How much has global warming contributed to recent enhanced hurricane activity?
No, hurricane Katrina was not caused by global warming. But observations and our scientific analysis of them strongly suggests that there is a non-trivial human influence making such storms more intense and damaging.
There is no doubt that climate is changing, and humans are partly responsible. Global mean air temperatures are running 1 degree F or more above pre-1970s values. While 1998 remains the warmest year on record, 2002, 2003 and 2004 follow closely behind. Sea level has gone up over 1¼ inches in the past decade, as the ocean waters warm and expand and glaciers melt. Using climate models, these changes have been definitively linked to increases in greenhouse gases in the atmosphere, most notably carbon dioxide, which has increased 32% in the past century and half of that increase has occurred since 1970. This increase is caused by human activities and especially the burning of fossil fuels.
As part of this global warming, tropical (30ºN-30ºS) sea surface temperatures have increased 0.9ºF since 1970, and going along with this is a related increase in total atmospheric moisture (water vapor) of 4% locally, or about 8% (with a range of uncertainty of 4 to 12%) once feedbacks on increased winds and intensity are factored in. Further, recent studies have definitively shown that hurricanes and activity has increased globally, especially the severe category 4 and 5 storms. The main fuel for hurricanes is the latent heat release as rain forms from water vapor.
These observed changes in sea temperatures and water vapor increase the energy available for storms and enhance the rainfall intensity. Observations clearly reveal increases in heavy rains in the United States (the top 5% of heavy rains in terms of intensity increased 14% in the U.S. over the 20th century) and many other places around the world, often at the expense of more moderate rains. In hurricane Katrina, rainfalls locally exceeded about 12 inches near New Orleans, and with a swath of heavy rains of 2 to 3 inches extending north to the Canadian border.
There is no doubt that environmental changes related to human influences on climate have changed the odds in favor of more intense storms and heavier rainfalls. A reasonable estimate of the effect of global warming on storms, such as hurricane Katrina, is for order 8% enhancement of heavy rainfall and fuel for the storms since 1970, which may well be the enhancement of flooding that can breach levees designed without this in mind.
Selected slides from Are hurricanes changing with global warming?: