A new study in the journal Science by a University of Minnesota Duluth professor argues that a recent slowdown in global warming is caused by decades-long variations in ocean temperatures.
The rate of global warming has slowed by nearly half since the late 1990s. Temperatures are still rising, but not at the rate that climate change models have predicted.
That in turn has fueled a debate between climate change scientists and those who deny a human link to global warming.
The new research by Byron Steinman, an assistant professor of geological sciences at UMD's Large Lakes Observatory, concludes that the slowdown in warming is caused by natural oscillations in ocean temperatures similar to the cycle of El Niño and La Niña, but takes place over several decades.
"The slowdown is largely a result of a negative trend in the Pacific Ocean, Pacific sea surface temperatures," he said. "When this trend reverses, we're very likely to see an accelerated rate of warming."
The study also linked recent droughts in North America and Africa to increased carbon dioxide emissions.
Steinman and his fellow researchers, climate scientists Michael Mann and Sonya Miller of Penn State University, developed a new method to isolate natural variability from external factors that influence climate change, such as greenhouse gas emissions. It combined historical observations and an analysis of more than 170 climate model simulations.
"We made what I feel is a pretty substantial step forward in that sense," he said, "in terms of isolating internal variability in the climate system."