Natural climate variables so far outweigh global warming impacts
By Summit Voice
SUMMIT COUNTY — The timing and amount of monsoon rains in the northern hemisphere have important economic and environmental ramifications, for example for farmers in Asia and the wildfire season in the southwestern U.S.
As a result, climate researchers have been trying to determine how the Earth’s steady warming will affect those seasonal rainfall patterns, and so far, the jury is still out. Some recent research has suggested that the timing of the North American monsoon might be delayed, while other studies have indicated that there could be an overall increase in monsoon precipitation.
In one of the latest studies, scientists with the International Pacific Research Center, University of Hawaii at Manoa, say that monsoon rainfall patterns appear to more influenced by natural long-term swings in ocean surface temperatures. The Interdecadal Pacific Oscillation or mega-El Niño-Southern Oscillation, which has lately been in a mega-La Niña or cool phase is one key factor, and shifts in the Atlantic Multidecadal Oscillation, also contributes to the intensification of monsoon rainfall.
The findings are published in the March 18 online publication of the Proceedings of the National Academy of Sciences.
The research team was led by meteorology professor Bin Wang, who, with his colleagues, found that, during the past 30 years, the summer monsoon circulation, as well as the Hadley and Walker circulations, have all substantially intensified. This intensification has resulted in significantly greater global summer monsoon rainfall in the Northern Hemisphere than predicted from greenhouse-gas-induced warming alone: Namely a 9.5 percent increase, compared to the anthropogenic predicted contribution of 2.6 percent per degree of global warming.
Most of the recent intensification is attributable to a cooling of the eastern Pacific that began in 1998.
“These natural swings in the climate system must be understood in order to make realistic predictions of monsoon rainfall and of other climate features in the coming decades,” Wang said. “We must be able to determine the relative contributions of greenhouse-gas emissions and of long-term natural swings to future climate change.”