A new generation of scientific instruments has begun scouring ocean depths for temperature data, and the evidence being pinged back via satellite warns that the consequences of fossil fuel burning and deforestation are accumulating far below the planet’s surface.
More than 90 percent of the heat trapped by greenhouse gas pollution since the 1970s has wound up in the oceans, and research published Monday revealed that a little more than a third of that seafaring heat has worked its way down to depths greater than 2,300 feet (700 meters).
Plunged to ocean depths by winds and currents, that trapped heat has eluded surface temperature measurements, fueling claims of a “hiatus” or “pause” in global warming from 1998 to 2013. But by expanding cool water, the deep-sea heat’s impacts have been indirectly visible in coastal regions by pushing up sea levels, contributing to worsening high-tide flooding.
“The heat’s going in at the surface, so it’s getting down pretty deep,” said Glen Gawarkiewicz, a Woods Hole Oceanographic Institution scientist who was not involved with the study. “With 35 percent of the heat uptake going below 700 meters, it really points out the importance of continued deep ocean sampling. It was a surprise to me that it was that large of a fraction.”
The research, published in Nature Climate Change, was led by Lawrence Livermore National Laboratory. It compared modeling results with data from a mishmash of sources, most notably from a nascent fleet of monitoring devices called deep Argo floats.
The researchers concluded that half of overall ocean warming has occurred since 1997—a date that they noted in their paper was “nearly coincident with the beginning of the observed surface warming hiatus.”
A combination of climate pollution, a recent change in a long-running cycle of the Pacific Ocean and the current El Niño has led to a spike in warming rates recorded at the surface of the planet. That followed a surface warming slowdown; 2014 and 2015 were the warmest years on record globally.
Research groups from around the world have deployed thousands of Argo floats to measure since around the year 2000 to take temperature, salinity and other measurements. Technological advances have allowed a small fleet of deeper-diving floats to be deployed more recently. Some of those have been built to dive as deep as 20,000 feet.
“Knowing how much the ocean is warming and how fast and where are all important for knowing how much the atmosphere is going to warm and how much seas are going to rise,” said Gregory C. Johnson, a National Oceanic and Atmospheric Administration scientist who works on that agency’s Argo float program.
Monday’s paper used the new deep-sea Argo data to expand on a paper published in 2014 by Lawrence Livermore and other researchers, which revealed high levels of warming in the ocean’s surface layer.
“The oceans as an energy store are really doing a lot of the work,” said Lawrence Livermore researcher Paul Durack, who helped produce the studies that were published Monday and in 2014. “The actual temperature change is relatively small, but due to the huge heat capacity of the oceans this equates to a very, very large heat content change.”