The world’s oceans recorded their second-highest average temperature for May, intensifying concerns among scientists that the planet’s largest carbon sink may be losing its capacity to buffer the worst effects of climate change, according to new data released by the EU’s Copernicus Climate Change Service.
The global average sea surface temperature reached 20.79°C last month — just 0.14°C below May 2024’s all-time record — capping what Copernicus described as an “alarming” two-year streak of rapid marine warming.
This development comes on the heels of a recent alert from the US National Oceanic and Atmospheric Administration (NOAA), which reported that atmospheric carbon dioxide concentrations had hit record seasonal highs. Globally, CO₂ levels peaked at 426 parts per million (ppm) in March, up from 423 ppm a year earlier, and even surpassed 430 ppm at Hawaii’s Mauna Loa Observatory. For context, levels were around 300 ppm just six decades ago.
The ocean has historically absorbed about 25–30% of atmospheric CO₂ and approximately 90% of the excess heat generated by greenhouse gas emissions. However, leading experts are now warning that this buffering capacity may be declining.
“We’re seeing stronger and stronger bursts of ocean warming — marine heatwaves — that are pushing us past records at an alarming rate,” said Michael Meredith of the British Antarctic Survey.
Julien Nicolas, senior scientist at Copernicus, noted that while the El Niño phenomenon partially fueled recent warming, the persistently elevated sea temperatures reflect a deeper, long-term trend. “As sea surface temperatures rise, the ocean’s capacity to absorb carbon diminishes,” he said. “This could accelerate the accumulation of greenhouse gases and intensify climate change.”
Some regions are being hit harder than others. In May, a marine heatwave off the coasts of the UK and Ireland surpassed previous records, while much of the Mediterranean Sea also experienced above-average temperatures. According to a recent Nature study, the record North Atlantic warming in summer 2023 was primarily caused by heat exchange with the atmosphere, not internal ocean dynamics — a trend likely to increase the frequency and severity of future heatwaves.
“This is global warming playing out before our eyes,” said Matthew England, co-author of the study and professor at the University of New South Wales. In some regions, sea surface temperatures are already 4°C above historical norms, he added.
Meanwhile, Copernicus’ broader climate data showed that May 2025 was the second warmest on record globally, although it marked the first month since July 2023 that average surface temperatures dipped slightly below the critical 1.5°C threshold over pre-industrial levels — coming in at 1.4°C above.
Weather extremes also persisted. North America, northern and central Europe, and parts of Asia were drier than normal, while southern Europe, the eastern US, and parts of Australia and Africa experienced above-average rainfall.
At the UN Ocean Conference in France this week, momentum continues to build for stronger marine protections. Fifty nations, including the EU bloc, officially ratified the High Seas Treaty, joining over 30 countries that previously committed to protect 30% of the world’s oceans by 2030.
As climate pressures mount, scientists and policymakers are increasingly calling for urgent action to strengthen ocean resilience, enhance monitoring, and curb emissions — warning that the time left to rely on the ocean as a safety net is running out.
