The main reason soaring greenhouse gas emissions have not caused air temperatures to rise more rapidly is that oceans have soaked up much of the heat. But new evidence suggests the oceans’ heat-buffering ability may be weakening.

For decades, the earth’s oceans have soaked up more than nine-tenths of the atmosphere’s excess heat trapped by greenhouse gas emissions. By stowing that extra energy in their depths, oceans have spared the planet from feeling the full effects of humanity’s carbon overindulgence.

But as those gases build in the air, an energy overload is rising below the waves. A raft of recent research finds that the ocean has been heating faster and deeper than scientists had previously thought. And there are new signs that the oceans might be starting to release some of that pent-up thermal energy, which could contribute to significant global temperature increases in the coming years.

The ocean has been heating at a rate of around 0.5 to 1 watt of energy per square meter over the past decade, amassing more than 2 X 10²³ joules of energy — the equivalent of roughly five Hiroshima bombs exploding every second — since 1990. Vast and slow to change temperature, the oceans have a huge capacity to sequester heat, especially the deep ocean, which is playing an increasingly large uptake and storage role.

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