Greenhouse gases

Guest commentary by Kerry Emanuel

Executive Summary

Chapter 6 of the draft DOE report examines whether global warming exacerbates extreme weather. It rightly notes that because events such as hurricanes are rare, detecting their response to climate change in short and imperfect historical records is extremely difficult—if not impossible. Yet the authors devote most of the remainder of the chapter to attempting just that. By omitting to frame such efforts in the context of theory and models, they commit three fundamental errors: 1) searching for trends where none were predicted, 2) neglecting important variables for which trends were predicted and 3) overlooking—or failing to acknowledge—that some predicted trends are of a magnitude that is not a priori detectable in existing noisy and short data sets. The draft report also overlooks recent literature on climate change effects on weather extremes, and quotes selectively and misleadingly from the most recent report of the Intergovernmental Panel on Climate Change (IPCC). For these reasons, I find much of Chapter 6 to be of questionable utility. There are at least three climate change-induced trends in hurricane-related hazards that were predicted theoretically, simulated by models, and confirmed by observations:

1. Hurricanes are producing more rain, causing increased flooding. As water, not wind, is the source of most damage and mortality in hurricanes, this is the most consequential scientific finding.
2. The proportion of hurricanes that reach high intensity is increasing.
3. Hurricanes are intensifying more rapidly.

There is no robust scientific finding that hurricane frequency is increasing or expected to increase. Thus, much of Chapter 6 of the DOE report is devoted to refuting a hypothesis unsupported by scientific consensus. The short section on tornadoes does not include other more destructive aspects of severe convective storms, such as hail and damaging straight-line winds, and as with the section on hurricanes, omits inferences from theory and models.

[This commentary is also available as a pdf file]