In reading some sections of the IPCC Fourth Assessment Report: Climate Change 2007 (usually just called AR4) while looking for sources for another project, one thing really stood out: it is full of optimism.

This optimism may surprise you. It certainly surprised me, given the severity of the environmental problems we must deal with in the next few years.

How is AR4 optimistic?

Some of the studies included in AR4 concerning the impact on humans caused by anthropogenic climate change take into account surprisingly optimistic progress and economic growth. In some of these studies the technical and economic improvement is strong enough that the average person may be better off in forty or ninety years, despite the human-caused climate changes and increasing population pressure. This gives lie to the frequently repeated canards about the IPCC as a bunch of pessimistic activists who don't believe in improvement and promote a doom and gloom vision of the future. Simply reading any of AR4 makes it clear that it is just a summary of current knowledge and best estimates---with probabilities---of likely future scenarios. Since economics and development are so fuzzy compared to climate science, there are a wide range of plausible scenarios for these human factors. Within this wide range, many studies are optimistic in their assumptions. Some of the studies included in AR4 include enough improvement that a larger proportion of people will be better off (or at least less likely to starve) in seventy years than when the report was written. But this isn't the whole picture.

Every silver lining's got a
Touch of Grey

It is vital to remember that the optimism in some studies doesn't excuse us from immediate action to avoid the problems of anthropogenic global warming and other human-caused environmental degradation. When the studies with beneficial assumptions are compared against themselves but with climate change removed, the versions with no climate change are more optimistic. That is, even the studies that include enough improvements that many people are expected to be better off in the second half of the century than now, more people would be better off if we stop causing climate change. Not all the studies are as optimistic. It is more likely that climate change will actually increase the number of hungry people in the second half of the century, if not sooner. And using this one measure---the availability of food for humans over the next ninety years---is an incomplete view of the impact of climate change: effects extend far beyond food resources and last far longer than ninety years.

The Fourth Assessment Report is in some ways surprisingly optimistic, but an understanding of the report calls for immediate action to avoid the expected negative impacts, and the likely disastrous impacts. Fortunately, since we know the cause of the damage we know what we must do to avoid causing more damage. Or, at least, what we must do in order to avoid causing more damage than we are already committed to causing. Beyond the mere pragmatic necessity of immediate action is the moral responsibility to act now: not all harm caused can be simplified to measurements of human health and wealth.