Distinct Rise in Global Ocean Temperatures Detected

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A global network of profiling floats that provides scientists the most accurate means of observing energy accumulation in the climate system has detected an increase in the temperature of the world’s oceans over a recent eight-year period.

Researchers led by Dean Roemmich, a physical oceanographer at Scripps Institution of Oceanography, UC San Diego, found that the top 2,000 meters (6,500 feet) of the world’s oceans warmed at a rate of 0.4 to 0.6 watts per square meter (W/m 2) between 2006 and 2013. The rate translates to a warming of roughly 0.005° C (0.009° F) per year in the top 500 meters of ocean and 0.002° C (0.0036° F) per year at depths between 500 and 2,000 meters. 

For perspective, Roemmich noted that the heat gain was the equivalent of adding the heat of two trillion continuously burning 100-watt light bulbs to the world’s oceans.

“The rate of ocean heat gain during the past eight years is not unusual – indeed many studies of ocean data over the past 50 years and longer have produced similar rates. What is new is that the rate and patterns of ocean heat gain are revealed over a period as short as eight years, thanks to the Argo array, that the warming signal is shown to extend to 2,000 meters and deeper, and that it is occurring predominantly in the Southern Hemisphere ocean south of 20° S,” said Roemmich.

“When we measure globally and deep enough, we see a steady rise in the earth’s heat content, consistent with the expected greenhouse gas-driven imbalance in our planet’s radiation budget,” said study co-author Susan Wijffels of Australian research agency the Commonwealth Scientific and Industrial Research Organization (CSIRO).

The study puts a widely reported “hiatus” in global surface air temperatures since 1998 into context. Roemmich said the study illustrates that the hiatus in warming of the sea surface and the lower atmosphere is not representative of the steady, continuing heat gain by the climate system. Scientists measure that heat gain in terms of increasing temperature averaged over the water column. 

The science team reports its findings in the Feb. 2 issue of the journal Nature Climate Change in a paper entitled “Unabated planetary warming and its ocean structure since 2006.”

The data come from the global Argo array, a network of 3,750 floats funded by NOAA, Australia’s Integrated Marine Observing System, the National Institute of Water and Atmospheric Research (NIWA) in New Zealand, and other international agencies that allows scientists to observe the basic physical state of all world oceans simultaneously. The Argo network of sensor-bearing profiling floats measures ocean water temperature, salinity, and velocity to a degree that had not been possible before the launch of the network 15 years ago. The floats were co-developed by Scripps research oceanographer Russ Davis in the 1990s and have given scientists the first global-scale observations of the oceans’ upper depths in history.

The most marked increase in heat content was found in the Southern Hemisphere, where the oceans make up a much higher proportion of surface area than in the Northern Hemisphere. The researchers reported that they used three statistical analysis methods to estimate global ocean heat content and that all three largely agreed, pointing to the robustness of the Argo network’s accuracy. Because nearly all of the excess heat in the climate system is retained in the oceans, the Argo network has provided scientists the most direct and accurate means of observing energy accumulation in the climate system.

“The ability to consistently detect a global ocean heat gain of 0.4 to 0.6 W/m 2 over the short 2006-2013 period is historically unprecedented,” said the researchers in the study. “Homogeneous global coverage, high data quality, and temporal resolution of seasonal and interannual fluctuations are key attributes for enabling the Argo-only analyses that underpin this result.”

Study authors besides Roemmich and Wijffels, who co-chair the Argo Steering Team, a panel of international scientists that directs the network, include John Church and Didier Monselesan of CSIRO, John Gilson of Scripps Oceanography, and Phil Sutton of NIWA.

New floats in the Argo network have been steadily added since 1999, deployed by research vessels and other volunteer ship and from aircraft. The network achieved comprehensive coverage of the world’s oceans in 2007, when 3,000 Argo floats began operating simultaneously. The Argo dataset is freely available online and at present is cited in more than 200 research papers per year. Scientists hope that over the long term Argo data will be used to enhance the understanding of ocean trends and cycles that play out over multiple decades.

A new generation of floats known as “Deep SOLO” is augmenting the current Argo network. Deep SOLO floats are capable of recording fundamental ocean data at depths up to 6,000 meters (20,000 feet).

Going forward, write the authors, “it is feasible to quantify the total energy budget of the ocean, reducing uncertainties and accounting for the possible sinks of missing energy, including the deep ocean.”

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