Different depths reveal ocean warming trends

http://www.bbc.com/news/science-environment-29474646

By Jonathan Webb
Oct. 5, 2014

The deeper half of the ocean did not get measurably warmer in the last decade, but surface layers have been warming faster than we thought since the 1970s, two new studies suggest.

Because the sea absorbs 90% of the heat caused by human activity, its warmth is a central concern in climate science.

The new work suggests that shallow layers bear the brunt of ocean warming.

Scientists compared temperature data, satellite measurements of sea level, and results from climate models.

Both the papers appear in the journal Nature Climate Change.

Specifically in the Southern Hemisphere where fewer measurements have been made, a team of researchers from the Lawrence Livermore National Laboratory in California investigated long-term warming in the top 700m of the ocean.

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Sea levels across the planet are accurately assessed by satellites, which bounce radio waves off the ocean surface. And sea level changes are closely related to ocean temperature, because the water expands as it warms up.

By combining these calculations, the scientists found that the rate of upper-ocean warming between 1970 and 2004 had been seriously underestimated. That inaccuracy is specific to the Southern Hemisphere, but is big enough, the scientists suggest, that global upper-ocean warming rates are also "biased low" - to the tune of 24% to 55%.

The researchers say the underestimation probably arose simply from the scarcity of measurements in the south.

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Dr Zika added that the heat content of shallow layers is a particular concern, as that warmth is more likely to return to the atmosphere. "If it were getting really dee

Meanwhile, at depths greater than 2km, temperatures appear relatively stable - at least as far as changes can be detected.

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They suggested that clearer conclusions would come from the next generation of floating probes, known as Deep Argo, the first of which were deployed off the New Zealand coast in June. These will plunge up to 6km beneath the waves.

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