"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 Union'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. Although others have suggested that the AMO, a cycle of from 50 to 70 years, is the significant contributing factor to the increase in number and strength of hurricanes, 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."
To determine the contributions of sea surface warming, the AMO and any other factors to increased hurricane activity, the researchers used a statistical method that allows them to subtract the effect of variables they know have influence to see what is left.
"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.
The cause of increased tropical Atlantic sea surface temperatures is the real question. 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 that global fluctuation did not account for everything. They first found what appeared, at least superficially, to be a significant influence from the AMO in the tropical Atlantic.
To test if the fluctuation was indeed due to the AMO, they looked only at data from before 1950. They found that the apparent AMO signal became indistinguishable from the statistical noise if the recent cooling trend between 1950 and the 1980s was not included.
"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.
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.
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.