Despite mixed signals from warming ocean surface waters, a new re-analysis of data from the depths suggests dramatic warming of the deep sea is underway because of anthropogenic climate change. The scientists report that the deep seas are taking in more heat than expected, which is taking some of the warming off the Earth's surface, but it will not do so forever.
"Some of the heat (from human-caused global warming) is going into melting sea ice and heating the surface, but the bulk is going into the oceans," said climate researcher Kevin Trenberth of the National Center for Atmospheric Research, a coauthor on a new research paper reporting on the deep ocean warming in the journal Geophysical Research Letters.
The study involved the bringing together of a diverse suite of data, ranging from satellite measurements of the surface waters to ship observations at all depths, instruments mounted on elephant seals, ARGO profilers (a large collection of small, drifting-robotic probes deployed worldwide), and data-gathering instruments moored in place. The data includes temperature, salinity, depth, and altimetry of the ocean surface, going back decades.
Piecing together different kinds of data from different times and sometimes from sparse data sets was the key challenge, Trenberth explains, but that is the specialty of his coauthors at the European Centre for Medium Range Weather Forecasts in the U.K "They have one of the most sophisticated data assimilation systems," Trenberth said. That has allowed for a new view of not only how the deep sea is heating up, but how winds and El Niño events play into it all.
Winds blowing on the oceans can drive water into the deep ocean as well as cause upwelling of deep waters, which can release massive amounts of heat. The 1998 El Niño year, for instance, was the hottest on record because the oceans were releasing a lot of heat from the ocean into the atmosphere, Trenberth explained to DNews.
The new re-analysis of ocean data is not the last word on what's happening in the deep seas, but the best estimate of what is happening.
"It's more than speculation and suggestion," agrees climate scientist Gavin Schmidt of NASA's Goddard Institute for Space Studies, "and it's probably right to a reasonable degree. The fact of the matter is we'll never be able to get data from below 400 meters in the middle of the Pacific Ocean" because there is not enough money invested in ocean sensors to cover such places. "So we have to use physics to fill in the gaps."
The bottom line, says Trenberth is that the heat of global warming is going to different places. "So global warming is continuing even though it's not always manifested as a strong surface temperature increase. It's just manifesting itself in different ways."