By Chris Mooney
Inventive new research has found a surprising way of investigating the relationship between hurricanes and climate change – by examining the history of Spanish shipwrecks in the Caribbean Sea during a planetary cool period in the late 17th and 18th century.
The result, based on comparisons between tree rings from the Florida Keys and a historical record of shipwrecks finds that there were far fewer hurricanes, or tropical cyclones, from 1645 through 1715, when the planet went through what is called the “Maunder Minimum.” This was an era in which very low sunspot activity correlated with relatively cooler temperatures here on Earth (the Maunder Minimum was part of a cooler period known to climate history as the “Little Ice Age”).
“We see a severe reduction in the hurricane activity that overlaps perfectly with the Maunder Minimum,” said Valerie Trouet, a researcher with the Laboratory of Tree-Ring Research at the University of Arizona, who conducted the work with colleagues from the University of Southern Mississippi and the University of Santiago de Compostela in Spain.
The result does not lead to any forecast when it comes to the hurricanes of the future, but at the same time, it’s certainly suggestive. After all, hurricanes derive their energy from the heat stored in tropical oceans. If seas are cooler – as they were between 1645 and 1715, when the Earth received less radiation from the sun – then there’s less explosive energy for storms to draw upon. If they’re warmer, as they are today, then all else being equal, there’s more opportunity for extreme storm intensification.
To study shipwrecks, the new research drew upon historical records from no less than 657 wrecks of Spanish ships in the Caribbean between 1495 and 1825 – a period that started just after Columbus’ first expedition to the new world. All of the storms that were inferred to have wrecked these ships occurred prior to the earliest year in today’s official Atlantic hurricane database, which is kept by the National Oceanic and Atmospheric Administration and begins in 1851.
The study weeded out historical shipwrecks attributed to other causes, like fire or human conflicts, but kept wrecks whose cause was not known. “It is known that storms were the main cause of the wrecking of ships, at least in the earlier part of the record,” says Trouet.
But to strengthen the analysis, the researchers also cross-referenced the shipwreck history with data from the rings of pine trees growing in the Florida Keys, which can also provide a record of hurricane history.
In general, strong winds and surging seas, characteristic of hurricanes, stun the growth of these pine trees, and this in turn shows up in the growth rings inside their trunks. And the research found that in the tree ring record, and the record of shipwrecks in the area, showed a “high synchronicity between years of damage at sea (shipwrecks and on land (pines).”
Therefore, overall, the researchers conclude that developments like cooler ocean temperatures, provided a “drastically unfavourable environment” for hurricanes in the Caribbean. Storm activity was reduced by 75 percent during the period, the study suggested.
This wasn’t only because of cooler sea temperatures, the study notes – in general, during the Maunder Minimum, there was also a prevalence of El Niño type conditions, which are known to suppress hurricane activity in the Atlantic basin in particular (thought they can enhance it elsewhere). That, too, might have contributed to the lows in shipwrecks and, by inference, hurricanes during the Maunder Minimum.
The study is historical in nature, and so does not provide any forecast of future hurricane activity, or any conclusion about how hurricanes may change going forward. However, as Trouet notes, “we know that there’s a very strong link between sea surface temperatures and tropical cyclone activity.” The new research clearly reinforces this – and with it, one of the key theoretical underpinnings behind the idea that storms should worsen, on average, in a warmer climate.
In fairness, many other factors affect hurricanes, ranging from temperatures at the top of the atmosphere to outbursts of dust from the coast of Africa. So it’s a complicated picture. But nonetheless, learning that hurricanes seem to have gone relatively quiet at a time of low solar activity and planetary cooling certainly adds to concern that in the era we’re heading into now, they could do the opposite.