The real issue, of course, is what will happen in the future. Although ultimately we want to know what the human and monetary risks are, we should start with something simpler, and it is natural now to ask how global temperature will evolve going forward.
Let’s begin with a simple-minded approach. Suppose we just extrapolate the relationship between temperature and CO2 concentration shown in Figure 3.2.1. Doing so gives a temperature increase of about 1.9 K (about 3.5 °F) per doubling of CO2. What’s wrong with that?
There are two problems. First, CO2 was not the only climate influence that changed over the past century or so. There were changes in other greenhouse gases, small changes in solar output, volcanic eruptions—which spewed sun-reflecting particles into the atmosphere, thereby cooling it—and manmade sulfur pollution, which does the same thing. So the temperature change reflects more than just greenhouse gas increases. Second, the world ocean acts as a huge buffer, absorbing most of the excess energy produced by increasing greenhouse gases. This causes the temperature of the planet to lag well behind changes in CO2 so even if the concentration of greenhouse gases leveled off right now, the planet would continue to warm for a while owing to the thermal lag effect of the ocean.