By the end of March, the surface temperature of the world's oceans was above anything seen in the 40 years that satellites have been measuring it. Records were "headed off the charts" and, as the heat refused to dissipate for more than a month, the Earth had entered "uncharted territory", scientists said.
The temperature at the ocean's surface - like that on land - is being pushed higher by global heating, but can jump around from one year to the next as weather systems come and go. But in the 2km below the surface, things are different.
The temperature down there has been on a relentless climb for decades.
"The heat-holding capacity of the ocean is mammoth," says Dr Paul Durack, a research scientist specialising in ocean measurements and modelling at the US Department of Energy's Lawrence Livermore National Laboratory. "The ocean captures more than 90% of the imbalance of energy that we're creating because of anthropogenic climate change." The ocean soaks up more of the direct energy from sunlight than land does. But as greenhouse gases trap more of the energy that's reflected allowing less to escape to space - the ocean tries to balance itself with the heat in the atmosphere above. The latest UN climate assessment laid out the heat gain. Between 1971 and 2018, the ocean had gained 396 zettajoules of heat.
How much is that? Scientists have calculated it is equivalent to the energy of more than 25bn Hiroshima atomic bombs. And that heat gain is accelerating.
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