Past Atlantic hurricanes linked to climate change

In the today's issue of the journal Nature, climate researchers including Jonathan Woodruff of the University of Massachusetts Amherst show that the frequency of intense hurricanes in the Atlantic Ocean over the last 1,500 years has been closely linked to long-term changes in the El Nino/Southern Oscillation (ENSO) and sea surface temperature. The finding could help with hurricane modelling and prediction in the future.

Establishing the link between hurricane variability and climate change over these longer timescales 'is a new viewpoint for us,' Woodruff explains. 'There's a randomness to hurricanes. But the fact that we can see trends that rise above that randomness is significant and a bit of a surprise. Our work indicates that hurricane activity has responded noticeably to past climate shifts. When considering future climate change over the next century, our results indicate that measurable changes in hurricane activity could occur, rising above the noise in the system.'

A relationship between ENSO, sea surface temperature and hurricane activity is seen in modern times, Woodruff says, but the historical record based on ships' logs and other observations is not long enough to assess variability on timescales longer than a few decades at best. 'Given the possible effects of continued climate warming on intense tropical cyclone activity, it's essential that we develop an understanding of how past climate change has affected tropical cyclone frequency, intensity and track on longer timescales,' the geologist says. 'This work is another step forward in understanding the complex relationship between climate variability and Atlantic hurricane activity.'

Woodruff and colleagues' study shows that a statistical climate model and actual palaeoclimate data cross-validate each other over the last 1,500 years during key intervals of climatic change.

Specifically, UMass Amherst's Woodruff and colleagues at Penn State and Woods Hole Oceanographic Institute prepared sedimentary reconstructions of hurricane-induced flooding, preserved in coastal ponds and salt marshes and collected as core samples from eight representative sites throughout the western North Atlantic, an approach known as palaeotempestology.

These environments are usually protected from the sea by barrier beach systems. They enjoy sustained quiet periods during which only fine-grained mud and organic materials build up on pond floors and marsh surfaces. But during hurricanes and other storms, these normally calm environments are overrun with ocean waves and storm surges that carry in coarser sand from the barrier beaches. The sedimentary record is thus one of fine-grained organic mud, interbedded with coarse-grained, storm-induced deposits. Such deposits serve as natural archives of past hurricanes, with storm reconstructions that can extend back for many thousands of years, Woodruff points out.

Although still limited to a few reconstructions at present, he and colleagues have now observed statistically significant trends in tropical cyclone activity emerging from palaeo-hurricane records. They compared these trends to data from a statistical model which independently predicted hurricane variability using paleo-reconstructions of climate factors known to influence hurricane activity, such as sea surface temperature, ENSO and the North Atlantic Oscillation.

The model predicted similar trends to those observed in the paleo-storm reconstructions, with an observed decrease in hurricane activity during the 'Little Ice Age' around 300 years ago, a time when sea surface temperatures were lower than today and El Nino events appear to have occurred more frequently. Likewise, a period of increased hurricane activity similar to present levels also occurred around 1,000 years ago during an interval known as the 'Medieval Climate Anomaly,' driven predominantly by increases in both sea surface temperature and the frequency of La Nina events.

Woodruff says the new finding 'is like so much in science―in hindsight it makes sense. When the evidence is supplied, it's simple enough to see the relationships, as in this case with the two independent records telling the same story. But until we had this evidence, things were much less clear.'