National Oceanic and
Atmospheric Administration
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Ocean warming trends dwarf cooling trends

Four global maps showing ocean heat anomaly trends for 1993-2019 with red to blue color bar

This figure shows linear trends for upper (0–700 m) ocean heat content anomalies for 1993–2019 for annual estimates of (a) the PMEL combined maps, and the in situ-only maps of (b) JMA, (c) IAP, and (d) NCEI

July 13, 2020

The ocean is the flywheel that propels the climate system, storing huge amounts of heat and carbon in a vast reservoir that covers 70 percent of Earth’s surface. Measuring changes in that heat content over time has been a subject of intense scientific research, with analyses growing more precise as observing systems expand and the duration of measurements grows. In a new article published in the journal Nature Climate Change, Gregory Johnson and John Lyman with PMEL's Large Scale Ocean Physics group analyzed four prominent ocean temperature datasets and found that over the past 52 years, the number of ocean regions showing long-term warming trends dwarfs the regions showing cooling trends.

“Ocean warming is tightly linked to increases in atmospheric greenhouse gases concentrations, so global ocean temperature trends are an important yardstick for measuring climate change,” said Johnson. “We noticed that as the time-series of upper ocean heat content we were analyzing grew over the years, more of the ocean area was covered by a statistically significant warming trend.”

Johnson and Lyman decided to step back and look at how temperature trends changed when estimated over varying time periods. To investigate those questions, the pair generated annual maps of ocean heat content anomalies in the upper 700 meters of ocean from 1993 to 2019 by combining sea-level data from satellite altimeters with ocean temperature data. They also analyzed annual maps of ocean temperature data from the upper 700 meters from 1968 to 2019 from three other research groups: NOAA’s National Center for Environmental Information, the Japan Meteorological Agency, and the Chinese Academy of Science’s Institute of Atmospheric Physics. 

For the 27-year period 1993 to 2019, they found 53 percent of the global ocean exhibited statistically significant warming trends versus 3 percent of the ocean showing significant cooling trends. For the 52-year period from 1968 to 2019, the imbalance grew markedly: 72 to 79 percent of the ocean area showed warming, while only 1 to 3 percent exhibited cooling. When trends were estimated over shorter 5-year periods, the areas and imbalances were much smaller, with 24% warming and 17% cooling. 

While the ocean has soaked up the overwhelming majority of greenhouse warming, it hasn’t come without a cost. Warmer water occupies more space than cooler water, and about 42 percent of sea level rise documented since 1993 is due to thermal expansion caused by global warming. Rising ocean temperatures are also associated with increased marine heatwaves, which in turn can devastate coral reefs, dislocate or disrupt distributions of important fish species, and increase vertical ocean stratification, which reduces nutrients in surface water, and decreases dissolved oxygen at depth, both of which affect ecosystems, fisheries, and possibly even ocean carbon storage. Rising ocean temperatures can also influence changes in the climate and promote rapid intensification of tropical cyclones. 

Read the full article on NOAA Research