Thursday, July 26, 2007

2007 Hurricane Season - Slow Start

The 2007 hurricane season seems to be off to a typically slow start. Most hurricanes form in August, September and October, peaking in mid September. “There’s absolutely nothing out of the ordinary,” Gerry Bell, a hurricane forecaster for the US National Oceanic and Atmospheric Administration, said of the Atlantic season’s first two months. “It’s not slow. It’s not fast.”

Forecasters are projecting 17 Atlantic storms, with nine becoming hurricanes and five reaching intense strength - Bill Gray, Colorado State University; 13 to 17 storms, seven to 10 hurricanes and three to five intense hurricanes - NOAA; and 14.7 storms, 7.9 hurricanes and 3.5 intense hurricanes - London-based Tropical Storm Risk. Private forecaster WSI Corp. has lowered its forecast to 14 storms from 15 and to six hurricanes from eight. A betting pool may be in order.

The low estimates are a result of a slower than anticipated rise in sea surface tempertures in the Atlantic Basin. “Because the ocean temperatures have not yet rebounded from the significant drop in late spring, we have decided to reduce our forecast numbers slightly,” said Todd Crawford, a WSI seasonal forecaster.

So how does will this season stand up to historical trends?

If we look at U.S. land-falling hurricanes over time, using periods long enough to reveal long-term trends, the most recent 50-year period, 1957 to 2006, produced 83 landfalling US hurricanes, 34 of them major. In contrast, during the 50-year period from 1900 to 1949, 101 hurricanes (22% more) made U.S. landfall, including 39 (or 15% more) major hurricanes.

What does this say about climate change inducing more and stronger torms?

William Gray and colleague Phil Klotzbach say “The hypothesis that increasing carbon dioxide in the atmosphere increases the number of hurricanes fails by an even wider margin when we compare two other multi-decade periods: 1925-1965 and 1966-2006. In the 41 years from 1925-1965, there were 39 U.S. land-falling major hurricanes. In the 1966-2006 period there were 22 such storms — only 56% as many. Even though global mean temperatures have risen by an estimated 0.4 Celsius and CO2 by 20%, the number of major hurricanes hitting the U.S. declined.” Gray, Hurricanes and Hot Air, Wall Street Journal Online, July 26, 2007; Page A12

Cross posted at Blue Marble Climate