Hurricanes and Climate Change

A lady I worked with said to me last week, “George, we’re getting an awful lot of hurricanes. Is that because of global warming?” Here’s my answer to her, and to you:
Recently a letter was sent to Sen. John McCain by ten scientists, including four state climatologists. McCain’s Senate Commerce Committee met this week to discuss climate change.

The major emphasis of the letter was whether hurricane frequency and intensity have increased in recent years, and what is likely to happen in the future. The letter included the following statements:

“As climate researchers, we wish to point out two misconceptions carried in media reports when it comes to assessing hurricanes and their relation to climate change. First is the erroneous claim that hurricane intensity or frequency has risen significantly in recent decades in response to the warming trend seen in surface temperature. Second is the claim that a future surface warming trend would lead to more frequent and stronger storms. We believe that both of these are demonstrably false.

“First, the recent surface warming trend observed over the last few decades is unaccompanied by an increased frequency of hurricanes. The National Hurricane Center reports that in the last century, the decade with the largest number of hurricane strikes in the U.S. was the 1940s, with a decline since that time. For the North Atlantic as a whole, according to the United Nations Environment Programme of the World Meteorological Organization, “Reliable data … since the 1940s indicate that the peak strength of the strongest hurricanes has not changed, and the mean maximum intensity of all hurricanes has decreased.” Recent history tells us that hurricanes are not becoming more frequent.

“Regarding the mid-latitudes, no drastic increase in storminess is seen. While several scientists have identified a slight increase in heavy precipitation events in the U.S., in other severe storm categories, the trends are slightly downward. These include thunderstorms, hail events, tornadoes, and winter storm activity. According to meteorological measurements, extreme weather is not increasing.

The second claim in news stories involves the question: if surface warming trends continue, are more or fewer severe storms likely?

Computer simulations suggest that in a warmer world most of the warming would occur in the polar regions. Atmospheric circulation, which crucially affects storms, is driven primarily by the temperature difference, or gradient, between the tropics and the poles. Warmer polar regions would reduce this gradient and thus lessen the overall intensity or frequency or both of storms – not just tropical storms but mid-latitude winter storms as well (such as blizzards and northeasters).

Again, longer periods of history bear this out. In the past, warmer periods have seen a decline in the number and severity of storms. This is well-documented in scientific journals for data extending back centuries or even millennia. If the surface temperature of the planet rises further in the future, it is likely that these declines will continue.

Even the IPCC agrees that the data indicate considerable variability, but without a significant trend: ‘Changes globally in tropical and extra-tropical storm intensity and frequency are dominated by inter-decadal to multi-decadal variations, with no significant trends evident over the 20th century. Conflicting analyses make it difficult to draw definitive conclusions about changes in storm activity, especially in the extra-tropics.’ (Climate Change 2001: The Scientific Basis, p. 5)

I spoke to another of my colleagues about this, and he seemed skeptical about the claim that hurricanes affecting the U.S. had become less numerous. So here are the numbers, straight from the National Hurricane Center, by decade – landfalling U.S. hurricanes:

Decade All hurricanes Strong hurricanes*
1900-1909 16 6
1910-1919 19 8
1920-1929 15 5
1930-1939 17 8
1940-1949 23 8
1950-1959 18 9
1960-1969 15 6
1970-1979 12 4
1980-1989 16 6
1990-1999 14 5
* Saffir-Simpson Categories 3 through 5

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