PrintThe 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 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.” Maybe our meteorological friends would be wise to “chill out” on this one. 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 warming from a doubling of CO2 is often stated to be between about 2oC (maybe as low as 1.5oC) 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 near 1.5oC 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.
Why Not Water Vapor?
Water vapor is the most important greenhouse gas in terms of the warming being provided now, but we usually don’t talk about water vapor with global warming. Why not? Simply, water vapor is almost entirely a slave to other things. Put some more CO2 up in the atmosphere, and the atmospheric concentration of CO2 remains high for centuries or millennia or longer 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 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, 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 out to lunch by focusing 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 warming than CO2, you can argue that 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, 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. 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” favorable 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 a number of other things and these are laws that are passed by and enforced by the government. So, if you don’t much like government, you may not want 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, 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), the cleanup becomes part of the economy, and life goes on. (Imagine life without toilets and sewers…)
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 the sooner we start, the better off humanity will be.