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Cooler Pacific Ocean Behind Global Warming "Pause"

Feb 28, 2015 01:34 PM EST
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Researchers reported Thursday that natural oscillations in the climate, which resulted in cooler Pacific Ocean waters, were partially behind the controversial global warming "pause."

However, this in no way suggests that the planet is now cooling, nor that there is any slowdown in human-caused global warming.

Climate experts have long debated over how the rate of rising global temperatures decreased between 1998 and 2013, while greenhouse gas levels simultaneously continued to rise during this period, even reaching a record high in 2013.

"We know that it is important to distinguish between human-caused and natural climate variability so we can assess the impact of human-caused climate change on a variety of phenomena including drought and weather extremes," Michael Mann from Penn State, who led the study, said in a statement. "The North Atlantic and North Pacific oceans appear to be drivers of substantial natural, internal climate variability on timescales of decades."

In order to better understand what causes climate variability in the Northern Hemisphere and the role the Pacific and Atlantic Oceans play, Mann and his colleagues used a combination of real-world observational data and state-of-the-art climate model simulations from the Intergovernmental Panel on Climate Change.

The study looked at the influences of the Atlantic Multidecadal Oscillation (AMO) - oscillations in North Atlantic sea-surface temperatures over periods of 50 to 70 years - and the Pacific Decadal Oscillation (PDO), which varies over broader timescales. These large circulations of water across the Pacific and Atlantic can cause the oceans, particularly in their upper layers, to experience natural cycles of warming and cooling, researchers explain.

And though these cycles, or oscillations, normally cancel each other out - that is, when the northern Pacific is warming, the northern Atlantic is cooling, and vice versa - these latest findings indicate that the AMO and PDO don't exactly match in their timing and magnitude.

In about the last decade, the level of northern Pacific cooling has been larger than that of warming in the northern Atlantic, leading to an "offset [of] anthropogenic warming over the past decade," Mann said.

According to Mann's team, this cooler Pacific Ocean and the strange global warming pause are linked to heat beneath the tropical Pacific and a tendency for sustained La Niña type conditions. The most likely explanation for these phenomena is the "random excursions" of the AMO.

However, the researchers note that this offset will probably reverse, and human-caused warming will increase in the near future. But when exactly in the next few decades that may happen remains to be seen.

The study's findings were published in the journal Science.

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