Storm Warning

The hurricane forecast is becoming clearer, and the news is not good

Out of a fog of meteorological data, an alarming picture of intensifying tropical cyclones and hurricanes is emerging. It can be summed up in two words: more Katrinas.

The likely effects of climate change on the number and intensity of tropical storms has been debated for years. Some argued that warmer sea surface temperatures would mean more and stronger storms; after all, the moisture from evaporating ocean water is the feedstock of cyclones, including hurricanes. Others predicted that climate change would alter heat distribution in the oceans to damp down storms.

Until recently, no one could say for sure which of these predictions would be correct. The haphazard business of spotting and measuring cyclones left trend analysis bedevilled by uncertainty over the quality of data. As late as last year, the consensus among meteorologists remained that no discernible trend could be distinguished from natural variations.

Now that has changed.

Three reports have found a clear signal amid the statistical noise. In June, Kevin Trenberth at the US National Center for Atmospheric Research in Boulder, Colorado, uncovered a rising intensity of hurricanes in the North Atlantic (Science, vol 308, p 1753). Last month, Kerry Emanuel at the Massachusetts Institute of Technology found a 50 per cent increase in the destructive power of cyclones in the past half-century (Nature, vol 436, p 686).

The good news is that there is no rising trend in the overall number of hurricanes, nor any sign that the worst storms are growing fiercer. But there is bad news: there has been a near doubling in the number of the strongest categories of hurricanes –- the category 4 and category 5 storms exemplified by Katrina. Equally dramatic is the discovery that the trend towards stronger cyclones occurs in every ocean, has been continuous for more than three decades, and closely tracks the rise in sea surface temperatures right across the tropics.

It is this near-uniform global picture that warns us the trend is genuine, rather than the result of natural variability. Local and regional conditions fluctuate all the time, but rarely does the whole of nature move swiftly in one direction unless there is some external cause. As the report's co-author, Judy Curry of the Georgia Institute of Technology in Atlanta, puts it: "We can say with confidence that the trends in sea surface temperatures and hurricane intensity are connected to climate change."

TWhat next? We all saw the economic, social and political havoc wrought by Katrina, and the new analysis suggests there is more to come. The consequences of other storms of similar force striking this year or next would be profound. Perhaps climate change, and the need to stop it, would at last be promoted from political nuisance to political necessity.

Source: NewScientist 24 September 2005