Over the past century, global measurements of the temperature at the Earth's surface have indicated a warming trend of between 0.3 and 0.6 degrees C. But many - especially the early - computer-based global climate models (GCM's) predict that the rate should be even higher if it is due to the man-made "Greenhouse Effect". Furthermore, these computer models also predict that the Earth's lower atmosphere should behave in lock-step with the surface, but with temperature increases that are even more pronounced
Unlike the surface-based temperatures, global temperature measurements of the Earth's lower atmosphere obtained from satellites reveal no definitive warming trend over the past two decades. The slight trend that is in the data actually appears to be downward. The largest fluctuations in the satellite temperature data are not from any man-made activity, but from natural phenomena such as large volcanic eruptions from Mt. Pinatubo, and from El Niño. So the programs which model global warming in a computer say the temperature of the Earth's lower atmosphere should be going up markedly, but actual measurements of the temperature of the lower atmosphere reveal no such pronounced activity.
So What is Going On?
The atmosphere is extremely complex in its behavior. Because of this, finding the correct explanation for the behavior we observe is complex as well. Virtually all scientists will agree that a doubling of the amount of carbon dioxide in the Earth's atmosphere should have some effect on the temperature of the Earth. But it is much less certain how or if we will recognize the effects of this increase. There are several reasons:
* First, the influence of a man-made doubling of the amount of carbon dioxide in the atmosphere is small compared to the Earth's natural cooling rate, on the order of only a percent.
* Second, there is a much more important greenhouse gas than carbon dioxide, namely water vapor. Water vapor over the Earth is extremely variable, both in space and in time.
* Third, the ways in which clouds and water vapor feed back and ultimately influence the temperature of the Earth are, at best, poorly understood.
* Fourth, while the whole Earth is indeed in a state that scientists describe as "radiative equilibrium," where the incoming sunlight equals the outgoing infrared radiation to provide a roughly constant overall temperature, the surface is far from this radiative balance condition. Evaporation and convection processes in the atmosphere transport heat from the surface to the upper troposphere, where it can be much more efficiently radiated into space since it is above most of the greenhouse-trapping water vapor. So in short, it is this convective overturning of the atmosphere - poorly represented in computer models of global warming - that primarily determines the temperature distribution of the surface and upper troposphere, not radiation balance.