Ocean-surface temperatures are breaking records

Here’s what that means

HUMANS HAVE long used the ocean as a dumping zone. Piles of rubbish have accumulated in the sea and endangered marine life. But apart from plastic, oceans and their inhabitants also bear the brunt of human-made emissions and a warming planet. Oceans have soaked up about 90% of the excess heat caused by greenhouse-gas emissions in recent decades. One symptom of this has been a gradual increase in the temperature of surface waters, and this year’s rise has been particularly alarming. On April 1st, average global sea-surface temperatures reached 21.1°C (70.0°F). The new record is more than half a degree above the global average between 1982 and 2011 (see chart).

< View the graphics chart at the source page. >

Sea-surface temperatures are in fact measured about a metre below the surface by satellites, and the readings are confirmed by ships and buoys. Temperatures usually reach a high in March or early April, after summer in the southern hemisphere—where most of the Earth’s water is located—warms the seas. This year’s average sea-surface temperature started from a relatively high point compared with previous years, because 2022 was already a hot year for ocean-surface temperatures. But in 2023, unlike previous years, they have also risen for longer than usual.

There is a lot of head scratching going on among scientists as to what is causing the spike in sea-surface temperatures this year. It is difficult to attribute the changes to a specific effect, because sea-surface temperatures vary naturally, and the unusual nature of the spike is making many reluctant to diagnose its underlying cause.

That said, the biggest role in ocean-surface temperatures is played by the El Niño-Southern Oscillation (ENSO), a natural phenomenon that influences weather patterns around the world. After an unusually long period of La Niña, one of ENSO’s two extreme phases, the Earth returned to its neutral phase at the beginning of March, according to America’s National Oceanic and Atmospheric Administration, a federal body. Global average air temperatures tend to be cooler when La Niña is active, in part because changing wind patterns mean more heat is stored in the deeper layers of the ocean. As a La Niña comes to an end, some of that heat gets released to the surface. The end of a three-year-long La Niña, therefore, may have contributed to the record-breaking sea-surface temperatures measured this year.

On top of this, an El Niño event, the warm stage of ENSO, may be on the horizon. The latest prediction by America’s National Weather Service gives a 62% chance of ENSO tipping into El Niño between May and July this year. Scientists at the Met Office, Britain’s weather service, also think the high sea-surface temperatures could mean a large El Niño event may occur. Even a weak El Niño will bring higher air temperatures in the short term and will be in stark contrast to previous years, when cool-trending La Niñas have masked the underlying warming caused by accumulating greenhouse-gas emissions.

In the long term, warmer ocean waters, both at the surface and deep down, have far-reaching consequences. They lead to more rapid melting of the ice sheets, coral bleaching, stronger storms and higher sea levels—owing to melting ice and also because water expands as it heats up. Hotter seas also absorb less heat and less carbon dioxide, which accelerates warming of the atmosphere. The consequences of warmer waters will be felt on land, too.

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