Atlantic Hurricanes and CO2

By: By Dr. William Gray (from
By: By Dr. William Gray (from

The official end of the 2008 Atlantic basin hurricane season occurred last Sunday (November 30). This year was an active and destructive season. My colleague, Phil Klotzbach and I were very happy to see that our forecasts for this year’s activity worked out well, as did NOAA’s seasonal hurricane forecast. See our website for a 53-page summary of this season’s activity. Although this is my 25th year of making these seasonal forecasts, Klotzbach should get most of the credit for the success of this year’s forecast.

President-Elect Barack Obama said last week that “storms are growing stronger with each passing hurricane season” (implying that this is due to CO2 increases). He is repeating what Al Gore has been saying for years and what was implied by thousands of media reports after the damaging Atlantic seasons of 2004-2005. Polls have shown that a relatively high percentage of US citizens think that human-induced global warming has increased hurricane activity.

Yes, the Atlantic has seen a very large increase in major hurricanes during the 14-year period of 1995-2008 (average 3.9 per year) in comparison to the prior 25-year period of 1970-1994 (average 1.5 per year). But, have rises in CO2 been, in any way, been responsible for the recent large upswing in Atlantic basin major hurricanes since 1995?

I and a number of my colleagues believe that this large increase in Atlantic major hurricanes is primarily due to the multi-decadal increase in the Atlantic Ocean Thermohaline Circulation (THC) that is driven by Atlantic salinity variations. These Atlantic multi-decadal changes have also been termed the Atlantic Multidecadal Oscillation (AMO). These increases are not a result of global surface temperatures or CO2 increases.

In the quarter-century period from 1945-1969 when the globe was undergoing a weak cooling trend, the Atlantic basin experienced 80 major (Cat 3-4-5) hurricanes and 201 major hurricane days. By contrast, in a similar 25-year period from 1970-1994 when the globe was undergoing a general warming trend, there were only 38 major hurricanes (48% as many) and 63 major hurricane days (31% as many). Atlantic sea surface temperatures and hurricane activity is related to but does not necessarily follow global mean temperature trends.

What made the 2004 and 2005 seasons so unusually destructive was not the high frequency of major hurricanes but the high percentage of hurricanes that were steered over the US coastline.

(see more with charts and graphs at

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