Global warming has long been predicted to make hurricanes more intense. Well, now we are seeing more intense hurricanes. Chris Mooney has a great post on the recent storm surge of Category 5 hurricanes, now that Felix has joined that once-elite club. He notes:

  • There have now been 8 Category 5 Atlantic hurricanes in the past 5 years (Isabel, Ivan, Emily, Katrina, Rita, Wilma, Dean, Felix).
  • There have been two Atlantic Category 5s so far this year; only three other seasons have had more than one (1960, 1961, 2005).
  • There have been 8 Atlantic Category 5 hurricanes so far in the 2000s; no other decade has had so many. The closest runner up is the 1960s with 6 (Donna, Ethel, Carla, Hattie, Beulah, Camille).

Some people, especially the Deniers, think this is all a coincidence, or the result of incomplete data from earlier years. Here’s why I don’t:

felix_radar.png

Global warming increases sea surface temperatures (SSTs), which are directly correlated with stronger storms. Indeed, tropical cyclones are threshold events — if sea surface temperatures are below 80°F (26.5°C), they do not form.

Some analysis even suggests there is a sea surface temperature threshold close to 83°F needed for the spawning major hurricanes. Global warming may actually cause some hurricanes and some major hurricanes to develop that otherwise would not have (by raising sea surface temperatures above the necessary threshold at the right place or time). This is especially true in the Atlantic, where sea surface temperatures appear to be closer to the threshold than other hurricane-forming basins.

Equally important, one of the ways that hurricanes are weakened is the upwelling of colder, deeper water due to the hurricane’s own violent action. But if the deeper water is also warm, it doesn’t weaken the hurricane. In fact, it may continue to intensify. Global warming heats both the sea surface and the deep water, thus creating ideal conditions for a hurricane to survive and thrive in its long journey from tropical depression to Category 4 or 5 superstorm.

I have a longer discussion of this in Hell and High Water — and a good nontechnical article can be found here. The bottom line is that we would expect global warming to increase the number of Category 5 storms. Indeed, a 2004 modeling study concluded:

One implication of the results is that if the frequency of tropical cyclones remains the same over the coming century, a greenhouse gas-induced warming may lead to a gradually increasing risk in the occurrence of highly destructive category-5 storms.

So the fact that we are seeing such an increase is no surprise.

Yes, it could all be a grand coincidence or the result of inadequate data — but as a scientist I apply Occam’s Razor. We have data that matches our theory. The simplest explanation is that the theory is right.

As NASA’s James Hansen put it in an important 2007 analysis, “to the degree that hurricane intensification of the past decade is a product of increasing SST in the Atlantic Ocean and the Gulf of Mexico, human-made GHGs probably are a substantial contributor.”

One final point: I am aware that the 2004 modeling study predicted it would be many decades before we saw the warming signal in hurricane intensity trends. You can take that as a sign that the data is wrong, if you want, but as I noted in “Are scientists overestimating — or underestimating — climate change? Part I,” scientists have underestimated recent sea ice loss, ice sheet mass loss, temperature rise, sea-level rise, and expansion of the tropics.

So I conclude that a more likely explanation is that the models are systematically underestimating key climate-cycle feedbacks and omitting crucial factors that will accelerate the pace of climate change beyond what the IPCC “consensus” believes.

This post was created for ClimateProgress.org, a project of the Center for American Progress Action Fund.