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The world’s ocean temperatures reached the second-highest level on record for May, capping an “alarming” two-year streak of rapid warming and fuelling concerns about the seas’ ability to absorb rising levels of carbon dioxide.
The EU earth observation service Copernicus said the global average sea surface temperature in May was 20.79C, 0.14C below the record in the same month in 2024.
The findings follow an alert from the US National Oceanic and Atmospheric Administration in recent days that atmospheric concentrations of carbon dioxide had hit their highest seasonal peak since records began.
The level of CO₂ in the atmosphere peaked globally at the mean of 426 parts per mn in March, up from 423 ppm a year ago, and surpassed 430 ppm at the Mauna Loa Observatory in Hawaii. The greenhouse gas concentration has risen from about 300 ppm over the past 60 years.
Scientists estimate the ocean has absorbed between a quarter and about 30 per cent of carbon dioxide released into the atmosphere and about 90 per cent of excess heat, helping to keep temperatures on land cooler.
But Michael Meredith, a scientist at the British Antarctic Survey in Cambridge, warned there were growing concerns that the ocean “may lose some of its capacity to buffer us against the worst extremes of climate change” by absorbing less warmth or carbon than previously.
“We’re seeing stronger and stronger bursts of ocean warming — marine heatwaves — that are pushing us past records at an alarming rate.”
Copernicus senior scientist Julien Nicolas said that while the 2023-24 marine warmth “was partly driven” by El Niño, a naturally occurring warming phenomenon across the Pacific, the fact that sea temperatures had continued at very high levels “underscores the long-term warming trend”.
“As sea surface temperatures rise, the ocean’s capacity to absorb carbon diminishes, potentially accelerating the build-up of greenhouse gases in the atmosphere and intensifying future climate warming,” he said.
While the sea surface temperature average was below the 2023 peak in late May and early June, records are still occurring in some parts of the ocean.
In the north Atlantic off the coast of the UK and Ireland, a marine heatwave developed in May that surpassed the previous peak. Most of the Mediterranean was also much warmer than average for May, Copernicus said.
A study published in Nature this month said the extreme warming in the north Atlantic Ocean during the summer of 2023 was driven by the exchange of heat between the ocean and the atmosphere, rather than heat transfer within the ocean. The researchers said such heatwaves were likely to worsen.
Matthew England, one of the study authors and Scientia professor at the University of New South Wales, said the sea surface temperature was already 4C above average in some areas around the UK and Ireland. This demonstrated “global warming playing out before our eyes”, he said.
At the UN Ocean conference in France this week, 50 parties including the EU bloc ratified the high seas treaty. More than 30 countries had already agreed a target in 2022 to protect 30 per cent of the world’s oceans by 2030.
In its latest monthly update, Copernicus also said surface temperatures remained high last month, ranking as the second warmest on record. But it marked the first month since last July where the average surface temperature rise was slightly below the threshold of 1.5C above pre-industrial times, coming in at 1.4C above the historical levels.
Much of North America, the Horn of Africa, central Asia, northern and central Europe, as well as southern regions of Russia, Ukraine and Turkey, were drier than usual in May. In contrast, it was wetter than average in most of southern Europe, the eastern US, south-eastern Africa and eastern and north-western Australia.
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