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World - Oceans were the warmest on record in 2021, for the 3rd year in a row

Last year was the hottest on record for the world's oceans for the third year in a row, according to new research, part of a long-term upward trend in ocean temperature that scientists say is overwhelmingly due to planet-warming fossil fuel emissions.

(CNN)Last year was the hottest on record for the world's oceans for the third year in a row, according to new research, part of a long-term upward trend in ocean temperature that scientists say is overwhelmingly due to planet-warming fossil fuel emissions.

The annual study, published Tuesday in the journal Advances in Atmospheric Sciences, found the past five years have been the hottest five on record for the oceans, dating back to the late 1950s.

"We want to stress that global warming is actually ocean warming, and ocean warming has serious consequences," said Lijing Cheng, lead author on the report and climate and environmental science professor with the Institute of Atmospheric Physics at the Chinese Academy of Sciences. "Ocean warming keeps breaking records, which is a reminder that the world needs action to combat climate change."

Ocean heat is a better indicator of the climate crisis than air temperature because natural cycles like La Niña and El Niño play a "relatively small" role in ocean warming, according to Kevin Trenberth, an author on the report and scientist at the National Center for Atmospheric Research in Colorado. And last year was a record even despite La Niña conditions in the Pacific Ocean — a cooler-than-average temperature pattern in the ocean around the equator.

Ultimately, researchers say humans have been the dominant cause of the relentless warming trend. Greenhouse gas emissions from fossil fuels trap heat in the planet's atmosphere, creating an energy imbalance. The oceans, in turn, absorb 90% of the excess heat, which has led to an alarming increase in temperature.

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