As Jeff Master, our favorite meteorologist and hurricane blogger, wrote on Saturday:

This year is now the only hurricane season on record in the Atlantic that has featured major hurricanes in five separate months. The only year to feature major hurricanes in four separate months was 2005, and many years have had major hurricanes in three separate months. This year’s record-setting fivesome were Hurricane Bertha in July, Hurricane Gustav in August, Hurricane Ike in September, Hurricane Omar in October, and Hurricane Paloma in November.

Because global warming will be cooking the Atlantic hurricane forming region year-round for the foreseeable future, we can expect this deadly record to be repeated many times. A recent study offered “the firmest evidence so far that global warming will significantly increase the intensity of the most extreme storms worldwide.”

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Tropical cyclones are threshold events: If sea surface temperatures are below 80°F (26.5°C), they do not form. Some analysis even suggests there is a sea-surface temperature “threshold [close to 83°F] necessary for the development of major hurricanes.”

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Global warming may thus actually cause some hurricanes and some major hurricanes to develop that otherwise would not have (by raising sea-surface temperatures above the necessary threshold at the right place or time).

The Wikipedia discussion of the current Atlantic hurricane season notes:

Although not an El Niño or La Niña year, 2008 is the second most destructive [Atlantic hurricane] season on record, behind only the 2005 season, with up to 52 billion in damage.

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And, of course, the season isn’t over — the devastation from Paloma is just beginning to be calculated, and more hurricanes may yet develop. Masters notes that Paloma is an unusually strong storm for this late in the season:

Paloma is now the second strongest November hurricane on record in the Atlantic. Hurricane Lenny of 1999, a Category 4 hurricane with 155 mph winds, was the strongest November hurricane on record. Paloma shares second place with Hurricane Michelle of 2001 (Cat 4, 140 mph) and Hurricane Greta of 1956 (Cat 4, 140 mph).

At the same time, Wikipedia notes, “the season also had the earliest known date for 3 storms to be active on the same day: Hurricane Bertha, and Tropical Storms Cristobal and Dolly were all active on July 20.”

Again, because of globally-warmed sea surface temperatures in the Atlantic and Caribbean, we expect more intense storms will be seen earlier and later in the season. The 2005 hurricane season was the most striking example of that trend, with Emily “the earliest-forming Category 5 hurricane on record in the Atlantic,” in July, and Zeta, the longest-lived tropical cyclone to form in December and cross over into the next year, where it became the longest-lived January tropical cyclone.

USA Today‘s Weather blog notes yet one more remarkable record set by the 2008 season:

Six straight named storms — Dolly, Edouard, Fay, Gustav, Hanna, and Ike — all made U.S. landfall for the first time on record.

Finally, remember what is coming on the business-as-usual (BAU) emissions path. The recent Nature study found:

The team calculates that a 1ºC increase in sea-surface temperatures would result in a 31% increase in the global frequency of category 4 and 5 storms per year.

What kind of BAU warming can we expect?

ornl-final.jpg 2050-ornl-final.jpg

And that is just 2050, with average warming of 2.6°C. The BAU path takes us to double that warming (or more) by 2100 — by which time sea levels will probably be three to six feet higher.

The time to act is now.

This post was created for, a project of the Center for American Progress Action Fund.