24 Jun 2013

Air pollution linked to Atlantic hurricane numbers

Weather Presenter

New research from the Met Office has raised the possibility that man-made aerosols, a form of industrial air pollution, may have impacted upon the number of Atlantic hurricanes.

Hurricane from space

The research, published in Nature Geoscience, suggests that aerosols may have suppressed the number of Atlantic hurricanes through the 20th century and even controlled the decadal changes in their numbers.

It has been found that aerosols make clouds brighter, causing them to reflect more of the sun’s energy back into space – having a cooling affect.

This impacts temperatures of the oceans and weather patterns in the tropics, making conditions less favourable for hurricanes.

In order for clouds to form, something called cloud condensation nuclei are needed for tiny water droplets condense onto. These come in the form of natural and man-made aerosols.

Man-made aerosols tend to be present in greater concentrations than natural aerosols, meaning that clouds forming from them contain more water droplets and are therefore brighter.

Dr Nick Dunstone, a Met Office climate prediction scientist and lead author of the research, said: “Industrial emissions from America and Europe over the 20th Century have cooled the North Atlantic relative to other regions of the ocean. Our research suggests that this alters tropical atmosphere circulation – making it less likely that hurricanes will form.”

He added: “Since the introduction of the clean air-acts in the 1980s, concentrations of aerosols over the North Atlantic have reduced and model results suggest that this will have contributed to recent increases in hurricane numbers.”

It has long been known that north Atlantic hurricane activity has marked long-timescale variability. There was a relatively quiet period between 1900-20 and 1970-80, with active periods between 1930-60 and since 1995.

The incorporation of changes in man-made aerosol emissions in the latest Met Office Hadley Centre model, which includes a comprehensive treatment of aerosol-cloud interactions, reproduces much of the decade-to-decade variability in Atlantic hurricane activity. This supports evidence of a link between the two.

Dr Ben Booth, a Met Office climate processes scientist and another co-author of the study says: “This study, together with work we published last year, suggests that there may be a greater role than previously thought for man-made influence on regional climate changes that have profound impacts on society.”

This study suggests that the number of Atlantic hurricanes in the coming decades will depend on future aerosol emissions and how they interact with natural cycles in the north Atlantic.