Cleaner air linked to more hurricanes Met Office scientist Dr Nick Dunstone, 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." He added that this should be weighed against the positive effects of cutting pollution levels, which has benefited human health and helped to prevent devastating drought in Africa. Co-author Dr Ben Booth, also from the Met Office, 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." Hurricanes are powerful rotating tropical storms that affect the Atlantic, Gulf of Mexico and eastern Pacific. In other parts of the world similar storms are called typhoons or cyclones. The storms form over warm ocean water and can be up to 600 miles across with spiralling winds reaching speeds of 200mph. Globally, an average of around 90 tropical storms occur each year. In the North Atlantic there has been a clear increase in the frequency of tropical storms and hurricanes, rising from an average of 10 per year between 1850 and 1990 to 15 per year between 1998 and 2007. The number of storms powerful enough to be classed as hurricanes has increased from five per year to eight per year during the same periods. Several studies have also shown a global trend towards hurricanes and other cyclonic storms becoming stronger and more destructive. The clearest evidence of growing storm intensity has been seen in the North Atlantic and Indian Ocean. There is an ongoing scientific debate about the link between increased tropical storm activity and global warming. In 2007 the Intergovernmental Panel on Climate Change (IPCC) said it was "more likely than not" that humans had contributed to hurricane intensification since the 1970s.