling and not-entirely-scientific discussion. Some people fear that they are going to lose their jobs, or lose a lot of money, if the problem-causing industry is changed, and these people respond to the scientists by arguing that there is no problem, that the problem that does not exist must be caused by nature rather than humans, and that this natural problem that does not exist would cost way too much to clean up, and that the clean up would involve taking actions that we all will hate and are probably illegal and are promoted by crazy people who hate us. A very common approach is to attempt to convince the public, or policymakers, that scientists are still having a big debate, even if they are not. It is fairly easy to find a few skeptics, fund them and promote their statements, and to “cherry-pick” certain results from the scientific literature and present them out of context.
Politics often feeds into this. Usually, if a problem is identified that affects a lot of people, the government ends up dealing with the problem. You are not allowed to tear out your sewer or septic system, poop in a pot, and dump it over the fence into my yard. Nor are you allowed to smoke in many public places now, or dump your trash in my yard, and laws such as these are passed by and enforced by the government after they are demanded by many people. So, if you don’t much like government, you may think that it is unwise to have the government trying to clean up a problem. And, if you can keep the argument focused on whether or not the science is good, rather than on possible wise responses to the problem, there is little danger that the government will do anything—we usually don't do much about a problem until we agree that there is a problem.
The press makes all of this worse, attempting to maintain "balance" by presenting “both sides” of a “scientific dispute,” even if one side is being manufactured and does not have much scientific basis of its own. Recent scholarship has demonstrated clearly that a reader of the mainstream press in the U.S. would have a very skewed view of the degree of scientific agreement over global warming, for example—many press outlets present a conflict that really doesn’t exist.
But, some forward-looking people also see the problem as a possibility—a new invention may make a lot of money and help a lot of people. And, history indicates that problems usually are followed fairly quickly by new inventions, the cost of dealing with the problem typically is much less than previously stated (often about 10% of the previously stated cost, and sometimes with the cleanup cheaper than the original as well as cleaner), the cleanup becomes part of the economy, and life goes on. (Imagine life without toilets and sewers, with people dumping their poop out the windows in the morning into the street the way we used to do…) (check out this clip, Toilets and the Smart Grid, from Earth: The Operators’ Manual, https://www.youtube.com/watch?v=KJqDQ41m_KI
The twin energy problems—finding replacements for the finite fossil fuels, and doing so before the world is changed too much in bad ways—are arguably the biggest environmental problems we have ever faced, but they can be solved. Because of the huge size, the solutions will take longer, and more inventions will be required than for the ozone hole or DDT or lead in gasoline. The scholarship is clear that speeding up our solutions will make us better off.