We are not content, though, merely to extrapolate from the past. We would like to make predictions based on a rigorous understanding of climate science.
The basic theory of the interaction between radiation and the atmosphere has been stable for about 100 years. We can state with almost perfect certainty that if we double CO2 concentration and allow nothing but temperature to respond (clouds, water vapor, vegetation, etc., are held fixed), the earth will warm up by about 1 kelvin (1.8 F). There is no controversy about this conclusion within the scientific community.
The problem, of course, is that these other features of the climate system do change. For example, as discussed previously, the water vapor content of the atmosphere varies, mostly in response to temperature itself. As the atmosphere warms, the concentration of water vapor increases. But water vapor is the most important greenhouse gas, and its increase leads to further warming. This is an example of a positive feedback in the system, and current understanding suggests that this factor alone more or less doubles the warming that occurs in response to increasing CO2. But the true physics of climate is not that simple, and the distribution of water vapor is affected by many other variables besides temperature, so even here there is uncertainty.
Much more problematic are clouds, which, regarding radiation, work both sides of the street. They account for most of the reflection of sunlight by our planet, thereby cooling it. But they also absorb and reradiate infrared radiation (see Figure 3.1.1) just like greenhouse gases, thereby exerting a warming effect. Which effect wins depends on the altitude and optical properties of the clouds. At present, there is no generally accepted theory for how clouds respond to climate change.
To this problem we can add many other issues that reflect the immense, almost overwhelming complexity of the climate system. As sea ice melts, a white surface is replaced by dark ocean waters, which absorb more sunlight (another positive feedback). In some places, jungles, which are relatively dark, may be replaced by deserts, which are highly reflective—a negative feedback. The rate at which the oceans absorb excess CO2 may itself change in response to changes in ocean temperature and concentration of dissolved CO2.