At first sight, the soil samples collected by geophysicist Jeffrey Donnelly at the Playa Grande lagoon in Puerto Rico don't look like much to get excited about. The cores sitting in his laboratory in Woods Hole, Massachusetts seem to be little more than a bit of moist, black clay. But look closely, and you can see the gray, porous layers of tiny grains; sand that swept into the lagoon from a nearby spit of land by powerful hurricanes. By examing the layers of sand in each of the 12 four-meter-long cores (13 foot) samples, they become an extensive climate timeline. Together, they represent the longest chronology of hurricane activity that currently exists anywhere in the world.
The samples have allowed hurricane historian Donnelly from the Woods Hole Oceanographic Institution (WHOI) to look more than 5,000 years into our planet's past. And what he found may have profound implications for our understanding of the effects of global warming on violent storms. The frequency of fierce storms that sweep into the Caribbean and onto the Puerto Rican island of Vieques varies considerably. There are stormy periods and more placid epochs -- and they alternate back and forth.
Donnelly and his colleague Johnathan Woodruff listed their results in a recent issue of the scientific journal Nature. Hurricanes, they wrote, regularly struck the lagoon between 5,450 and 3,650 years ago. This period of intense hurricane activity was interrupted only briefly by a 150 year respite. After that period, there were only few hurricanes - until about 2,550 years ago, when an interval characterized by a relatively high number of strong hurricanes began, continuing until the next quite phase, which began about 1,050 years ago. But during the last 300 years, the lagoon has once more been exposed to a higher number of violent hurricanes - just as the unpleasant storms have been multiplying elsewhere as well.
The increase in super-strong hurricanes of category four and five -- as strong as storms get -- has been difficult to ignore in recent years. They have left a trail of death, destruction and billions of dollars in damage in their wake. Hurricane Katrina's 2005 destruction of New Orleans was perhaps the most devastating, but storms like Wilma (2005) and Ivan (2004) aren't far behind.
The seeming up-tick in violent hurricanes, one has heard over and over again in recent years, can be blamed on global warming. According to this hypothesis, climate change increases ocean surface temperatures, which fuels the transformation of weak winds to mighty hurricanes. Researchers reported last summer that humans -- by virtue of carbon emissions leading to global warming -- share at least some of the responsibility for the record hurricane year of 2005.
The surface temperature of the oceans really is increasing, Donnelly told SPIEGEL ONLINE. "But that only favors the development of hurricanes," he says. It isn't, in other words, the whole story behind the many hurricanes in recent years.
"We happen to be living in an active hurricane phase," the geologist says dryly as the most recent tropical storms along the east coast of the United States replay on his desk-sized computer screen. Hurricanes are natural, he says.
Scientists have only been keeping an eye on the surface temperatures of the oceans and on hurricanes since the 1960s, Donnelly points out. "That's too brief a period and too unreliable to really reveal trends in the activity of strong tropical storms." The analysis of soil samples he describes in his Nature article may turn out to be more reliable.
Split Apart instead of Joining Forces
Donnelly also believes he knows the reason why the intensity of hurricanes varied so greatly during the past 5,000 years. Changes in the West African monsoon -- a process of air circulation that occurs over a vast area -- and in El Nino are the reason, he says. This climate phenomenon results from a temperature increase in the waters of the Pacific Ocean every three to eight years. The resulting warm currents play havoc with the winds above, resulting in drought and severe storms across the globe.
In periods during which El Nino occurs more frequently, the number of hurricanes drops dramatically, Donnelly explains. The reason? The climate phenomenon causes the masses of air above the ocean to sheer off in different directions. They split apart instead of joining forces for a storm -- the dreaded funnel collapses. The data from Puerto Rico also shows something else, according to Donnelly: "When El Nino declines, the hurricanes come back." That seems to be what is happening now.
The US, meteorologists have recently warned, is facing an active hurricane season this year. The National Oceanic and Atmospheric Administration (NOAA) has since qualified this warning, but the US weather experts are still assuming a 75 percent likelihood of an above-average hurricane season. El Nino developed only weak to medium intensity in the tropical Pacific last December and January, so that hurricane activity in the northern Atlantic is currently increasing, according to meteorologists. More powerful storms, more hurricanes and more landfalls are being predicted. In fact, the first tropical storm of the year developed unusually early.
Global warming seems to have an effect on El Nino too. But the question is: What kind of effect?
"We don't know whether climate change will lead to more of El Nino or to more hurricanes after all," Donnelly says. Properly predicting changes in hurricane intensity, the scientist adds, requires a closer examination and a better understanding of El Nino, as well as of the effects of global warming on the West African monsoon.