NBC10.com - Local News - Glenn's Blog: Hurricanes And Global Warming

The first thing I'm going to do is not talk about Katrina in this discussion. Hurricane Katrina is such a highly charged, emotional subject that it is simply unfair to even imply that global warming had anything to do with it. The best-known part of the Katrina disaster was in New Orleans, and that disaster was caused by failed levees, incomplete evacuation, and lack of adequate response at the city, state, and federal level. And now to the subject at hand.
There are two basic schools in this debate: hurricanes are increasing and/or getting stronger because of global warming; or hurricanes are increasing because this is part of a natural cycle. There is no doubt that the number of tropical storms and hurricanes have increased dramatically since 1995. In the past 11 years, we have had 163 named storms in the Atlantic Basin, with 44 of them becoming Category 3 or stronger (major, or intense hurricanes). In the previous 11 years, there were 102 tropical storms and only 15 Category 3+ hurricanes. If we go back over the past century, we find what clearly appear to be cycles of active and inactive hurricane periods in the Atlantic. We know there was a very active period in the 1940s into the early 60s, followed by a very inactive period, followed by the current record-setting period.
Dr. Bill Gray, the pioneering meteorologist from Colorado State, who has been predicting hurricane season activity for decades, is convinced about the cycle. He correlates the cycle to changes in the underwater ocean circulation around the world, called the "thermohaline" or "great ocean conveyor". When it slows, ocean temperatures are lower in the Atlantic, leading to fewer storms; when it speeds up, warm ocean water is carried to higher latitudes and leads to more storms. It's simple. It's logical. And believers in this theory include one of Gray's disciples, Chris Landsea (another famous hurricane researcher), and Max Mayfield, Director of the National Hurricane Center. These experts also believe that there is no significant evidence that tropical storms and hurricanes have shown increases in intensity in recent decades.
Kerry Emanuel from MIT leads the other side of the argument, with Michael Mann of Penn State, and researchers from the GFDL in Princeton, NJ. There have been several studies in the past year showing not only a significant increase in hurricane intensity, but also specifically refuting the argument about the cycles. These new studies have gotten lots of publicity, which has then led to attacks on Dr. Gray, Max Mayfield, and the head of NOAA (Max's boss) for ignoring the evidence of global warming on hurricanes.

I have read summaries of these studies, and reviews of them by respected climate scientists, and am not so sure about my previous beliefs in the hurricane cycle and that global warming was not affecting hurricanes. I am also having a hard time understanding how Gray and Mayfield can be so sure that global warming has NOTHING to do with the recent record-setting hurricane seasons. Gray is on record as saying that he believes the whole belief in global warming is a hoax, so he has no credibility on the issue, as far as I'm concerned (a lot of climate scientists say even worse things about Gray). Max Mayfield is an old friend of mine; someone I worked with at the National Hurricane Center in the 1970s. He has always been a respected scientist, and my personal feelings toward him lead me to not be as critical of him as I would be of a stranger who said the same things. I am not aware of Max's overall beliefs in global warming, or whether he has been under any kind of pressure from above to agree with Gray. If the director of NHC suddenly said that hurricanes are increasing in number and/or intensity because of global warming, the debate would be over.
How many more extremely active hurricane seasons would be needed to convince skeptics that something is happening that is bigger than cycles? The argument will be settled over the next 20 years. If we suddenly go back to a long, inactive period, the cycle theory will be much more powerful. If we keep getting active hurricane seasons, with lots of major hurricanes, global warming will be seen as a clear factor. Of course, people don't want to wait that long. In the meantime, the evidence seems to be pointing toward a global warming connection of some sort, so I would say that it's more likely than not that hurricanes are tending to be more intense (as global models have predicted, by the way). If ocean temperatures continue to rise, the average hurricane will likely become stronger.
The evidence for an increase in the numbers of hurricanes is less compelling. Worldwide numbers have not changed that much. And if global warming causes more El Nino's, for example, that could actually lead to fewer hurricanes, since El Nino suppresses tropical activity in the Atlantic. If wind patterns change, leading to more wind shear, fewer tropical storms would form. I'll stay uncommitted on this one.
Glenn Schwartz
Chief Meteorologist, NBC10
6/6/06
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