Harbinger.........of climate change?

IRENE'S IMPACT

The occurrence of Hurricane Irene is reviving an old question: are hurricanes getting worse because of human-induced climate change?

The short answer from scientists is that they are still trying to figure it out. But many of them do believe that hurricanes will get more intense as the planet warms, and they see large hurricanes like Irene as a harbinger.

While the number of the most intense storms has clearly been rising since the 1970s, researchers have come to differing conclusions about whether that increase can be attributed to human activities.

“On a longer time scale, I think – but not all of my colleagues agree – that the evidence for a connection between Atlantic hurricanes and global climate change is fairly compelling,” said Kerry Emanuel, an expert on the issue at the Massachusetts Institute of Technology.

Among those who disagree is Thomas R Knutson, a federal researcher at the US government’s Geophysical Fluid Dynamics Laboratory in Princeton, NJ. The rising trend of recent decades occurred over too short a period to be sure it was not a consequence of natural variability, he said, and statistics from earlier years are not reliable enough to draw firm conclusions about any long-term trend in hurricane intensities.

“Everyone sort of agrees on this short-term trend, but then the agreement starts to break down when you go back longer-term,” Knutson said.  He argues, essentially, that Emanuel’s conclusion is premature, even while conceding that evidence for a human impact on hurricanes could eventually be established.

While scientists from both camps tend to think hurricanes are likely to intensify, they do not have great confidence in their ability to project the magnitude of that increase. One climate-change projection, prepared by Knutson’s group, is that the annual number of the most intense storms will double over the course of the 21st century. But what proportion of those would actually hit land is another murky issue. Scientists say climate change could alter steering currents or other traits of the atmosphere that influence hurricane behaviour.

Storms are one of nature’s ways of moving heat around, and high temperatures at the ocean surface tend to feed hurricanes and make them stronger. That appears to be a prime factor in explaining the power of Hurricane Irene, since ocean temperatures in the Atlantic are well above their long-term average for this time of year.

The ocean has been getting warmer for decades, and most climate scientists say it is because greenhouse gases are trapping extra heat from the sun. Rising sea-surface temperatures are factored into both Knutson’s and Emanuel’s analyses, but they disagree on the effect that warming in remote areas of the tropics will have on Atlantic hurricanes.

Rising air temperatures
Air temperatures are also rising because of greenhouse gases, scientists say. That causes land ice to melt, one of several factors leading to a rise in sea level. That increase, in turn, is making coastlines more vulnerable to damage from the storm surges that can accompany powerful hurricanes like Irene. Overall damage from hurricanes has skyrocketed in recent decades, but most experts agree that is mainly because of excessive development along vulnerable coastlines.

In a statement they issued five years ago, Emanuel, Knutson and eight colleagues called this “the main hurricane problem facing the United States,” and they pleaded for a reassessment of state and federal policies that subsidise coastal development – a reassessment that has not happened in the intervening years.

“We are optimistic that continued research will eventually resolve much of the current controversy over the effect of climate change on hurricanes,” they wrote at the time. “But the more urgent problem of our lemming-like march to the sea requires immediate and sustained attention.”