Ancient storms could help predict shifts in tropical cyclone hotspots

To get a better sense of how climate change might alter the patterns of major ocean storms, shifting the parameters of tropical cyclone hotspots, scientists reconstructed 3,000-years of storm history in the Marshall Islands.

The analysis showed that during the Little Ice Age, storms more frequently struck Jaluit Atoll in the southern Marshall Islands.

The findings, published Monday in the journal Nature Geoscience, suggest differences in ocean warming strongly influence Pacific storm patterns.

By analyzing differences in sediment size, researchers were able to pinpoint the timing of extreme weather events. The data showed that prior to the Little Ice Age, storms hit Jaluit Atoll roughly once per century, but between 1350 and 1700 AD, the islands were struck by four cyclones per century -- a significant increase.

By studying the affects of ancient climate change on storms patterns in the Northern Pacific, researchers were able to predict how modern climate change, or global warming, will influence tropical storm patterns in the decades ahead.

"Atmospheric circulation changes due to modern, human-induced climate warming are opposite of the circulation changes due to the Little Ice Age," lead study author James Bramante said in a news release.

"So we can expect to see the opposite effect in the deep tropics -- a decrease in tropical cyclones close to the equator. It could be good news for the southern Marshall Islands, but other areas would be threatened as the average location of cyclone generation shifts north," said Bramante, a researcher at the Woods Hole Oceanographic Institution in Massachusetts.

Bramante were able to reconstruct the region's storm history by studying sediment layers excavated from "blue holes," ancient caves that collapsed and became underwater sinkholes.

When large ocean storms strike, they churn up the water and marine sediments, causing previously stratified sediment layers to mix. By identifying the mix of large and small sediment particles created by Typhoon Ophelia, which devastated the atoll in 1958, researchers were able to locate the sediment signatures of ancient storms.

Radiocarbon dating helped researchers work out the timing of ancient storms in the region, revealing an uptick in cyclone frequency during the Little Ice Age. By analyzing tree rings, coral cores and fossilized marine organisms, scientists were able to reconstruct ancient changes in climate conditions.

When scientists plugged their data into climate models, they determined shifts in the behavior of equatorial trade winds drove significant changes in the frequency at which tropical cyclones formed. The shifts also drove changes in the intensity and paths of North Pacific storms.

Scientists suggest their models can be used to predict how rising ocean and atmospheric temperatures will alter the location of current tropical cyclone hotspots.

"Through the geologic archive, we can get a baseline that tells us how at-risk we really are at any one location," said study co-author Jeff Donnelly.

"It turns out the past provides some useful analogies for the climate change that we're currently undergoing. The earth has already run this experiment. Now we're trying to go back and determine the drivers of tropical cyclones," said Donnelly, a WHOI senior scientist.

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