In papers written before Hurricane Katrina various researchers - including Kerry Emanuel of the Massachusetts Institute of Technology (Nature 436 686) and Peter Webster of Georgia Tech and co-workers (Science 309 1844) - reported that hurricanes have steadily become stronger over the last 25 years, and particularly in the last few years. This could be due to higher sea surface temperatures providing "fuel" for the hurricanes. Generally, the sea surface temperature must be above about 26°C for hurricanes to form and intensify.

Using satellite data from the Tropical Rainfall Measuring Mission Instrument, Kafatos and co-workers analysed how sea surface temperatures in the Gulf of Mexico have varied over the last 30 years. They found that mean sea surface temperatures were over 30°C throughout August this year, with a patch of 33°C next to the state of Louisiana (figure A). Furthermore, sea surface temperatures in the Gulf reached a record 0.8°C above normal compared to previous years (figure B). Hurricane Katrina caused devastation in New Orleans and the surrounding area when it reached the Louisiana coast on 29 August.

In addition to sea temperatures, hurricane behaviour is dictated by two other phenomena: surface latent heat flux, which is related to water evaporating and condensing; and sensible heat flux, which leads to changes in the temperature of the atmosphere. Inside a hurricane, latent heat can be converted into kinetic energy to produce secondary circulation which, in turn, can intensify the hurricane. Kafatos and co-workers found that daily variations of surface latent heat and sensible heat fluxes increased significantly during the intensification period of hurricane Katrina, so further adding to its strength (figure C).

"The Gulf waters should be continuously monitored using satellites to discern whether the increase continues in the following years," says Kafatos. "If this turns out to be the case, it will mean similar catastrophic events in the future."

However, further evidence is needed before climate scientists can be sure that there is a direct connection between global warming and the recent increases in hurricane intensity. For instance, the increase in the temperature of the sea needs to extend deep below the surface, whereas most satellites can only measure the temperature of a thin layer of water at the surface.