ResilienceClimate change and Hurricane Katrina: what have we learned?
Theory and computer models show that the incidence of the strongest hurricanes — those that come closest to achieving their potential intensity — will increase as the climate warms, and there is some indication that this is happening. Global warming, however, is occurring far too fast for effective human adaptation. Adapting to the myriad changes expected over the next 100 years is such a daunting prospect that otherwise intelligent people rebel against the idea even to the extent of denying the very existence of the risk. This recalcitrance, coupled with rising sea levels, subsiding land, and increased incidence of strong hurricanes, all but guarantees that New Orleans will have moved or have been abandoned by the next century.
Three weeks and three days before Hurricane Katrina devastated New Orleans ten years ago, a paper of mine appeared in the scientific journal Nature showing that North Atlantic hurricane power was strongly correlated with the temperature of the tropical Atlantic during hurricane season, and that both had been increasing rapidly over the previous thirty years or so. It attributed these increases to a combination of natural climate oscillations and to global warming.
Had Katrina not occurred, this paper and another by an independent team would merely have contributed to the slowly accumulating literature on the relationship between climate and hurricanes.
Instead, the two papers inspired a media firestorm, polarizing popular opinion and, to some extent, scientists themselves, on whether global warming was in some way responsible for Katrina. While the firestorm was mostly destructive, benefiting only the media, it had a silver lining in inspiring a much more concerted effort by atmospheric and climate scientists to understand how hurricanes influence and are influenced by climate.
We have learned much in the intervening years.
Sea level and storm surges
An obvious point is that slowly rising sea levels increase the probability of storm-induced surges even when the statistics of the storms, such as top wind speed, themselves remain stable. Storm surges are physically the same thing as tsunamis but driven by wind and atmospheric pressure rather than the shaking seafloor, and they typically arrive near the peak of the storm’s fury.
As with Katrina and Sandy, they are often the most destructive aspects of hurricanes. Had Sandy struck New York a century ago, there would have been substantially less flooding, as sea level was then roughly a foot lower. As sea level increases at an accelerating pace, we can expect more devastating coastal flooding from storms.
Potential intensity
What about the storms themselves? Hurricanes are giant heat engines driven by the thermodynamic disequilibrium between the tropical oceans and atmosphere. This disequilibrium drives a strong flow of heat from the ocean to the atmosphere and is a direct consequence of the greenhouse effect: the tropical atmosphere is so opaque to infrared radiation that the sea surface cannot cool very much by directly radiating heat to space. Instead, it cools mostly by the evaporation of water, the same mechanism by which our sweaty bodies cool on a hot day.
