Hurricanes, referred to as "acts of God" by insurers and seen by others as harbingers of the effects of climate change, are becoming more common as an unexpected side-effect of efforts to reduce pollution, research indicates.
A study by British scientists has produced evidence that there is a direct link between a decline in industrial pollution across the Atlantic and an increase in the number of deadly storms battering the coasts of America and the Caribbean.
In a sign that efforts to reduce humankind's negative impact on the planet can themselves produce harmful results, research by the Meteorological Office suggests that for much of the 20th century, sooty particles generated by industrialisation made conditions unfavourable for hurricanes.
But efforts to improve air quality since the 1980s seem to have once more unleashed natural forces that lead to the formation of gigantic storms, such as Hurricane Sandy which devastated parts of New York last October and was the second costliest in United States history.
The process at the heart of the increase is the climactic interaction between tiny airborne liquid droplets, known as aerosols, and clouds.
The British researchers found that pollutants, spread through the atmosphere as aerosols, have the effect of brightening clouds, causing more of the sun's energy to be reflected back into space. The knock-on effect of this process on ocean temperatures and circulation patterns makes it harder to create hurricanes. When the aerosols are removed this brake on storm-generating conditions is released.
Dr Nick Dunstone of the Met Office, who led the study published in the journal Nature Geoscience, said: "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."
The US$68 billion cost of Sandy was superseded only by Hurricane Katrina, which devastated New Orleans and caused damage worth US$80 billion.
Between 1850 and 1990, there were an average of 10 Atlantic tropical storms a year, half of which were powerful enough to be classified as hurricanes. Between 1998 and 2007, this figure rose to about eight hurricanes a year.
The researchers underlined that the increase in hurricanes had to be balanced with positive effects from the reduction in pollution, in particular the reduction of droughts in Africa and wider benefits for human health.
Dr Ben Booth, co-author of the report, said: "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."