In the Blue Holes of the Bahamas, Secrets of Hurricanes Past

Katrina. Harvey. Maria. Dorian. In recent years, hurricanes have killed thousands of people and caused billions of dollars in damage. But getting a handle on how frequently these destructive storms have pummeled the planet is tough because records stretch back only about a century and a half.

Now, researchers have assembled a 1,500-year history of hurricanes in the Bahamas, based on sand and shell fragments pulled up from submarine caverns known as blue holes. Their results, published in October in Paleoceanography and Paleoclimatology, show that hurricane activity has varied over time. In fact, recent hurricane activity in the Bahamas has been low compared with historical highs, despite intense activity elsewhere in the Atlantic arena. The fluctuations are likely driven by changes in atmospheric and oceanic circulation and volcanism, the scientists suggest.

In the aftermath of Hurricane Dorian and the destruction it caused in parts of the Bahamas, this historical record is “a wake-up call, said Lisa Kennedy, a geographer at Virginia Tech who was not involved in the research. “We are only beginning to understand hurricane patterns and processes in the context of long-term history.”

Blue holes form when carbonate rock erodes, collapses and fills with water; they are revered among divers for their deep, clear waters. They are also important keepers of the scientific record, as hurricanes wash coarse material like sand, gravel, shells and pieces of coral into them.

“I can immediately look at it and say, ‘There’s a hurricane layer,’” said Lizzie Wallace, a paleoclimatologist in the Massachusetts Institute of Technology/Woods Hole Oceanographic Institution Joint Program in Oceanography.

In 2014, Ms. Wallace’s colleagues collected sediment cores from the bottom of blue holes on South Andros Island.

Back at Woods Hole, the team used radiocarbon dating to date the ages of mangrove leaves they found scattered within the cores. By interpolating between these age markers, Ms. Wallace and her collaborators estimated when each layer of hurricane-transported debris was deposited. They focused on 51 layers in their longest core record — nearly 60 feet long — from a blue hole called AM4. The oldest layer was deposited around 500 A.D.

Hurricane activity in the Bahamas has been far from constant, Ms. Wallace and her team showed. For example, storms were frequent from the 7th to the 9th centuries — more than six occurred per century, on average. But most of the 19th century was quiet; the researchers found no hurricane debris dating from roughly 1840 to 1915.

“We found these active and quiet intervals,” Ms. Wallace said.

These cores don’t capture every hurricane, the scientists noted. Rather, they reflect only the most intense storms, those with winds exceeding 111 miles per hour, or stronger than Category 3 on the Saffir-Simpson Scale, that pass within about 30 miles of South Andros Island.

These periods of relative activity and inactivity appear in core records from other places like the Gulf of Mexico, said Jeffrey Donnelly, a paleoclimatologist at Woods Hole Oceanographic Institution and Ms. Wallace’s thesis adviser. “It suggests not randomness but some broader-scale climatic forcing.”

Changes in atmospheric and oceanic circulation and volcanism are possible culprits, the researchers suggest.

The Intertropical Convergence Zone, the region in the tropics where trade winds from the northern and southern hemispheres converge, drives convection that can kick-start a hurricane. If the zone shifts north, more hurricanes will be recorded at northern latitudes.

Volcanic eruptions can also have an effect by pumping out aerosols that cool the planet. Cooler ocean surface temperatures have been linked to reduced hurricane activity.

Only two hurricanes appear in the South Andros Island cores since 1851, the year the U.S. government started keeping records. That average frequency, of just over one hurricane per century, is far lower than in previous periods, said Dr. Donnelly. “Much of the last 1,500 years has been much more active than anything we’ve seen in the last hundred.”

If the past is an indicator of the future, this lull will be temporary, the researchers suggest. And, they add, an uptick in hurricane activity will affect more than just the Bahamas: Many of the storms that strike the Caribbean continue on to hit the Gulf of Mexico.