In the first week of 2021, 2020 was declared the joint-warmest year ever recorded, tying with 2016 for this dubious distinction. A new report now establishes that the world’s oceans heated up to record levels in the past year as well. The report, Upper Ocean Temperatures Hit Record High In 2020, was published on 13 January in the Advances In Atmospheric Sciences journal. Prepared by a team of scientists from the US, China and Italy, the report states that not only was 2020 the hottest year on record for the global ocean, but also that the five hottest years have occurred since 2015. The researchers also found that the ocean heat content (OHC) of the upper 2,000m has been increasing since the 1950s, and has seen a sharp spike since the 1980s. The researchers found that between 1986-2020, the ocean heated up at a rate that is eight times higher than between 1958-85.
[4/8] The geo-pattern of 2020 OHC relative to 1981-2010 shows warming throughout most of the world’s oceans with higher rates of warming in both the northern and southern Atlantic (except southeast of Greenland), and in localized zones of the Pacific, Indian and Southern Oceans pic.twitter.com/ZXqcOc0usP— Lijing Cheng (@Lijing_Cheng) January 13, 2021
It has been well established that the global ocean has absorbed over 90% of the excess heat generated due to man-made climate change since at least 1970. This has resulted in a variety of climate risks, from stronger tropical storms to marine heatwaves and coral bleaching, and dwindling fish stocks as acidity levels rise in the ocean. Most of these effects were starkly visible in 2020. There were a record 29 tropical hurricanes in the Atlantic Ocean, while cyclones Amphan, Nisarga and Nivar battered India. Marine heatwaves off Australia and in the northern Indian Ocean caused major bleaching events in the Great Barrier Reef and in coral reefs in the Gulf of Mannar in the Bay of Bengal.
As the report points out, last year’s record ocean heat was despite the fact that 2020 saw the onset of the cooling La Niña (a weather pattern over the Pacific Ocean) conditions six months into the year. This is certainly a marker of longer-term climate change impacts. Analysing long-term data, the study found that in the northern Indian Ocean (comprising the Arabian Sea and the Bay of Bengal), the OHC started rising after 2000, partly due to increased heat coming in from the Pacific Ocean. The report warns that on top of the heat already present in the ocean, further emissions will mean excess heat will continue to enter the ocean. “In other words, the excess heat already in the ocean, and heat likely to enter the ocean in the coming years, will continue to affect weather patterns, sea level, and ocean biota for some time, even under zero carbon emission conditions,” it states.
The report dovetails with another study published recently in the Annual Reviews journal, stating that the global ocean is the hottest it has been for the past 1,000 years, and heating faster than at any time in the past 2,000 years.