Sunday, March 12, 2017

Oceans are storing up staggering amounts of heat​​​​​​​

A diver films a reef affected by bleaching off Lizard Island in the Great Barrier Reef. (AFP/Getty Images)

Chelsea Harvey writes for The Washington Post:

The world is getting warmer every year, thanks to climate change — but where exactly most of that heat is going may be a surprise.

As a stunning early spring blooms across the United States, just weeks after scientists declared 2016 the hottest on record,
it’s easy to forget that all the extra warmth in the air accounts for only a fraction of the heat produced by greenhouse gas emissions. In fact, more than 90 percent of it gets stored in the ocean. And now, scientists think they’ve calculated just how much the ocean has warmed in the past few decades. 
A new study, out Friday in the journal Science Advances, suggests that since 1960, a staggering 337 zetajoules of energy — that’s 337 followed by 21 zeros  — has been added to the ocean in the form of heat. And most of it has occurred since 1980.
“The ocean is the memory of all of the past climate change,” said study co-author Kevin Trenberth, a senior scientist at the National Center for Atmospheric Research. 
The new value is a number that significantly exceeds previous estimates, Trenberth noted. Compared with ocean warming estimates produced by the Intergovernmental Panel on Climate Change, the new values are about 13 percent greater. This is the result of a new methodology for estimating ocean warming, involving a series of steps “that really make this paper different than previous ones,” Trenberth told The Washington Post. 
In previous decades, there have been a lot of challenges associated with monitoring temperature changes in the ocean. Before the year 2000 or so, most monitoring instruments had to be deployed from ships. This mean that scientists only had the most reliable data for parts of the world that lie along major shipping routes. 
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In the past 15 years, though, scientists have developed the “Argo” network, a system of free-drifting devices that are designed to periodically adjust their buoyancy, so they can sink several thousand meters into the sea, collect measurements, and then rise back up to the surface. There are now about 3,500 of these devices deployed throughout the world’s oceans, leading to a much better dispersal of observations. 
The new study, which was led by Lijing Cheng of the Chinese Academy of Sciences and included other scientists from that institution, from the National Center for Atmospheric Research, and from the National Oceanic and Atmospheric Administration, employs a new methodology for using both the recent Argo measurements and past observations from ships to produce a continuous series of estimates from 1960 to 2015.
The scientists incorporated an updated database of pre-Argo measurements that have been corrected for certain biases, as well as information from climate models, and extended existing observations of ocean conditions taken at specific locations to larger areas of the sea. They then conducted a comparison of recent Argo data with measurements created using their new methodology and found that the method produces true-to-life results. 
The results suggest that the ocean has been sucking up more heat than previous research has indicated. In fact, according to Trenberth, the new estimates help explain observations of global sea-level rise that scientists have had difficulty accounting for until now.

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