Extinct species left fossils, ancient humans left cave paintings and tools. What did the storms of centuries past leave behind? Sand.
It may not sound like much, but it's enough for ancient hurricane hunters like Dr. Jeff Donnelly to go on. Donnelly is Associate Scientist with Tenure in the Department of Geology and Geophysics at Woods Hole Oceanographic Insitution, and he’s the lead author on a new study showing that, over the past two thousand years, Cape Cod has – at times – been battered with storms of surprising frequency and intensity.
To retrace the history of hurricanes, Donnelly and his fellow marine geologists turn to the sediment at the bottom of coastal ponds - the quiet, back corners of coastal ponds, to be precise. Most of what ends up there is fine, silty mud. But an intense storm can wash sand from the nearby beach over the dunes and hundreds of meters back into the pond. The result is a layer of relatively course sand that marine geologists can use to estimate the date and the strength of the storm.
Salt Pond, in Falmouth, MA, has yielded a storm record that goes back two millenia, and paints a picture of a New England very different from anything seen since the region was colonized by Europeans. Between 250 and 1150 AD, storms estimated to be solidly Category 3 hurricanes hit the area, on average every forty years. A few hundred years later, between 1400 and 1675, the storms were even more frequent and intense.
Then, things went relatively quiet. The only modern storm to even show up in the record from Salt Pond is Hurricane Bob, which was a Category 2 hurricane when it hit New England in 1991. (Other historic storms, like the hurricane of 1938, likely hit too far to the west to show up in Falmouth.)
The active periods are, in many ways, quite different from each other. In the earlier, storms originated in the tropics and worked their way up the coast. In the latter, the tropics were relatively quiet, but the Bahamas, the Carolinas, and the northeast were active. The common thread is warm ocean waters. In particular, the first millenium was a period of deep tropical warming similar to what is happening now as a result of human-caused climate change.
Donnelly says this study yields two lessons. First and foremost, we've developed our coastlines without any appreciation for our true vulnerability to hurricanes. Secondly, with ocean temperatures now even higher than they were in the first millenium, we could be in for some truly wild weather in decades to come.