As the pundits, politicians, and some scientists love to say, the predictions of global warming are based on models that do not accurately reflect the com­plexity of climate systems. However, the predictions of models for yearly global

average surface temperatures have been fairly accurate, within the margin of error, even going back to the First Assessment Report in 1990. Since that time, of course, computer technology has advanced enormously so many more factors can now be taken into consideration, including aerosols, reflection of the sun from snow cover (albedo), and some limited effects from cloud cover, though this is an area that is still developing. So, let’s look at the predictions of models that consider human effects in producing greenhouse gases and models that do not consider these effects but only consider natural effects such as solar irradiance.

The actual results of models show that the observed temperatures since about 1970 cannot be modeled without considering the radiative forcing due to green­house gases (Figure 1.9). The models based solely on natural factors indicate that we should be in a slight cooling trend, but in fact global temperatures have been rising, as clearly demonstrated in this chapter. The observed results fall right in the middle of the error margins of the model predictions that include human effects. This should give substantial confidence that the models are in fact accu­rately considering the most important factors leading to global warming, and that the most important factor is human effects from greenhouse gases.