Shipwreck records and tree rings unveil Caribbean hurricane history – and clues to the future

Valerie Trouet, University of Arizona; Grant Harley, The University of Southern Mississippi, and Marta Domínguez Delmás, Universidad de Santiago de Compostela

The U.S. is currently in a decade-long hurricane “drought”: no major hurricanes of category 3 or higher have made landfall in the U.S. since Hurricane Wilma hit Florida in 2005. With damage costs for U.S. hurricanes from 1970 to 2002 estimated at US$57 billion (in 2015 dollars), this landfall drought is fortuitous for coastal communities and insurance companies alike.

The 10-year hurricane pause is the longest since 1851, the period for which reliable hurricane records are available through the National Hurricane Center’s Hurricane Database (HURDAT). The current lull emphasizes how fickle hurricane systems can be, which complicates accurate hurricane predictions under human-caused climate change.

Is there a way to use the record of past hurricanes to better predict the future?

Climate models for the future generally agree that globally, tropical cyclones will become more intense but less frequent in the 21st century. Projections on the regional level, however, vary greatly and have large uncertainties.

For the North Atlantic Basin, the main uncertainties are due to the chaotic nature of the climate system and to our limited understanding of how tropical cyclones – known as hurricanes in the U.S. – respond to changes in how much energy is in the atmosphere. Such changes in the so-called energy budget – the amount of energy that enters the atmosphere and how much is lost to space – can be caused by greenhouse gas emissions, but also by changes in the strength of the sun’s radiation.

In a recently published study, we combined documentary shipwreck data and tree-ring data to extend the tropical cyclone record for the Caribbean back over the last 500 years. These two new proxies and the extended Caribbean tropical cyclone record allow us to look at past big swings in the atmosphere’s energy budget and how tropical cyclones have responded to them.

By providing insight into hurricane dynamics of the past, we aim to give climate scientists better tools for predicting how climate change will affect hurricane intensity and frequency with more regional accuracy.

Poring over shipwreck records and tree rings

All three of us are tree-ring scientists. We use the rings in trees and wood to study the climate, the forests and human societies of the past. All three of us remember exactly the moment and the place when the idea for this research project emerged: it was on the last night of the second Ameridendro conference, held in Tucson, Arizona, in May 2013, and we were sharing conversation and a beer on the patio of Hotel Congress.

Grant mentioned a set of tree-ring samples that he collected from approximately 250-year old slash pine trees on the Florida Keys.

When hurricanes pass near to the Keys, strong winds and storm surges cause these pines to grow less and to form narrow growth rings. The pine trees are intolerant of salt (storm surge), and strong winds cause branches and needles to break off trees, which results in narrow growth rings for a few years after a storm.

South Florida slash pine section from Big Pine Key, Florida Keys, U.S. Growth of the tree is from right to left. ‘Bands’ of narrow growth years can be detected throughout the section and represent the growth suppression caused by hurricane disturbance (strong winds, storm surge).
Grant Harley

Old, remnant stumps and logs from South Florida slash pines were cut with chainsaws to extend the hurricane suppression chronology back to the early 1700s, Big Pine Key, Florida Keys, U.S.
Grant Harley, John Sakulich

Marta told us stories of the dendro-archeological work she’s done on shipwrecks: by tree-ring dating the wood recovered from shipwrecks, information can be inferred about the provenance of the wood, timber procurement for shipbuilding, past forest management practices, and woodworking techniques. She also mentioned a shipwreck database that contained comprehensive information about the when, where and why of past shipwrecks.

All three of us remember the exact moment, because our minds collided and an exhilarating idea arose: could we combine the shipwreck record with the Florida Keys tree-ring record to recreate past tropical cyclone activity?

[Continue reading to learn just how good this idea was…]