Hurricane Harbingers

Are hurricanes becoming stronger and more numerous?


New Orleans is only three-fifths the city it was before Hurricane Katrina hit. In 2005, the Big Easy was home to 455,000 people. Today just 273,000 dwell there. The city drowned and 1,300 people died because a Category 3 hurricane (wind speeds 111-130 miles per hour) overwhelmed poorly designed Army Corps of Engineers levees. Now, with Hurricane Dean churning its way across the Caribbean aiming for the Mexico's Yucatan peninsula, Federal Emergency Management Agency (FEMA) bureaucrats are rushing "federal assets" to south Texas just in case Dean veers northward.

The 2005 hurricane season was the most active on record. It started early and ran late, featuring 27 named storms, of which 15 were hurricanes. In May 2006, the National Oceanic and Atmospheric Agency's (NOAA) National Hurricane Center predicted that "a very active hurricane season" was looming. According to NOAA's May 2006 forecast, the season would likely see 13 to 16 named storms, with eight to 10 becoming hurricanes, of which four to six could become "major" hurricanes of Category 3 strength or higher. On August 8, 2006, NOAA lowered the number of forecasted storms slightly to 12 to 15 named storms, with seven to nine becoming hurricanes, and three or four becoming major hurricanes. NOAA also predicted a high likelihood (75 percent chance) that 2006 would be an above-normal Atlantic hurricane season.

Fortunately for coastal residents, NOAA was way off the mark. In 2006, there were only 5 hurricanes, of which two were major, and just 4 named tropical storms. NOAA noted, "On average, the north Atlantic hurricane season produces 11 named storms, with six becoming hurricanes, including two major hurricanes." Basically, 2006 turned out to be a slightly below average year. What happened?

Several factors affect hurricane formation. Sea surface temperatures must be above 82 degrees Fahrenheit. During 2006, a number of factors combined to keep hurricanes at bay. Ocean temperatures were lower than in 2005 because of persistent easterly winds kept drawing up cooler water from the ocean's depths. In addition, recent research indicates that the numerous dust storms flowing off the Sahara in 2006 somehow contaminated the hurricane nurseries off the Cape Verde Islands. But probably the biggest factor was that the eastern Pacific Ocean flipped from La Nina to El Nino conditions. During an El Nino, water temperatures off the coast of South America warm up and this has global effects on climate. One important effect is that El Nino increases wind shear over the Caribbean which inhibits hurricane formation by decapitating them.

Controversial recent research suggests that man-made global warming will produce more and more intense hurricanes in the future. In August 2005, Massachusetts Institute of Technology tropical storm researcher, Kerry Emanuel published an analysis in Nature that claimed that Atlantic hurricanes had become much more powerful as a result of higher average sea surface temperatures (SSTs) since the mid-1970s. A month later, researchers including Peter Webster at the National Center for Atmospheric Research (NCAR) and Greg Holland at the Georgia Institute of Technology (GIT) published a study which also found that the number of category 4 and 5 hurricanes had increased over the past 35 years, mostly in the Pacific and Indian Oceans. The study also noted that the overall number of hurricanes had actually decreased around the world, except in the north Atlantic. The researchers argued that tropical ocean SSTs rose by approximately 0.5 degrees celsius between 1970 and 2004, fueling more intense hurricanes.

In July, Holland and Webster published a new study in the Philosophical Transactions of the Royal Society which found that the number of tropical storms in the Atlantic have doubled over the past century due to global warming. The study claims that the number of tropical cyclones averaged six per year between 1905 and 1930, rose to 9.4 annually between 1931 and 1994, and jumped to 14.8 per year since 1994. In each period a bit more than half of the storms became hurricanes. The researchers find that the trend of more hurricanes results from an average sea surface temperature increase of 0.7 degrees Celsius in the eastern Atlantic of over the past century. They conclude, "[W]e are led to the confident conclusion that the recent upsurge in tropical cyclone frequency is due in part to greenhouse warming, and this is most likely the dominant effect."

On the other hand, National Hurricane Center researcher Chris Landsea is not so sure. He argues in the May 1 issue of the journal EOS that Holland and Webster have underestimated the number of hurricanes that occurred in the early part of this century. Why? Because many storms would have been missed since there were far fewer observers to note them earlier in the century. Landsea concludes that "improved monitoring in recent years is responsible for most, if not all, of the observed trend in increasing frequency of tropical storms." Landsea slices the frequency data differently, arguing that there have been alternating multi-decadal quiet and active periods, e.g., quiet up to 1925, active between 1925 and 1970, quiet from 1970 to 1994, and active since.

Paradoxically, man-made global warming might offer some relief from hurricane assaults. A study published in the journal Geophysical Research Letters in April suggests that man-made global warming will increase wind shear over the Caribbean and Atlantic. If this finding is confirmed, such an increase in wind shear would tend to reduce the number and intensity of Atlantic hurricanes.

So what's up for this year? Until Dean spun up, the 2007 hurricane season had been pretty quiet. In May, NOAA predicted the Atlantic would experience 13 to 17 named storms, with seven to 10 becoming hurricanes, of which three to five could become major hurricanes of Category 3 strength or higher. On August 9, NOAA revised its forecast for the 2007 hurricane season downward slightly to 13 to 16 named storms, seven to nine hurricanes, and three to five major hurricanes. However, agency scientists boosted the chance that the 2007 Atlantic hurricane season would be above normal from a 75 percent chance to 85 percent. Dean is expected to hit Mexico as a Category 5 storm with winds of over 155 miles per hour.

On top of any effects that global warming may have on the future intensity and number of hurricanes, many researchers believe that we have also entered into a multi-decadal period in which hurricane activity is naturally higher. The bad news is that this period of higher activity could persist for the next four decades. So more people and their property will be at risk simply because they have chosen to move into and build in harm's way. Between 1960 and 1994 the populations of the eight most hurricane prone states—North Carolina, South Carolina, Georgia, Florida, Alabama, Mississippi, Louisiana, and Texas—increased by 80 percent. Even worse, coastal populations of these states rose by 103 percent.

Nevertheless, it is interesting to note that despite the tremendous violence of hurricanes, they have killed only about 15,000 Americans since 1900 and 8,000 of those died in the great Galveston storm on September 8 of that year. After 1985, the number of Americans who died as a result of hurricanes averaged 16 per year (in comparison, about 67 people are killed by lightning and 65 by tornadoes each year). While hurricanes kill relatively few people (with the horrific recent exception of Katrina), the damage they do to property is escalating. "There is a huge upward trend in hurricane damage in the U.S., but all or almost all of this is due to increasing coastal population and building in hurricane-prone areas," notes MIT researcher Kerry Emanuel.

A new study in the journal Natural Hazards Review normalized storm damage by estimating how much damage a past storm would cause if it struck the same area now. The normalization procedure attempts to take into account increased wealth, population and an inflation factor, though not everyone agrees that it succeeds in doing so. In one example, the researchers compared the damage caused by the 1979 Hurricane Frederic which came ashore in Alabama. In 1979, the storm caused $2.3 billion in damages and would cause more than $10 billion today. As the study notes, "Unless action is taken to address the growing concentration of people and properties in coastal areas where hurricanes strike, damage will increase, and by a great deal, as more and wealthier people increasingly inhabit these coastal locations."

But what action should be taken? Well, one idea is to eliminate incentives like the Federal government's National Flood Insurance Program. Since 1968, the NFIP collected $1.1 billion in premiums and paid out an average of $1 billion to cover flood losses annually. So far, so good—until Katrina. The NFIP borrowed $18.5 billion to pay off Katrina claims and, according the Government Accountability Office (GAO), the program has no prospect of ever collecting enough in premiums to pay it back. As the GAO notes, "Because of the catastrophic nature of flooding and the inability to adequately predict flood risks, private insurance companies have largely been unwilling to underwrite and bear the risk of flood insurance." One sensible way to discourage people from living and building in hurricane prone areas would be to eliminate federal flood insurance.

Nevertheless, the coasts will remain population and development magnets and a richer society will be able to afford better hurricane defenses, such as, stronger buildings, higher levees, and protective surge barriers. But what's the best way to pay for them? One proposal would be instead of depending on "federal assets," local jurisdictions should be pushed to pay for and maintain their own hurricane defenses. If New Orleans needs new and better levees, then the city's citizens should pay for them. If New Orleans residents refuse to tax themselves enough to do so that means that it doesn't make economic sense to live and work there. One proof of the adequacy of their levees would be the willingness of private insurers to offer flood policies to residents. The same logic applies to all coastal counties. Ultimately, instead of retreating from the shore, I believe that we will instead learn how to live with stronger storms.

Ronald Bailey is Reason's science correspondent. His book Liberation Biology: The Scientific and Moral Case for the Biotech Revolution is now available from Prometheus Books.