“When you have this long-term upward trend, you’re getting records broken almost every year, and it’s this monotonous increase,” said John Abraham, a co-author of the study and a professor at the University of St. Thomas in Minnesota. “We’ve built up so much greenhouse gas that the oceans have begun to take in an increasing amount of heat, compared to what they previously were.”
The team analyzed data from a worldwide network of buoys in seven ocean basins. Overall, it found that the upper 2,000 meters (6,562 feet) of Earth’s oceans absorbed more than 227 excess zettajoules of energy, compared with the 1981-2010 average. Last year broke the previous record set in 2020 by at least 14 zettajoules.
Additionally, the team found that ocean waters have been steadily warming since 1958, with each decade warmer than the last. Warming has significantly increased since the 1980s. Over recent decades, portions of the Atlantic Ocean, Pacific Ocean and Indian Ocean have warmed the most.
Through climate model experiments, the researchers showed that the warming pattern since 1979 was mainly attributed to increased anthropogenic greenhouse gas concentrations. The longer-term trends brought on by human activity are also overpowering short-term climate fluctuations, such as La Niña and El Niño, which can have regional effects.
“Ocean stores more than 90 percent of the Earth’s net heat gain due to greenhouse gases. Thus, ocean warming is a fundamental indicator of the climate change,” Lijing Cheng, lead author and associate professor at the Chinese Academy of Sciences, wrote in an email. “The record ocean warming in 2021 is strong evidence that global warming continues.”
The 2021 record isn’t surprising, said ocean researcher Linda Rasmussen, who was not involved in the study. Mainly, Rasmussen said, that is because the major driver of ocean warming has not changed.
“The accelerating increase in anthropogenic greenhouse gases that is warming the atmosphere continues apace,” Rasmussen wrote in an email. “Because the ocean still absorbs the vast majority of the excess heat, it would be surprising if the trend didn’t continue.”
Last year, the record warmth manifested in several extreme weather events. Warmer water provides more energy, or fuel, for tropical storms, increasing their intensity and longevity. Following a record-breaking 2020 Atlantic hurricane season, 2021 brought another intense stretch of storms.
Hurricane Ida caused intense flooding and thunderstorms, ranking as the fifth-most expensive hurricane on record, with damage estimated at $75 billion. Hurricane Nicholas and Tropical Storms Elsa and Fred also inflicted billions of dollars’ worth of damage.
The increase in ocean heat also raises air temperatures, allowing more moisture to enter the warmer atmosphere. For every 1.8 degrees of warming, heavy rain events will intensify by about 7 percent. 2021 marked one of the wettest years on record for the East Coast, thanks to a slew of tropical storms and summer thunderstorms.
The unusual December tornadoes that struck several states can also be traced to the warm waters. In December, record warm temperatures in the Gulf of Mexico created an atmosphere more reminiscent of spring than winter. As such, two tornado outbreaks occurred in the southern and central United States in the same week.
The influx of ocean heat is also affecting the frequency and severity of marine heat waves, or a period of unusually high ocean temperatures, Rasmussen said. For instance, a series of marine heat waves have occurred in the North Pacific — including the infamous “blob” in 2014-16 that disrupted food chains by reducing phytoplankton productivity, starving many animals. The North Pacific was home to another warm “blob” in 2021, a continuation from an anomaly in 2020.
Like hurricanes or droughts, marine heat waves can be difficult to tie to one specific cause. However, Rasmussen said, observations and analysis of more than 100 years of data in Southern California have shown that marine heat waves in the region have become more severe with time.
“The coastal ocean temperatures that have broken records repeatedly in recent years would not have broken records without the underlying warming trend that has been in place for many decades,” wrote Rasmussen, a retired researcher at Scripps Institution of Oceanography. “Marine heat waves are one of the phenomena that are expected to increase as the ocean as a whole warms.”
Outside of extreme weather events, ocean warming is also affecting sea level rise. As the ocean warms, the water expands and raises sea levels. Last year, sea levels continued rising. Global sea levels are expected to continue rising at a rapid pace in coming decades through thermal expansion as well as contributions from glaciers.
“Ocean warming is destabilizing Antarctic ice shelves from underneath, which could lead to the collapse of large pieces of the ice sheet such as the Thwaites glacier, threatening massive . . . sea level rise,” Michael Mann, a co-author of the study and a climate scientist at Pennsylvania State University, wrote in an email. “This finding really underscores the urgency of acting on climate now.”
Despite the record ocean warmth, scientists with the Copernicus Climate Change Service of the European Union announced that global surface air temperatures in 2021 were the fifth-warmest on record. Surface air temperatures for the past seven years were the hottest on record, with 2020 and 2016 tied for the warmest.
Abraham said air and land temperatures tend to fluctuate much more than the ocean’s. Like air temperatures around your house fluctuating on a day-to-day or weekly basis, air temperatures around the planet experience similar changes. To tease out larger climate trends against natural variability, scientists analyze surface air temperatures on the scale of decades.
Water, however, is much denser than air and holds heat much better than the atmosphere. It takes a much longer time for the oceans to either cool down or heat up, especially given that they cover more than 70 percent of Earth’s surface. Abraham and his colleagues were able to discern climate trends from natural variability with less than four years of data.
“If you want to know how fast the Earth is warming, you have to measure the oceans,” Abraham said. “Since most of the global warming heat ends up in the oceans, we like to say that ‘global warming is ocean warming.’ ”