Traces of ancient hurricanes on the seafloor are a warning for coastal areas

The year 2020 had the most tropical cyclones in the Atlantic, with 31, and 2021 had the third-highest, after 2005. The past decade saw five of the six most destructive Atlantic hurricanes in modern history.

Then a year like 2022 comes along, with no major hurricane landfalls until Fiona and Ian struck in late September. The Atlantic hurricane season, which ends November 30, has had eight hurricanes and 14 named storms. It’s a reminder that small sample sizes can be misleading when assessing trends in hurricane behavior. There is so much natural variability in hurricane behavior year to year and even decade to decade that we need to look much further back in time for the real trends to come clear.

Fortunately, hurricanes leave behind telltale evidence that goes back millennia.

Two thousand years of this evidence indicates that the Atlantic has experienced even stormier periods in the past than we’ve seen in recent years. That’s not good news. It tells coastal oceanographers like me that we may be significantly underestimating the threat hurricanes pose to Caribbean islands and the North American coast in the future.

The natural records hurricanes leave behind

When a hurricane nears land, its winds whip up powerful waves and currents that can sweep coarse sands and gravel into marshes and deep coastal ponds, sinkholes, and lagoons.

Under normal conditions, fine sand and organic matter like leaves and seeds fall into these areas and settle to the bottom. So when coarse sand and gravel wash in, a distinct layer is left behind.

Imagine cutting through a layer cake—you can see each layer of frosting. Scientists can see the same effect by plunging a long tube into the bottom of these coastal marshes and ponds and pulling up several meters of sediment in what’s known as a sediment core. By studying the layers in sediment, we can see when coarse sand appeared, suggesting an extreme coastal flood from a hurricane.

With these sediment cores, we have been able to document evidence of Atlantic hurricane activity over thousands of years.

We now have dozens of chronologies of hurricane activity at different locations—including New England, the Florida Gulf Coast, the Florida Keys, and Belize—that reveal decade- to century-scale patterns in hurricane frequency.

Others, including from Atlantic Canada, North Carolina, northwestern Florida, Mississippi, and Puerto Rico, are lower-resolution, meaning it is nearly impossible to discern individual hurricane layers deposited within decades of one another. But they can be highly informative for determining the timing of the most intense hurricanes, which can have significant impacts on coastal ecosystems.

It’s the records from the Bahamas, however, with nearly annual resolution, that are crucial for seeing the long-term picture for the Atlantic Basin.

The Bahamas are exceptionally vulnerable to the impacts of major hurricanes because of their geographic location.

In the North Atlantic, 85 percent of all major hurricanes form in what is known as the Main Development Region, off western Africa. Looking just at observed hurricane tracks from the past 170 years, my analysis shows that about 86 percent of major hurricanes that affect the Bahamas also form in that region, suggesting the frequency variability in the Bahamas may be representative of the basin.

A substantial percentage of North Atlantic storms also pass over or near these islands, so these records appear to reflect changes in overall North Atlantic hurricane frequency through time.

By coupling coastal sediment records from the Bahamas with records from sites farther north, we can explore how changes in ocean surface temperatures, ocean currents, global-scale wind patterns, and atmospheric pressure gradients affect regional hurricane frequency.

As sea surface temperatures rise, warmer water provides more energy that can fuel more powerful and destructive hurricanes. However, the frequency of hurricanes—how often they form—isn’t necessarily affected in the same way.

Some of the best locations for studying past hurricane activity are large, near-shore sinkholes known as blue holes.

Blue holes get their name from their deep blue color. They formed when carbonate rock dissolved to form underwater caves. Eventually, the ceilings collapsed, leaving behind sinkholes. The Bahamas has thousands of blue holes, some as wide as a third of a mile and as deep as a 60-story building.

They tend to have deep vertical walls that can trap sediments—including sand transported by strong hurricanes. Fortuitously, deep blue holes often have little oxygen at the bottom, which slows decay, helping to preserve organic matter in the sediment through time.