New research from the U.K. Met Office suggests aerosols associated with industrial pollution may have suppressed the number of Atlantic hurricanes over the 20th Century.
Aerosols can occur naturally in the form of volcanic plumes, clouds or fog, but are also particulate pollution and soot from burning coal and oil. Aerosols make clouds brighter -- reflecting more solar energy back into space -- impacting ocean temperatures and tropical circulation patterns.
The study focused on particulate pollution from North America and Europe generated mainly from burning fossil fuels. Researchers then created weather simulations covering the period 1860 to 2050.
"Industrial emissions from America and Europe over the 20th Century have cooled the North Atlantic relative to other regions of the ocean," Dr. Nick Dunstone, a Met Office climate prediction scientist and lead author of the research, said. "Our research suggests that this alters tropical atmosphere circulation - making it less likely that hurricanes will form.
North Atlantic hurricane activity is known to have long-timescale variability. "We saw relatively quiet periods between 1900-20 and then again from 1970-80, and active periods between 1930-60 and since 1995," Dr. Doug Smith, co-author of the study, said. "On average, active periods have 40% more hurricanes."
When the authors include changes in man-made aerosol emissions, which includes a comprehensive treatment of aerosol-cloud interactions, they can reproduce much of the decade-to-decade variability in Atlantic hurricane activity, supporting a link between the two.
"Since the introduction of 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," Dunstone said. "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."
Previous studies have suggested that while the number of tropical storms is not projected to increase, their intensity is. For 2013, the US National Oceanic and Atmospheric Administration (NOAA) has predicted 13 to 20 "named" storms, with an estimated seven to 11 hurricanes and three to six major hurricanes.