Human induced climate change, rather than naturally occurring ocean cycles, may be responsible for the recent increases in frequency and strength of North Atlantic hurricanes, according to Penn State and Massachusetts Institute of Technology researchers.
“Anthropogenic factors are likely responsible for long-term trends in tropical Atlantic warmth and tropical cyclone activity,” the researchers report in an upcoming issue of the American Geophysical Society’s EOS.
Michael E. Mann, associate professor of meteorology and geosciences, Penn State, and Kerry A. Emanuel, professor of atmospheric sciences, MIT, looked at the record of global sea surface temperatures, hurricane frequency, aerosol impacts and the so-called Atlantic Multidecadal Oscillation (AMO) Ð an ocean cycle similar, but weaker and less frequent than the El Nino/La Nina cycle. Their statistical analysis and modeling indicate that it is only the tropical Atlantic sea surface temperature that is responsible, tempered by the cooling effects of some lower atmospheric pollutants.
“We only have a good record of hurricanes and sea surface temperature for a little more than the last 100 years,” says Mann, who is also director of Penn State’s Earth System Science Center. “This means we have only observed about one and a half to two cycles of the AMO. Peer-reviewed research does suggest that the signal exists, but it is difficult to estimate the period and magnitude of the oscillation directly from observations.”
“There appears to be a strong historical relationship between variations in tropical Atlantic sea surface temperature and tropical cyclone activity extending back through the 19th century,” say Mann and Emanuel.
“This pattern of late 20th century cooling has been attributed in past work to the anthropogenic production of tropospheric aerosol,” note Mann and Emanuel in their paper. This human-caused cooling, they found, was masquerading as part of an apparent natural oscillation.
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.
When Mann and Emanuel use both global temperature trends and the enhanced regional cooling impact of the pollutants, they are able to explain the observed trends in both tropical Atlantic temperatures and hurricane numbers, without any need to invoke the role of a natural oscillation such as the AMO.
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, the industrialized world is doing much better at controlling pollution. North America and Europe have both reduced the amounts of aerosols they put into the atmosphere. The cooling effect has been decreasing since the 1980s.
Absent the mitigating cooling trend, tropical sea surface temperatures are rising. If the AMO, a regional effect, is not contributing significantly to the increase, than the increase must come from general global warming, which most researchers attribute to human actions.
For more information Contact: Vicki Fong
vfong@psu.edu
+1814-865-9481 or visit http://www.essc.psu.edu/
