8th October 2014

Ocean warming in Southern Hemisphere has been greatly underestimated

The evidence for global warming continues to pour in. A new study of ocean heat content shows that temperatures have been greatly underestimated in the Southern Hemisphere. As a result, the world's oceans are now absorbing between 24 and 58 per cent more energy than previously thought.

Like a fleet of miniature research vessels, more than 3,600 robotic floats provide data on upper layers of the world's ocean currents.

Scientists from Lawrence Livermore National Laboratory in California, using satellite observations and a large suite of climate models, have found that long-term ocean warming in the upper 700 metres of Southern Hemisphere oceans has been greatly underestimated.

"This underestimation is a result of poor sampling prior to the last decade, and limitations of the analysis methods that conservatively estimated temperature changes in data-sparse regions," said LLNL oceanographer Paul Durack, lead author of a paper in the 5th October issue of the journal Nature Climate Change.

Ocean heat storage is important because it accounts for over 90 percent of excess heat associated with global warming. The observed ocean and atmosphere warming is a result of continuing greenhouse gas emissions. The Southern Hemisphere oceans make up 60 percent of the world's oceans.

The researchers found that climate models simulating the relative increase in sea surface height between Northern and Southern hemispheres were consistent with highly accurate altimeter observations. However, the simulated upper-ocean warming in Northern and Southern hemispheres was inconsistent with observed estimates of ocean heat content change. These sea level and ocean heat content changes should have been consistent, suggesting that until recent improvements in observational data, Southern Hemisphere ocean heat content changes were underestimated.

Since 2004, automated profiling floats called Argo (pictured above) have been used to measure global ocean temperatures from the surface down to 2,000 m (6,560 ft). These 3,600 floats currently observing the global ocean provide systematic coverage of the Southern Hemisphere for the first time. Argo float data over the last decade, as well as earlier measurements, show that the ocean has been steadily warming, according to Durack.

"The Argo data is really critical," he said. "Estimates that we had until now have been pretty systematically underestimating the changes. Prior to 2004, research has been very limited by poor measurement coverage. Our results suggest that ocean warming has been underestimated by 24 to 58 percent. The conclusion that warming has been underestimated agrees with previous studies. However, it's the first time that scientists have tried to estimate how much heat we've missed."

Given that most of the excess heat associated with global warming is in the oceans, this study has important implications for how scientists view the Earth's overall energy budget. Heat currently stored by the oceans will eventually be released, causing land temperatures to accelerate and triggering more extreme climate events.

"We continue to be stunned at how rapidly the ocean is warming," said Sarah Gille, a Scripps Institution of Oceanography professor who was not involved in the study. "Even if we stopped all greenhouse gas emissions today, we'd still have an ocean that is warmer than the ocean of 1950, and that heat commits us to a warmer climate. Extra heat means extra sea level rise, since warmer water is less dense, so a warmer ocean expands."

"An important result of this paper is the demonstration that the oceans have continued to warm over the past decade, at a rate consistent with estimates of Earth’s net energy imbalance," says Prof. Steve Rintoul, from Australia’s Commonwealth Scientific and Industrial Research Organisation. "While the rate of increase in surface air temperatures slowed in the last 10 to 15 years, the heat stored by the planet, which is heavily dominated by the oceans, has steadily increased as greenhouse gases have continued to rise."

These new results are consistent with another new paper that appears in the same issue of Nature Climate Change. Co-author Felix Landerer of NASA's Jet Propulsion Laboratory, who contributed to both studies, says, "Our other new study on deep-ocean warming found that from 2005 to the present, Argo measurements recorded a continuing warming of the upper-ocean. Using the latest available observations, we're able to show that this upper-ocean warming and satellite measurements are consistent."

In related news, a report by Edinburgh's Heriot-Watt University – based on the work of 30 experts – finds that ocean acidification has increased by 26% since pre-industrial times. It is now causing nearly $1 trillion of damage to coral reefs each year, threatening the livelihoods of 400 million people.

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