HURRICANE SEASON: Warming revs into action

As I write this, tropical storms and hurricanes are lined up across the Atlantic Ocean like aircraft on final approach to Hartsfield-Jackson International Airport. Tropical Storm Fay produced prodigious rainfall, and Hurricane Gustav prompted the largest evacuation in Louisiana’s history. With six to eight weeks left in the hurricane season, storm activity has reignited a debate that began after the 2005 hurricane season on whether global warming is fueling stronger or more numerous hurricanes.

The 2007 Intergovernmental Panel on Climate Change stated that “warming of the climate system is unequivocal.” Unlike previous reports that treaded carefully on the side of scientific caution, the nonpartisan panel of scientists established a strong degree of certainty in the most recent report on climate change. Special interest groups and provocative media coverage continue to suggest a widespread skepticism among climate scientists that does not exist.

A warmer atmosphere and ocean impact global weather patterns. This will almost certainly result in global changes in floods, droughts and hurricanes. Such events have profound implications for agricultural productivity, energy supply, human health and the economy.

At the 2008 Georgia Climate Summit in May, Peter Webster of Georgia Tech noted a link between warmer ocean temperatures after 1970 and increased hurricane activity. Webster also highlighted recent findings that future hurricanes will have stronger winds and produce more rainfall.

Research led by James Elsner of Florida State University found that since 1981 winds have become stronger in the strongest hurricanes.

Bill Lau and colleagues at NASA, presenting findings at the American Meteorological Society’s annual meeting, showed that major hurricanes, greater than a category 3, are producing more rainfall.

Along with colleagues at UGA, I conducted research showing that major hurricanes produce the most rainfall at or near landfall, although weaker storms contribute more to seasonal totals. Inland freshwater flooding is often a greater safety threat than high winds and storm surge.

Because of natural cycles, it is often difficult to establish if these storms are increasing over time. Indeed, leading National Oceanic and Atmospheric Administration scientists continue to note that we have been in an active hurricane phase since the mid-1990s. Atmospheric phenomena, such as El Nino and dust blowing over the Atlantic Ocean from Africa, may affect storm activity.

But hurricanes are like car engines, and their fuel supply is warm water. In a warmed climate, hurricanes are running on what might be considered high-octane gasoline, rather than regular. Elsner estimated that every 1-degree (Celsius) increase in ocean temperature causes a 31 percent rise in the number of category 4 and 5 hurricanes. Some climate models project that sea surface temperatures could rise 2 to 3 degrees (Celsius) by the year 2100.

A few cautionary notes: First, no individual study should be considered to be the final word. Second, a lack of hurricane data before 1900 reduces our ability for meaningful comparisons with the most recent 30-year period. Third, global warming causes other changes in jet stream and wind patterns. Do such changes enhance or suppress the hurricanes and tropical storms? We have very little understanding of how African dust affects hurricanes. New aircraft, satellite and ocean observations of heat content, dust and winds will help tremendously here.

Finally, it’s shortsighted to only study storms affecting the United States. In a so-called “quiet” year such as 2006, when fewer storms made landfall here, many other storms raged in the Atlantic and in other ocean basins around the world.

But make no mistake: As global temperatures rise, we’ll be on hurricane watch for decades to come.

> Dr. J. Marshall Shepherd is associate professor and meteorologist in the geography department at the University of Georgia. Dr. Thomas L. Mote, a UGA climatologist and professor of geography, also contributed.