16th January 2021

Upper ocean temperatures hit record high in 2020

New record high temperatures in the world's oceans have been reported, measured from the surface level down to a depth of 2,000 metres.

Even with a COVID-19-related dip in global carbon emissions due to limited travel and other activities, the world's oceans continued a trend of breaking temperature records in 2020. A new study, by 20 scientists from 13 institutes around the world, reports the highest ocean temperatures in recorded history from surface level to a depth of 2,000 metres (6,561 ft).

The report, published this week in the journal Advances in Atmospheric Sciences, concludes with a plea to policymakers and others to consider the lasting damage warmer oceans can cause as they attempt to mitigate the effects of climate change.

"Over 90% of the excess heat due to global warming is absorbed by the oceans, so ocean warming is a direct indicator of global warming – the warming we have measured paints a picture of long-term global warming," said Lijing Cheng, PhD, from the Institute of Atmospheric Physics at the Chinese Academy of Sciences. "However, due to the ocean's delayed response to global warming, the trends of ocean change will persist at least for several decades, so societies need to adapt to the now unavoidable consequences of our unabated warming. But there is still time to take action and reduce our emissions of greenhouse gases."

Cheng and his colleagues developed a method for calculating the temperatures and salinity of the oceans down to 2,000 metres using data taken from all available observation devices in the World Ocean Database. These detailed measurements are overseen by the National Oceanic Atmospheric Administration and the National Center for Environmental Information.

The researchers found that, during 2020, the upper 2,000 metres of the world's oceans absorbed 20 more zettajoules than in 2019. To give an idea of how astronomically large that number is, 20 zettajoules can be written as 20,000,000,000,000,000,000,000 joules. That amount of heat could boil 1.3 billion kettles, each containing 1.5 litres of water.

"Why is the ocean not boiling?" Cheng asked. "Because the ocean is vast. We can imagine how much energy the ocean can absorb and contain, and, when it's released slowly, how big the impact is."

The study has reported other impacts – such as ocean salinity pattern amplification, and more stratification due to the upper layer warming quicker than the deeper sections. Both changes could cause harm to ocean ecosystems.

"The fresh gets fresher; the salty gets saltier," Cheng said. "The ocean takes a large amount of global warming heat, buffering global warming. However, associated ocean changes also pose a severe risk to human and natural systems."

Cheng noted the 2020 wildfires that ravaged Australia, parts of the Amazon region, and the west coast of the United States.

"Warmer oceans and a warmer atmosphere also promote more intense rainfalls in all storms, and especially hurricanes, increasing the risk of flooding," Cheng said. "Extreme fires like those witnessed in 2020 will become even more common in the future. Warmer oceans also make storms more powerful, particularly typhoons and hurricanes."

Cheng and his team will continue to monitor the ocean temperatures and the impacts warming has on other oceanic characteristics, such as salinity and stratification.

"As more countries pledge to achieve carbon neutrality in the coming decades, special attention should be paid to the ocean," Cheng said. "Any activities or agreements to address global warming must be coupled with the understanding that the ocean has already absorbed an immense amount of heat, and will continue to absorb excess energy in the Earth's system until atmospheric carbon levels are significantly lowered."

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