The last four decades have seen an increase in the intensity and number of tropical cyclones that have severe environmental and socioeconomic impacts in large areas around the globe.

Francesco SR Pausata of the Université du Québec à Montréal in Canada, previously at Stockholm University, and colleagues compared four mid-Holocene model runs with different vegetation cover over the Sahara and airborne dust loadings to pre-industrial climate simulations.

The first mid-Holocene model run only accounted for changes in the Earth’s orbit around the Sun but left the Sahara unvegetated and dust emissions as high as in the pre-industrial case. Overall this resulted in little change in the number of tropical cyclones relative to pre-industrial conditions, but a shift in their locations led to more of them making landfall in the Gulf of Mexico/Caribbean Sea and South-East Asia.

A second model run included changed orbital parameters with a green Sahara and reduced dust emissions. This markedly increased cyclone activity in tropical regions around the globe. Two additional model runs tested the impact of a green Sahara and reduced dust emissions separately.

Decreased dust emission from the Sahara alone increased sea surface temperatures and cyclone generation, particularly over the Caribbean and the Atlantic Ocean. The model accounting only for a vegetated Sahara showed a much larger impact on global climate. In this run, the decreased albedo of the vegetation compared to bare soil strengthened the West African monsoon and affected cyclone activity across the tropics.

The team’s results show that including changes in Saharan landcover and dust emissions is essential for the prediction of tropical cyclone activity. Until now, these changes have mostly been overlooked in the modelling of future climate as carbon dioxide concentrations rise. Yet incorporating them reveals that a regreening of the Sahara and Sahel regions may increase the vulnerability of the Caribbean, the Gulf of Mexico and the United States east coast to tropical cyclones to a larger degree than previously thought.

Pausata and colleagues reported their findings in Proceedings of the National Academy of Sciences.

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