The rate of ocean warming has nearly doubled since 1992 compared with the previous three decades. And the warming has reached deeper waters, scientists reported Friday.
The findings are important because the world's oceans provide one of the best records of the excess energy trapped on Earth by increased greenhouse gases, largely from the burning of fossil fuels. As the seas heat up from climate change, the water expands and rises, causing coastal flooding and, in Antarctica, ice shelves to disintegrate.
"From this [study] we can better understand the effects of natural and man-made variability to the climate system," said co-author Tim Boyer of NOAA's Ocean Climate Laboratory. "Decision-makers can gauge what needs to be done to ameliorate the situation, or, if not that, to plan for the consequences of the excess heat." The study was published in the journal Science Advances.
The researchers, from NOAA, the National Center for Atmospheric Research, the University of St. Thomas and the Chinese Academy of Sciences, found that between 1960 and 2015 total ocean warming was 13 percent greater than the most recent estimate by the Intergovernmental Panel on Climate Change (IPCC), whose periodic reviews influence the actions of governments.
After 1992 the rate at which the oceans warmed nearly doubled. Most of the warming is in the tropical Atlantic and Pacific Oceans, as well as the Southern Ocean, surrounding Antarctica.
According to the study, ocean warming now accounts for as much as 50 percent of global sea level rise. That's compared to 30 to 40 percent estimated by the IPCC and other studies, said co-author Kevin Trenberth, a climate scientist with the National Center for Atmospheric Research.
"This directly affects our understanding of sea level," Trenberth said. "And the regional information is critical for climate forecasts and understanding future global warming impacts."
The researchers used data from a network of 3,500 robotic floats that measure ocean temperatures and salinity to a depth of 2,000 meters. The Argo floats, operated by the World Climate Research Program, were first used in 2000. About 800 floats are now deployed every year, widely dispersed across the world's oceans.
The readings helped the scientists correct and validate temperature records from older, less reliable measurements and also enabled them to fill in gaps in geography and time, creating a "remarkably accurate record going back to 1960," according to Trenberth.
They found that the warming accelerated in the mid-1970s. In the 1990s, the warming spread to deeper waters, from 700 to 2,000 meters. The biggest increases in ocean heat content were in those deeper layers, showing "that the deep ocean has played an increasingly important role in the ocean energy budget since 1998," according to the study. The Atlantic Ocean heat content increase was about 3.5 times greater than the Pacific, despite being less than half the size.
"The Southern Ocean seems to be warming as much or more as just about anywhere," Trenberth explained. "Strong winds in the region play a key role. They pull the surface water away from Antarctica and push the warmer water down. That creates a return flow of warmer, saltier water toward Antarctica, where it's eroding ice shelves from beneath."
The world's oceans absorb more than 90 percent of the heat trapped by greenhouse gases, storing it for centuries. Eventually some of the heat is released to the atmosphere and warms adjacent land masses.
The ocean warming directly influences ocean ecosystems and seasonal currents.
"Higher sea surface temperatures are continually reinforced by the extra sub-surface heat, and hence the ocean influences surface weather and climate especially through more intense rains," the study said. Ocean warming also strengthened the 2015-2016 El Niño and contributed to record global heat in 2016. "The result was an all time record hurricane season in 2015 and record heat-waves, droughts and wildfires around the world."
In some areas, the heat build-up is forming a dense layer of oxygen-poor surface water, which affects ocean organisms like plankton. That layer prevents cooler, nutrient-rich water from reaching the surface.
The increased warming in deeper waters is a particular concern for marine life, said Stephanie Henson, a senior scientist at the UK's National Oceanography Centre.
"Anything contributing to ocean warming will affect marine ecosystems," said Henson, who co-authored a March 7 study in Nature Communications on global warming impacts to ocean life. Her research found that global warming will affect 86 percent of the world's oceans by 2050 if greenhouse gas emissions aren't dramatically cut soon.
The two studies are not directly related, but the deeper warming tracked by Trenberth's team is "interesting for humans from a food point of view," Henson said. Large tuna and other commercially valuable fish, as well as keystone species like sharks and whales, spend parts of their lives in that zone, where temperatures have generally been stable for thousands of years.