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Ocean Heat Reveals More About Climate

Courtesy of NOAA

A chief question for many climate scientists continues to be: How much has Earth really warmed in recent decades? In a new study, an international group of scientists seeks answers from the world’s oceans. Their analysis in a new article published by the open-access journal Science Advances furthers the discussion.

Researchers want to know more about the ocean because it absorbs most of the planet’s heat. The ocean acts as a sponge for excess heat in Earth’s system, more than 90 percent of the excess. So, determining reliable estimates of ocean heat contributes significantly to global warming research.

Researchers from the Institute of Atmospheric Physics at the Chinese Academy of Sciences in Beijing, China, the National Center for Atmospheric Research in Boulder, NOAA’s NCEI, and the University of St. Thomas in Minnesota, took a look at the ocean’s heat history to approach the question of global warming.

Their work builds on recent investigations of water temperature data from the surface to 2,000 meters deep in the oceans, crucial in understanding Earth’s temperatures. The magnitude and location of ocean heat content have become an area of active scientific research.

Honing in on Ocean Temperature

Global ocean heat content (OHC) assessments come from subsurface temperature profiles taken over time from many locations around the ocean. But, because of its vastness, remoteness, and susceptibility to bad weather, the ocean has historically challenged even the most ardent scientific efforts at data collection. Since 1960, changes have occurred in collecting ocean temperatures that improve coverage and accuracy over time. But, variability in data coverage leaves scientists with the task of creating models and methodologies that accurately reflect what has and is happening to the ocean, and hence, any related repercussions to the planet.

For the recent study, the scientists scrutinized the connection between the ocean and Earth’s energy imbalance (EEI), which drives global warming. The ocean absorbs all but a small percentage of the EEI, so a measure of change in OHC equals a measure of the majority of excess heat in Earth’s system.

The researchers created new methodologies to follow heat content that extend and improve on existing mapping strategy. They built a gridded temperature field from the ocean surface to 2,000 meters below. They use temperature data from the beginning of 1960 through the end of 2015 and construct an ocean “energy budget” for that time period. The group looked farther back in the record but determined that it wasn't adequate enough prior to approximately 1960. The scientists relied on newer observations from a fleet of Argo floats to verify past conditions, taking into account variability and sampling error.

Study Reveals the Ocean Warms Up

From the mapping methodology, developed by lead author Lijing Cheng and coauthor Jiang Zhu of the Institute of Atmospheric Physics, the scientists found that monthly OHC changes indicated significant warming in the past 56 years. This falls in line with a stronger ocean warming trend that has existed since the late 1980s. The analysis indicates accelerated heat input into the oceans for the upper 700 meters and from 700 meters to 2,000 meters, although with the latter generally after 1990. Indeed, an upward trend in the OHC from 1991 to 2015 is four times stronger than the preceding years to 1960.

The findings indicated that at depths below 700 meters the ocean may play an important role in heat absorption, starting from 1998. The study, along with another recent study in 2016 (Yan et. al), coauthored by two scientists on the present paper, both pose uncertainties about where in the ocean the Earth’s excess heat may be going.

Graph of ocean heat content from 1960 to 2015 by basin
These figures show ocean heat content changes from 1960 to 2015 for different ocean basins by depth in meters. (A) 0–2,000 meters, (B) 0–700 meters, and (C) 700–2,000 meters. Courtesy of authors and Science Advances.
Access a  larger version of the graph.

According to the new paper, ocean warming occurs by differing amounts geographically. “It is evident that all six ocean basins have experienced significant warming since 1998 but that heat was mainly stored in the southern oceans, the tropical/subtropical Pacific Ocean, and the tropical/subtropical Atlantic Ocean from 1960 to 1998,” the authors write.

The Atlantic Ocean shows the largest increase, 3.5 times greater than the Pacific Ocean. The authors describe this geographic long-term warming as “continuous and monotonic” and reveals a “robust footprint of global warming.”

The authors note that understanding how this heat has been transported or redistributed into the ocean remains an important research topic for future study. Additionally, the researchers recommend further analysis of temperatures in deep oceans where, prior to 1980, change is less understood.