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

An aerial view of the southern tip of Fongafale island in Tuvalu. The low-lying South Pacific island nation is particularly vulnerable to sea level rise, which is caused in part by warming ocean temperatures. 

(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.
      "The impacts are perhaps subtle but profound," Trenberth told CNN. "To stop this [trend], we really need to get to net-zero [emissions], and many countries have plans but not enough actions to support those. In the meantime, we must prepare better and build resilience."

      The far-reaching impact of warmer oceans

      Warmer oceans supercharge weather patterns to create more powerful storms, hurricanes and intense rainfall which leads to deadly flooding.
      The US witnessed devastating impacts from Hurricane Ida in Louisiana and Mississippi, as well as the record-breaking rainfall and deadly flooding its remnants brought as it tracked over the Northeast and New York City. In December, Super Typhoon Rai — known locally as Odette — claimed hundreds of lives in the Philippines during the holiday season.
      "Warm ocean provides fuel to storms, so ocean warming will naturally support stronger and longer-lasting storms," Cheng said.
      The rapid rise in ocean temperature threatens marine life and is a significant contributor to sea level rise.
      "Ocean warming — aside from causing coral bleaching and threatening sea life and fish populations that we rely upon for roughly 25% of our protein intake globally — is destabilizing Antarctic ice shelves and threatens massive meters of sea level rise if we don't act," Michael E. Mann, a climate scientist at Pennsylvania State University and one of the authors of the study, told CNN. "So this finding really underscores the urgency of climate action now."
      As the oceans heat up, the water expands and sea level rises. Cheng said about one third of the total sea level rise during the 20th century was due to ocean warming alone. In addition to more high-tide flooding, sea level rise threatens coastal freshwater supply with saltwater intrusion, and makes coastal communities and infrastructure more vulnerable to storm surge.
      Cheng said part of the reason action is so urgently needed is because the oceans will continue warming for decades after fossil fuel emissions are slashed.
      "Many countries have set goals to become carbon neutral around the middle of this century," he said. But "ocean warming will continue after 2060 and sea level will keep rising."
        Trenberth explained that although reaching net-zero emissions would end Earth's energy imbalance, "the oceans are slow to respond." Heat will continue to "percolate to greater depths; sea level keeps rising for a lot longer," he said.
        Even if countries make good on their pledges to reduce emissions soon, researchers say further ocean warming will continue and communities must prepare for the far-reaching consequences. The report's authors noted that the long-term risks that come with warming oceans — including sea level rise and more frequent flooding — should be accounted for in "engineering design, building codes, and modifications to coastal development plans."