Scientists discount global warming effect on hurricanes

Two scientists from Colorado State University who are known for their regular hurricane season forecasts say the frequency and intensity of hurricanes are not affected by global warming.

Philip Klotzbach and William Gray released their April forecast last week which calls for a very active Atlantic hurricane season this year. However, the two scientists see the active season as a result of a multi-decadal atmospheric cycle, not because of global warming.

‘Although global surface temperatures have increased over the last century and over the last 30 years, there is no reliable data available to indicate increased hurricane frequency or intensity in any of the globe’s seven tropical cyclone basins, except for the Atlantic over the past 12 years,’ their forecast report states.

While the Atlantic has seen a very large increase in the number of major hurricanes since 1995 compared to the 25 immediately preceding years, the scientists said there have been equally active cyclical periods before.

The report states that global mean ocean and Atlantic sea surface temperatures rose about 0.4° Celsius between the 50-year periods of 1900-1949 and 1956-2005. Despite the rise in mean temperature, there was actually a decline in the number of tropical cyclones making landfall in the United States in latter time period.

There were 189 named storms, 101 hurricanes and 39 major hurricanes that made landfall in the United States between 1900 and 1949. This compares to the 165 names storms, 83 hurricanes and 34 major hurricanes that made a US landfall between 1956 and 2005.

There are other statistics that point away from global warming having an effect on hurricanes, the scientists say.

In the quarter-century between 1945 and 1969, when the globe was undergoing a weak cooling trend, the Atlantic basin experienced 80 major hurricanes and 201 days when major hurricanes were in existence. By contrast, in a similar 25-year period between 1970 and 1994, when the globe was undergoing a general warming trend, there were only 38 major hurricanes and 63 major hurricane days.

‘Atlantic sea-surface temperatures and hurricane activity do no necessarily follow global mean temperatures trends,’ the report stated.

Even if sea-surface temperatures continue to rise, the scientists do not believe it will have a big affect on hurricanes.

‘We have no plausible physical reasons for believing that Atlantic hurricane frequency or intensity will change significantly if global temperatures continue to rise.’

Instead, any increase in the frequency or intensity of hurricanes, like the increase in major storms over the past 12 years, will be part of cyclical changes called the Atlantic Multidecadal Oscillation.

‘The large increase in Atlantic major hurricanes is primarily the result of the multi-decadal increase in Atlantic Ocean thermohaline circulation that is not directly related to global temperature increase or to human-induced greenhouse gases,’ the report states, adding that changes in ocean salinity are believed to be the driving mechanism for the cycles.

Klotzbach and Gray do not believe people should read too much into the very active and destructive Atlantic hurricane seasons of 2004 and 2005.

‘The activity of these two years was unusual, but well within the natural bounds of hurricane variation,’ their report states. ‘In addition, following the two very active seasons of 2004 and 2005, 2006 had slightly below-average activity, and no hurricanes made landfall in the United States.’

The report also points out that although 2005 set records with 27 named storms, 15 hurricanes and seven major hurricanes, there were comparable years of tropical cyclone activity. In 1933 for instance, during another cycle of high hurricane activity, there were 21 named storms that tracked west of 60°W. There was no satellite technology back then, so hurricanes were recorded by surface observations, which made recording storms entirely east of 60°W more difficult.

Going back to 1875, the National Hurricane Centre best track data base shows that six previous hurricane seasons had more days when hurricanes were active than 2005. These years were 1878, 1893, 1926, 1933, 1950 and 1995. In addition, five previous years – 1893, 1926, 1950, 1961 and 2004 – had more days when major hurricanes existed.

‘Although the 2005 hurricane season was certainly one of the most active on record, it is not as much of an outlier as many have indicated,’ the report stated.

Even if global warming is not causing the increases, the bad news is Klotzbach and Gray believe the Atlantic will have a very active season in 2007, and even more active than they thought back in December 2006. The scientists are now predicting 17 named storms, nine hurricanes and five major hurricanes. The numbers are almost identical to the predictions released last month by another known hurricane season forecaster, Tropical Storm Risk out of the United Kingdom.