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 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 per year in the top 500 meters of ocean and 0.002°C 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,” said Roemmich.
“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.”
“When we measure globally and deep enough, we see a stead 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 the spatial structure of ocean warming since 2006.”
The data comes from the global Argo array, a network of 3,750 floats funded by NOAA, Australia’s Integrated Marine Observing System (IMOS deployments of just 30 floats per annum along with other national and international deployments have resulted in 10% of the global array of 3,800 floats delivering a continuous data stream for the Australian region), the National Institute of Water and Atmospheric Research 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.
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.