Monday, September 11, 2006
PNAS Abstract : Previous research has identified links between changes in sea surface temperature (SST) and hurricane intensity. We use climate models to study the possible causes of SST changes in Atlantic and Pacific tropical cyclogenesis regions. The observed SST increases in these regions range from 0.32°C to 0.67°C over the 20th century. The 22 climate models examined here suggest that century-timescale SST changes of this magnitude cannot be explained solely by unforced variability of the climate system. We employ model simulations of natural internal variability to make probabilistic estimates of the contribution of external forcing to observed SST changes. For the period 1906-2005, we find an 84% chance that external forcing explains at least 67% of observed SST increases in the two tropical cyclogenesis regions. Model "20th-century" simulations, with external forcing by combined anthropogenic and natural factors, are generally capable of replicating observed SST increases. In experiments in which forcing factors are varied individually rather than jointly, human-caused changes in greenhouse gases are the main driver of the 20th-century SST increases in both tropical cyclogenesis regions.
Since last year's devastating hurricane season, few issues have been more contentious than whether human-driven global warming is responsible for the increased intensity and frequency of these storms. Research reported online this week in the Proceedings of the National Academy of Sciences points the finger pretty directly, showing that human activities have warmed the oceans and thus helped breed stronger hurricanes.
Hurricanes are born in the warm waters of the tropical Atlantic and Pacific oceans, which are both getting warmer. Over the 20th century, ocean surface temperatures increased by between 0.32 degrees Celsius in the Pacific tropical region and 0.67 degrees C in the Atlantic tropical region. This has correlated with a twofold increase in category-4 and -5 hurricanes over the last 30 years (ScienceNOW, 17 August). Some researchers maintain that these changes in sea surface temperature (SST) are within the natural variability of climate. Others say that the human-caused climate change is the culprit.
To figure out just how much people are to blame, atmospheric scientist Ben Santer of Lawrence Livermore National Laboratory in California and colleagues compared observed SSTs with the predictions of 22 global climate models. They ran the different models under various physical scenarios, including changes in solar irradiance, volcanic eruptions, and increased sulfate aerosols and greenhouse gas emissions. Only model simulations that included the known human-caused increases in greenhouse gases replicated the observed rise in SST. In total, the team found an 84% probability that two-thirds of the observed temperature changes were caused by human activities. "There is no way of explaining the observed increases without positing a large human impact on these ocean temperatures," Santer says.
Kerry Emanuel, an atmospheric scientist at the Massachusetts Institute of Technology in Cambridge, agrees that natural variability isn't the culprit. And because warmer seas provide more fuel to the engine of tropical storms, Emanuel says, "it would suggest that there are going to be stronger hurricanes."
But don't expect an enormously bigger wallop. Chris Landsea, a research meteorologist with the National Hurricane Center in Miami, Florida, expects hurricanes to increase their strength only about 1% over the next 100 years. Most researchers agree that the biggest issue with hurricane damage now is not necessarily the strength of the storms but the intense urban development that has emerged along U.S. coasts.