of the world differently, as summarized in the maps in Figures 22 and 23 on pp. S72–S73 in Supplement 10. According to the IPCC, a 2 C° (3.6 F°) warming appears to be inevitable because we have waited too long to prevent it, ignoring warnings from a number of prestigious committees of climate scientists for more than 25 years. Such a temperature increase is probably manageable. But as temperatures increase beyond this level, the projected harmful effects and costs of the resulting irreversible climate change will escalate rapidly (Figure 19-7, middle and right). Climate scientists warn that a 4 C° (7.2 F°) warming will threaten human civilization as we know it and much of the earth’s biodiversity. If we do little or nothing, the projected warming could be 5 C° (9 F°) by the end of this century. Scientists warn that the resulting irreversible changes in global climate could lead to a 33–50% decrease in global food production, a significant drop in the human population, and widespread loss of biodiversity. It is very important to distinguish between shortterm daily and annual changes in the weather in an area and long-term changes in an area’s climate, based mostly on temperature and precipitation. Climate is described by an area’s weather factors averaged over several decades to thousands of years. It is clear that the average temperature of the lower atmosphere has increased during the last 35 years (Figure 19-B). But this does not necessarily mean that the area where you live is getting hotter each year. Some years the weather will be warmer, and some years it will be cooler. Thus, a warmer-than-average year cannot necessarily be attributed to climate change from global warming, and a cooler-than-average year is not necessarily a sign of global cooling or a sign that global warming is not taking place. The latter is a common misconception that people get when they do not distinguish between weather and climate. Nevertheless, average atmo spheric temperatures are clearly on the rise. The good news is that we can avoid or sharply reduce the projected harmful effects of global warming and the resulting climate change if the world takes strong global emergency action, beginning now, to sharply decrease greenhouse gas emissions and to stabilize greenhouse gas concentrations in the 450–550 ppm range, and thus to slow the rate of climate change. Climate scientists urge immediate emergency action, because if we exceed certain tipping points, we will set into motion irreversible climate change that will last for hundreds to thousands of years. In 2007, economist and climate change expert Nicholas Stern prepared a report for the British government that formed the basis for much of the information in Figure 19-7. In 2008, Stern said that he had underestimated the threat from global warming. According to Stern,