Hurricanes and climate change

Two new studies point to a strong link between the observed increase in Atlantic hurricane activity and human-induced climate change (the image is tropical storm Alberto on June 13). The basic concept is the same as suggested before: climate change is increasing sea surface temperatures (SSTs) in the tropical Atlantic, and the higher temperatures contribute to more intense and possibly more frequent storms. These new studies drive home the point.

The first study, by hurricane expert Kerry Emanuel and climatologist Michael Mann published in the journal EOS last week, challenges the notion that the upswing in Atlantic hurricane activity is due solely to natural climate cycles. Sea surface temperatures (SSTs) in the north Atlantic tend to oscillate over a 60-80 year period, called the Atlantic Multidecadal Oscillation (AMO), probably due to internal variability in the thermohaline circulation (a part of the ocean “conveyor belt” often mentioned in climate stories and described in An Inconvenient Truth). Scientists often attribute the variability in hurricane activity in the past, especially the increase in recent decades, to this natural cycle. Emanuel and Mann use historical data to show that the increasing trend in Atlantic SSTs is more than an upswing in the AMO, and is likely to be caused by human-induced climate change. They add that it is possible that the emissions of aerosols (NOx, SOx, etc), which lead to regional cooling, may have masked even more warming.

Emanuel originally presented these ideas at the same meeting last fall where a speech by NASA scientist Jim Hansen's drew the ire of government officials [and led to all that news coverage of attempts to quiet government scientists]. During the presentation, Emanuel expressed concern that the line "changes in hurricane activity are due to natural cycles, not global warming" was appearing in every news story about Hurricane Katrina because government scientists were not permitted to say otherwise. The comment drew a round of applause, pretty much unheard of at a scientific meeting.

The second study, by Kevin Trenberth and Dennis Shea at the National Center for Atmospheric Research, estimates the contribution of different factors to the increase in Atlantic SSTs (see the press release). They find that human-induced climate change is responsible for almost half increase in SSTs that fuelled the strong 2005 hurricane season (difference between last fall and the 1901-1970 average). Like the Emanuel and Mann paper, they find the AMO has had less influence on the recent rise in Atlantic SSTs than previously suspected.

Neither of the studies is claiming any particular storm was caused by global warming. Climate science always comes down to probabilities. In the case of hurricanes, warmer waters increase the probability that a tropical depression will grow into a hurricane (simplistic explanation, yes, many other factors like vertical wind shear are obviously important too). You could think of all these climate studies and forecasts like advice from your doctor. For example, say the doctor tells you to quit smoking. The doctor knows that smoking won't absolutely ensure you'll get lung cancer. Even if it did, the doctor couldn't possibly tell you exactly when or where the cancer would form. But the doctor knows the science, knows the statistics, and concludes smoking will dramatically worsens your odds.