Hurricane peak not unique

Historical estimates suggest that global warming could boost the number of hurricanes.

By Natasha Gilbert

A surge in the number of Atlantic hurricanes over the past 10 years is not unusual and could be part of a naturally occurring millennial peak, say US researchers1. The finding suggests that any increase in hurricane activity due to global warming would add to the current peak.

The study, published in Nature today, found a peak in hurricane activity across the tropical Atlantic around 1000 AD, that approached levels seen today. In 2005, there were a record 15 Atlantic hurricanes, including hurricane Katrina, which devastated the city of New Orleans in Louisiana. These peaks contrast with lulls in hurricane frequency the study identified before and after the 1000 AD peak, when 8 or 9 hurricanes occurred each year.

Michael Mann, a climate scientist at Pennsylvania State University in University Park and the study's lead author, says that the results suggest that the annual number of hurricanes will continue to increase as a result of global warming.

Previous research has shown that warm sea surface temperatures could encourage hurricanes to form. The historical peak in hurricane activity coincided with periods of high sea surface temperatures, says Mann.

"This tells us that the relationship between sea surface temperatures and cyclone activity seems to be robust and gives support to the debate that we are likely to see an increase in tropical cyclone activity in response to global warming," he says.

Muddy waters

The team compiled existing and fresh data on the number of hurricanes to hit land across the Atlantic basin over the past 1500 years by looking at sediment deposits at eight sites, including the Caribbean, the US Gulf coast and the mid-Atlantic coast. When hurricanes hit land they often dump ocean sediment into lagoons and embayments. Using radiocarbon dating, scientists can date these sediment layers to give an estimate of when and how many hurricanes occurred.

In a separate analysis, the researchers used a statistical model based on modern data from 1851 to 2006 to estimate historical hurricane activity. The researchers modelled the effect of three climate conditions on the yearly formation of hurricanes: sea surface temperatures; the El Niño/Southern Oscillation, a warming of surface waters in the tropical eastern Pacific Ocean; and the North Atlantic Oscillation — a measure of variations in the jet stream across the North Atlantic.

Both analyses show a peak in hurricane activity around 1000 AD, where conditions produced "a perfect storm" of influences on hurricane activity, says Mann.

Mann says that if sea surface temperatures continue to rise as a result of global warming, the world can expect to see more hurricanes. But this increase could be tempered if climate change doesn't increase the El Niño effect, something that's debated in current climate-change models. Debate also rages in the hurricane-research community over whether the increase in hurricanes seen over the past decade is accurate or just due to improved techniques in counting the storms.

Chris Landsea, a hurricane researcher at the National Hurricane Centre in Miami, Florida, says that other research has shown that 3 or 4 hurricanes were missed in annual counts from the late nineteenth century. This would "nullify" the peak in activity seen over the past ten years, he says.

But Mann says that the statistical model used in his study takes into account the possibility that historical hurricane counts could have been inaccurate, yet the results still show a peak in activity over the past decade.

Urs Neu, a hurricane researcher at the Forum for Climate and Global Change at the Swiss Academy of Sciences in Bern, says that Mann's paper is "an important step forwards" and provides "the most comprehensive information" possible from existing records.

Neu says that more sediment sites in the tropical Atlantic basin need to be tested for evidence of hurricane activity to improve the historical estimates. He adds that other climate factors thought to influence hurricane activity, such as the West African monsoon, need to be considered.

Mann says he is "sceptical" that the West African monsoon is affecting hurricane activity in a way that the study has not already captured. "But this is an open area of research and I would not say that our statistical model is complete," says Mann.