Researchers attribute global warming slowdown to Pacific, Atlantic

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A new study has concluded that the recent slowdown in manmade global warming over the past decade is a ‘false pause’ resulting from the offsetting by cooling from natural cycles in the Pacific and Atlantic oceans – an effect that will reverse in coming decades.

Researchers over at Penn State University have attributed the slowdown in climate warming to natural oscillations in the climate, which are internal to the climate system and which necessarily do not signal any slowdown in human-caused global warming.

“Our findings have strong implications for the attribution of recent climate changes,” said Michael Mann, Distinguished Professor of Meteorology, Penn State. “Internal multidecadal variability in Northern Hemisphere temperatures likely offset anthropogenic warming over the past decade.”

In the study, the researchers distinguished between human-caused and natural climate variability in a bid to assess the impact of human-caused climate change on a variety of phenomena including drought and weather extremes. The researchers found that North Atlantic and North Pacific oceans are driving substantial natural, internal climate variability on timescales of decades.

AMO Spatial Pattern

Mann, Byron A. Steinman, assistant professor of geological sciences, University of Minnesota-Duluth and a former Penn State National Science Foundation postdoctoral fellow and Penn State researcher Sonya K. Miller reveal in their paper that the latest global warming hiatus or pause is a “false pause” and warn that the cooling effect is a result of natural cycles in Pacific and Atlantic oceans. This effect will reverse in the coming decades and global warming will accelerate again.

The researchers looked at Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO) – the two key factors that play a significant role in global temperatures. The researchers stripped out ‘external forces’ on those oscillations including volcanoes and the burning of fossil fuels, which are known to have significant impact, in a bid to work out how much they varied naturally, or internally.

Researchers note that the the internal or natural variability plays a significant role and has a huge influence over the span of several decades on temperatures in the northern hemisphere. They estimated that the variability could have a 0.15 C warming or cooling effect and in recent years, it is having a cooling effect.

“We find that internal multidecadal variability in northern hemisphere temperatures, rather than having contributed to recent warming, likely offset anthropogenic warming over the past decade,” the authors note.

In a blog post Mann noted: “Our conclusion that natural cooling in the Pacific is a principal contributor to the recent slowdown in large-scale warming is consistent with some other recent studies, including a study I commented on previously showing that stronger-than-normal winds in the tropical Pacific during the past decade have lead to increased upwelling of cold deep water in the eastern equatorial Pacific.”