TCS Daily

'Cane Mutiny

By Roy Spencer - August 18, 2006 12:00 AM

What a difference one year makes. With the anniversary of Hurricane Katrina's landfall (August 29, 2005) rapidly approaching, who would have predicted that we would now be in the middle of a near-normal Atlantic hurricane season? Weren't the global warming pundits' predictions for this hurricane season that it would be just as bad -- maybe even worse! -- than last year?

Yet, now at mid-August, we have had only three named tropical storms, compared to nine by this date last year. Normally, we would have had one hurricane by now, and we have not had any so far, so by that measure we are actually below normal.

Hurricanes require warm sea surface temperatures (SSTs), and last year the tropical Atlantic sea surface temperatures were running well above normal. Global warming was the explanation given by most 'experts' the media interviewed. And since global warming will only get worse, those SSTs were expected to just keep on increasing.

But now those same regions that had anomalously warm SSTs last year are -- gasp! -- near normal. The accompanying graphic shows large areas in the tropical Atlantic even a little cooler than normal.

This is not the only surprisingly cool SST story. A new scientific article now accepted for publication in Geophysical Research Letters shows that the globally averaged upper ocean cooled dramatically between 2003 and 2005, effectively erasing 20% of the warming that occurred over the previous 48 years!

The rapidity of this observed temperature change is beyond what computerized climate models can explain. This is perplexing for modelers, who tend to believe that their models contain all of the important physics of the problem.

But it is not so surprising for those of us who believe that there are stabilizing feedbacks in the climate system that keep the Earth's temperature from varying too much. If there weren't such stabilizing mechanisms, the climate system would have spun out of control long ago. Up until relatively recently, climate models would 'drift' warmer or cooler over multidecadal runs of the models, indicating that it is a much higher level of complexity that must be understood in order to accurately model the inherent stability of the climate system.

Take clouds and water vapor as examples. Both are known to have dominant roles in determining average global temperatures. Yet, how clouds will respond to the warming tendency from the slowly increasing carbon dioxide content of the atmosphere remains unclear.

Similarly, how precipitation systems will respond to warming is also uncertain. Precipitation is what keeps the atmosphere from filling up with water vapor from surface evaporation, and that water vapor controls at least 90% of the Earth's greenhouse effect. It has been shown theoretically that an increase in 'precipitation efficiency' (how efficiently precipitation removes water vapor from the atmosphere) causes a cooler and drier climate.

Hurricanes are believed to be the most efficient systems for removing atmospheric water vapor. They also remove large amounts of excess heat from the ocean. Even if some carbon dioxide-induced warming leads to a small increase in hurricane intensity (a plausible possibility) they are an important and natural part of the climate system. Coastal residents must be prepared for major hurricanes, with or without global warming. Katrina was only a strong Category 3 hurricane at warming certainly isn't needed to explain that storm.

Forecasting the effects of climate change is a risky business. Predictions of another near-record hurricane season in 2006 might have seemed like a pretty good bet to those who have great faith in climate models and their understanding of global warming. But Mother Nature always seems to find a way to demonstrate to mankind that we don't understand as much as we think we do, and that the effect of mankind on the climate system might well be buried in the 'noise' of natural variability.

While it is still quite possible for this hurricane season to end up being above normal in activity, the unexpectedly cooler SSTs should humble long-range forecasters at least a little.

Rather than focusing on how manmade global warming might affect hurricanes, we need to address the real problem: how our coastal areas that grew so rapidly in recent decades with an influx of people and infrastructure are increasingly vulnerable to the inevitable return of major land falling hurricanes -- with or without global warming.

Dr. Roy Spencer is a principal research scientist for the University of Alabama in Huntsville and the U.S. Science Team Leader for the Advanced Microwave Scanning Radiometer (AMSR-E) on NASA's Aqua satellite.

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