Three weeks ago, on this website, Robert Krulwich and I discussed new research documenting the 50 percent increase in the power of tropical hurricanes since the 1970s. The author of the paper, Dr. Kerry Emanuel of MIT, blamed a significant part of this increase on global warming.
But was Hurricane Katrina caused by global warming? Will devastating storms like Katrina become a normal occurrence for residents of the Gulf Coast? And as Louisiana, Mississippi, and Alabama begin the rebuilding process, what environmental lessons can be learned from this tragedy?
None of these questions has an easy answer. Hurricanes remain enigmatic monsters. For many years, virtually all scientists denied any link between global warming and hurricane activity. Hurricanes, they said, were governed by a separate cycle of oceanic and atmospheric activity, which seemed to shift about every 25 years. According to this theory, the recent spate of devastating storms has been caused by the fact that the Atlantic seems to be trapped in a particularly bad hurricane cycle: Nine of the last 11 years have seen higher than normal hurricane activity.
But that isn't the whole story. While most scientists agree that the number of hurricanes is not caused by global warming, there is an increasing body of evidence suggesting that the severity of hurricanes is directly linked to rising global temperatures. Intuitively, this makes sense. Global warming directly leads to warmer ocean surface temperatures. Warmer ocean surface temperatures cause more water to evaporate, which leads to storms containing both more water and more energy.
Emanuel's study revealed the frightening increase in hurricane strength. Using statistical data collected from storms from as far back as the 1850s, Emanuel concluded that roughly half of the 50 percent increase he observed in hurricane strength was due to storms lasting longer, and half because their peak wind speeds have been increasing. In other words, hurricanes are simply getting stronger in every possible way.
While Emanuel's study was the first to directly link global warming and hurricane strength, other studies have focused on specific changes in hurricane activity. Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research, published a paper earlier this summer in the journal Science that has direct bearing on what has happened in New Orleans. In his review of the statistical literature, Trenberth notes that during the 20th century precipitation for U.S. hurricanes increased by 7 percent. Even more ominously, since 1995 this increase in precipitation has become much more dramatic. As Katrina tragically shows, hurricanes do most of their damage with water. While New Orleans survived the actual storm relatively intact—thanks in large part to more stringent building codes designed to withstand 155 mph winds—it was unprepared for the ensuing flood.
Furthermore, over the last 100 years the number of storms with the heaviest precipitation has increased over 15 percent, with the largest increase coming for the most devastating 1 percent of storms. Such observations confirm virtually all atmospheric models of global warming, which predict an increase in "extreme precipitation events."
In the face of such grim statistics, what can residents of hurricane-prone regions do to defend themselves against the next Katrina? The first lesson is that cities must plan and prepare for a Category 5 hurricane. New Orleans officials were aware that their levees would most likely only function for a hurricane of Category 3 strength or less. Katrina was at the upper end of Category 4. As noted above, such powerful storms appear to be becoming more common.
Sadly, the Gulf States have lost much of their best natural defenses against hurricanes. As the NOVA scienceNOW broadcast segment pointed out, in the last 70 years, over 2,000 square miles of Louisiana wetlands have vanished, depriving New Orleans of an essential buffer. A functioning wetland ecosystem would have absorbed much of the water that ended up instead in the city streets.