Oceans turn up the AC on global warming

Science | Daniel James Devine

Oceans turn up the AC on global warming


The world’s oceans acted like giant air conditioners during the last decade, according to a new study. By absorbing heat from the air, the oceans played a major role in thwarting a projected rise in global temperatures from 2000 to 2010.

With the high seas serving as reservoirs of warmth, though, it’s possible ocean currents could eventually return the heat back into the atmosphere, researcher Virginie Guemas told Reuters. Guemas is the lead author of the study, published at the website of Nature Climate Change on Sunday.

The researchers point to natural cycles of ocean currents beginning around the turn of the century—such as a La Niña event in the Pacific Ocean—for bringing cool seawater to the surface and absorbing excess heat from the atmosphere. Ocean heat uptake is one of several factors, along with sun cycles and water vapor in the stratosphere, which scientists blame for a recent slowdown in global warming.

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According to NASA data, 10 of the hottest years since global temperature records began in 1880 have occurred since 1998. Yet, the rise in temperature has been less extreme than climate models predicted: In spite of a 58 percent rise in greenhouse gas emissions around the world since 1990, the global average temperature has changed little in the past decade.

According to an unpublished United Nations report leaked late last year, the climate forecast models the UN has relied upon since 1990 have consistently overestimated global warming by two to five times the actual observed rate.

The UN uses climate predictions to press for an international treaty to slow the release of carbon dioxide into the atmosphere. The first such treaty, the Kyoto Protocol, expired in 2012 and failed to contain emissions because many developed nations broke their promises to cut back on carbon. Kyoto never placed limits on the emissions of developing nations, such as China and India, where industry—and pollution—has risen rapidly.

At the most recent UN climate change conference, held in Qatar last December, a few developed nations (not including the United States, Japan, or Canada) agreed to extend their Kyoto carbon cuts for a few more years. The UN hopes to convince world powers to sign another emissions-reduction treaty by 2015. If past negotiations are any indication, it has little chance of success.

The Nature Climate Change paper follows on the heels of another study, published last month in Geophysical Research Letters, concluding that about 30 percent of ocean heat uptake since 1998 has occurred in deeper waters, below about 2,300 feet. The authors said the recent rates of deep-water warming “appear to be unprecedented.”

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