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Shipwrecks shed light on bygone climate

March 12, 2016, by Tim Radford

Evidence from vessels lost at sea increases knowledge of hurricanes.
Image: Ivan Alvazovsky via Wikimedia Commons

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Evidence from shipwrecks centuries ago helps scientists learn more about the pattern of Caribbean hurricanes at the time.

LONDON, 12 March, 2016 Researchers have established a new chronology of Caribbean hurricanes – with help from tree rings and silent testimony from a momentous period in world history: the wrecks of 657 Spanish ships from 1495 to 1825.

Formal records of the savage hurricanes that sweep into the US coasts date only from 1850, so to look further back researchers rely on proxy evidence: signs in the natural world that indicate devastating windstorms and torrential rain.

Tree rings carry a precise record of annual growth that delivers indirect evidence of seasonal variation, but Valerie Trouet, a dendrochronologist at the University of Arizona, and colleagues reckoned they could back up their findings with chronicles from the Spanish Main.

This included a tally of the losses of caravels, galleons, barques, Indiamen, privateers and men-of-war that dates from the second voyage of Christopher Columbus to the year Simon Bolivar became the Liberator of Peru.

The researchers report in the Proceedings of the National Academy of Sciences that although piracy, buccaneering and open warfare mark the history of the Caribbean and the Gulf of Mexico, most shipwrecks could be blamed on tropical cyclones or hurricanes. Spanish power in Cuba was secured in 1640 after a hurricane devastated a Dutch fleet poised to attack Havana.

Thorough record

Since the conquest and colonisation of the region supplied Spain with silver, gold and slaves, the records of successful and lost voyages were carefully kept.

Tree rings from the Florida Keys told one part of the story. Limited summer growth could be interpreted as evidence that, although huge levels of rain had fallen, trees could take less advantage because their leaves had been stripped by the winds.

But the pattern of wrecks and losses at sea over time also provided evidence of the natural pattern of hurricane activity during the centuries when atmospheric carbon dioxide levels were lower.

Hurricanes become more probable as sea surface temperatures rise, and these have been rising steadily as humans have stepped up greenhouse gas emissions with increasing and increasingly prodigal use of coal, oil and gas.

But the challenge for climate scientists – and insurance giants is to work out what global warming, driven by rising carbon dioxide ratios in the atmosphere, will mean for tropical windstorms in the future.

“We didn’t go looking for the Maunder Minimum. It just popped out of the data”

One set of projections has forecast that hurricanes will be fewer but fiercerwhile another has suggested that climate change means catastrophic winds and storm surges such as Hurricane Katrina, which hit New Orleans in 2005, could happen every other year

It would help to know the pattern of hurricane happenings that predate the Industrial Revolution. And the latest study provides a guide.

“We’re the first to use shipwrecks to study hurricanes in the past,” Dr Trouet said. “By combining shipwreck data and tree-ring data, we are extending the Caribbean hurricane record back in time, and that improves our understanding of hurricane variability.”

Dr Trouet recently used her tree-ring laboratory to answer questions about the pattern of drought in California. Her latest study has confirmed a close match between tree growth records and marine losses in the last 500 years.

It also identified a dramatic reduction in hurricane activity between 1645 and 1715, during a “Little Ice Age” when sunspot activity was at its lowest, and Europe experienced a series of more-than-usually cold winters.

This period is known by climate scientists as the Maunder Minimum. “We didn’t go looking for the Maunder Minimum,” Dr Trouet said. “It just popped out of the data.” – Climate News Network

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