Using a sediment core taken from the Great Blue Hole off the coast of the Central American state of Belize, researchers from the universities of Frankfurt, Cologne, Göttingen, Hamburg and Bern have analyzed the local climate history of the last 5,700 years.

Investigations of the sediment layers from the 30-meter-long core revealed that storms have increased over the long term and that tropical cyclones have become much more frequent in recent decades. The results were published under the title “An annually resolved 5700-year storm archive reveals drivers of Caribbean cyclone frequency” in the journal Science Advances.

The Great Blue Hole is up to 125 meters deep and approximately 300 meters wide, situated in the very shallow Lighthouse Reef, an atoll off the coast of Belize. The hole was formed from a stalactite cave that collapsed at the end of the last ice age and then became flooded by the rising sea level as a result of the melting of the continental ice masses.

In earlier studies, investigations on shorter sediment cores led by Professor Dr. Eberhard Gischler and Dr. Dominik Schmitt from the University of Frankfurt had shown that the sediments at the bottom of the Great Blue Hole allow for a very accurate reconstruction of the tropical cyclones in the region, the strongest of which are known as hurricanes.

The storms stir up particles on the atoll, which are carried into the Great Blue Hole and form distinctive layers at its bottom. The layers differ significantly in their composition from the normal sediments and can be dated very precisely based on the annual stratification of the normal sediments.

Motivated by these initial results, researchers from the University of Cologne and the University of Frankfurt deployed a coring platform at the Great Blue Hole in the summer of 2022, making it possible to recover its entire sediment record for the first time.

“Our drilling technology was originally designed for lakes and has been used several times in the Russian Arctic,” explained geologist Professor Dr. Martin Melles, who leads the research team from the University of Cologne. In this case, it was towed by boat about 70 kilometers from the mainland to the Great Blue Hole and anchored there.

The extracted sediment core reflects three phases of deposition: 12,500 years ago, a lake became formed in the collapsed cave, into which many organic remains from a surrounding rainforest were washed. Then, 7,200 years ago, the sea level after the last ice age rose to the edge of the Great Blue Hole. The water body became brackish and the surrounding rainforest was replaced by mangrove swamps. A complete inundation of the Great Blue Hole with marine reef growth instead of terrestrial vegetation around the rim then began 5,700 years ago.

Since then, marine sediments are deposited with almost no interference, documenting the tropical storms in the region in the form of storm layers.

The results show that a total of 574 storm layers have formed in the Great Blue Hole over the past 5,700 years. This is by far the longest annualized time series of tropical cyclones in the Caribbean. It adds many millennia to the meteorological data and human records, which go back only 175 years. It thus provides detailed insights into the natural fluctuations in storm frequency at times when humans had not yet intervened extensively in the climate system.

The storm frequency over the past 5,700 years shows a long-term increase, which is overlaid by short-term fluctuations. These two trends can be explained by known changes in solar irradiance caused by changes in the circular motion of the Earth’s axis and solar cycles. The reconstructed storms also show a frequency for the last few decades that is many times higher than the frequency in the older sediments.

“This maximum frequency cannot be attributed to either the long-term increase or the short-term fluctuations over the last 5,700 years, so it is likely to be a consequence of the current climate change caused by human activity,” explained Professor Melles. “Our geological results thus support the predictions of some climate models that progressive global warming will lead to a further increase in the frequency of tropical storms.”