Aerosols may even control multi-decadal cycle, says Met Office research

Hurricane Sandy

Research from the UK’s Met Office suggests aerosols may have suppressed the number of Atlantic hurricanes over the 20th century and even controlled the decade-to-decade changes in the number of hurricanes.

Researchers found that aerosols made clouds brighter, causing them to reflect more energy from the sun back into space. This affects ocean temperatures and tropical circulation patterns, effectively making conditions less favourable for hurricanes.

This interaction between aerosols and clouds is a process that is now being included in some of the latest generation climate models.

“Industrial emissions from America and Europe over the 20th century have cooled the North Atlantic relative to other regions of the ocean,” the research claimed. “Our research suggests that this alters tropical atmosphere circulation – making it less likely that hurricanes will form.”

“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. On the other hand, the reduction in aerosols has been beneficial for human health and has been linked to the recovery of Sahel rains since the devastating drought in the 1980s.”

It has long been known that North Atlantic hurricane activity has distinct long-timescale variability, known as the North Atlantic multi-decadal oscillation.

“We saw relatively quiet periods between 1900 and 1920 and then again from 1970 to 1980, and active periods between 1930 and 1960 and since 1995,” said Met Office research fellow and co-author of the study Doug Smith. “On average, active periods have 40% more hurricanes.”

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

“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,” said Met Office climate processes scientist and another co-author of the study Ben Booth.

Taken at face value, the study suggests the number of Atlantic hurricanes over the next couple of decades will depend on future aerosol emissions and how they interact with natural cycles in the North Atlantic.