Over 100,000 years ago, giant boulders weighing many tons were washed high onto a coastal cliff in the Bahamas. Some studies suggest they were put there by “super storms” — tempests far more powerful than anything seen in recorded human time — generated by a climate warmer than that of today.
A new study asserts that storms indeed placed the boulders there. But, it says, storms of intensities seen today, combined with a few meters increase in sea level during this warm time, would have been sufficient to achieve the feat. The study appears this week in the journal Proceedings of the National Academy of Sciences.
Coastal regions worldwide are threatened by climate change and resulting sea-level rise. Warmer climate conditions may also increase the strength of storms. Researchers are looking to events in the distant past to help project what might happen in the future, and the last warm period, between around 128,000 and 116,000 years ago, is often used as a comparison. This was the last time during which the climate was warmer than in the pre-industrial era, and sea level was higher than it is today.
Some researchers say that the North Atlantic was characterized at that time by “super storms,” stronger than any seen since instruments have been used to record weather starting in the 19th century. On the east coasts of Bermuda and the Bahamas, deposits including giant boulders testify to apparent storm waves that engulfed the land. The lead author of the study, Alessio Rovere of the University of Bremen and an adjunct scientist at Columbia University’s Lamont-Doherty Earth Observatory, said, “We wanted to know how much energy is required for a storm wave to move such huge boulders from the cliff edge to their present positions, and whether modern storms possess the energy to accomplish this feat.”
On the Bahamian island of Eleuthera, seven massive boulders are positioned atop a 15-meter-high cliff. They are so gigantic that it is almost inconceivable that they could have been deposited there by waves; according to several previous studies, they were transported there by super storms.
Two of the larger boulders are known in the region by the names “cow” and “bull.” Rovere and his team measured these two boulders and estimated their volumes and densities. They calculated a weight of 383 tons for the “cow” and 925 tons for the “bull”. The researchers fed the field data into a computer model and adjusted for sea level, which may have been up to 15 meters higher at the time of deposition than today. They simulated waves generated by three modern-era storms that hit Eleuthera especially hard: the “Perfect Storm” of 1991; Hurricane Andrew a year later; and Hurricane Sandy in 2012. The researchers then modeled a scenario in which these storm waves strike the cliffs of Eleuthera. This allowed them to calculate the wave height and length, as well as the current speed, that would have been required to move the rocks.
“Our results indicate that a super storm was not necessary to explain the present positions of the two boulders,” said Rovere. “Even if we assume a sea level only six meters higher than that of today, waves such as those produced by Hurricane Sandy would have been sufficient to transport the boulders “cow” and “bull” to their present positions.” Current projections say that sea level will rise about a meter by the end of this century, but some models put it much higher. In the future, said Rovere, even if the intensity of storms does not increase, sea-level rise alone might be enough to increase the frequency of waves strong enough to move such boulders today.
Other authors of the study include Lamont-Doherty researchers Michael Sandstrom, William D’Andrea and Maureen Raymo, along with scientists at a half-dozen other institutions.
The study was funded with partial support from WSL PURE in partnership with Columbia University’s Center for Climate and Life.