A new report in the scientific journal Nature indicates that the last decade has seen, on average, more frequent hurricanes than any time in the last 1,000 years. The last period of similar activity occurred during the Medieval Warm Period.
The study is not definitive, but it is a unique piece of work that combines an analysis of sediment cores from inland lakes and tidal marshes with computer modeling and finds a "striking consistency" between the two, the authors suggest.
The use of sediment cores to place and date ancient storms – called "paeleotempestology" – is becoming an increasingly useful tool in the broader effort to try to reconstruct the history of hurricane activity in order to better predict a future potentially influenced by climate change.
"You don't want to go into the business of predicting the future without knowing the history, which does tell us what's possible and tests our understanding," says Richard Alley, a Pennsylvania State climate change researcher. "When people build models to predict hurricanes in the future, one way you know it works is to wait 100 years and say, 'See.' Or you run it against the previous 1,000 years."
The massive hurricanes that have battered the Gulf Coast during the past decade have become a focal point of the climate change debate. Some scientists have suggested that the growing ferocity and frequency of storms is tied to rising ocean temperatures, some of which may be caused by human activity.
"There is some evidence that [climate change] is adding to hurricane strength, but there's no evidence it's tied to increased hurricane frequency," he says. "Questions about whether hurricanes will increase in intensity or frequency are really difficult to answer from a modeling standpoint. [That's why] it is very important to try to have an understanding of how storm frequency and intensity has varied in the past."
So far, written history about storms like the Galveston hurricane of 1900 have provided data about strength and impact on human communities. But further back in time, the written maritime record is spotty, making it unreliable as a gauge of older storms.
The research team has gotten around that by using sediment cores from a lagoon as well as tidal marshes behind barrier beaches. Hurricane storm surges drive sand into the muddy marshes, and over time that becomes a layer of sand sandwiched between mud. Scientists can date when that sandy layer formed. They can also apply the same techniques to some coastal lakes.
Mann and his coauthors from the University of Massachusetts and the Woods Hole Oceanographic Institution studied eight coastal locations where hurricanes make frequent landfall – seven in the US and one in Puerto Rico. From these samples, they extrapolated the frequency of ancient storms. The North Atlantic has produced an average of 17 storms a year during the past decade – twice the number from most years in the last millennium, according to the study.
There are some uncertainties in the analysis, the team acknowledges. But the sediment cores and modeling seem to corroborate each other. Says Alley: "We're eventually going to have good models of our climate system, and when policy makers say, 'Why should we believe that model?' we can say, 'Because we ran it for the last 1,000 years, and it matches what happens.' "