More and stronger North Atlantic hurricanes may be attributed to human-induced climate warming rather than natural ocean cycles, scientists reported on Tuesday.
According to a study released by researchers at the Pennsylvania State University and Massachusetts Institute of Technology, human-induced factors "are likely responsible for long-term trends in tropical Atlantic warmth and tropical cyclone activity."
These findings were published in an upcoming issue of the American Geophysical Society's journal EOS.
Earlier studies suggested that the Atlantic Multidecadal Oscillation(ADO), an ocean cycle of 50 to 70 years, may contribute significantly to recent increase in number and strength of hurricanes.
But in this new research, scientists indicated that the tropical Atlantic sea surface temperature is the only responsible factor. They reached this conclusion after analyzing the record of global sea surface temperatures, hurricane frequency, aerosol impacts and the ADO.
The cause of increased tropical Atlantic sea surface temperatures is the real question, they said. One contributor must be overall global sea surface temperature trends.
The researchers looked at the sea surface temperature record in the tropical Atlantic and compared it to global sea surface temperatures. They found that the tropical temperatures did closely follow the global temperatures, but global fluctuation did not account for everything.
For example, the cooling of tropical Atlantic during 1950-1980 can be attributed to human-produced aerosol pollutants, noted the team led by Michael Mann, an associate professor at the Pennsylvania State University.
While some gases, such as carbon dioxide and methane in the upper atmosphere create the greenhouse effect associated with global warming, other pollutants such as sulfur dioxide and nitrogen oxides in the lower atmosphere cool the Earth's surface by reflecting sunlight, the researchers explained.
This human-caused cooling, they found, was masquerading as part of an apparent natural oscillation.
Linking the global temperature trends with the enhanced regional cooling impact of the pollutants, the researchers were able to explain the observed trends in both tropical Atlantic temperatures and hurricanes.
Because of prevailing winds and air currents, pollutants from North American and Europe move into the area above the tropical Atlantic. The impact is greatest during the late summer when the reflection of sunlight by these pollutants is greatest, exactly at the time of highest hurricane activity, they said.
Without taking into account the mitigating effect of pollutants, the results were higher than what had actually occurred. This suggests that the cooling from pollutants in the atmosphere tempered the rise of sea surface temperatures and hurricane numbers.
However, according to the researchers, the industrialized countries is doing much better at controlling pollution. North America and Europe have both reduced the amounts of aerosols they put into the atmosphere, thus the cooling effect has been decreasing since the 1980s.
Absent the mitigating cooling trend, tropical sea surface temperatures are rising. If the regional AMO is not contributing significantly to the increase, than the increase must come from general global warming, which most researchers have linked with human actions, they concluded.