Paris (afp) — Oceans have absorbed the vast majority of the warming caused by burning fossil fuels and shielded societies from the full impact of greenhouse gas emissions.

But this crucial ally has developed alarming symptoms of stress — heatwaves, loss of marine life, rising sea levels, falling oxygen levels and acidification caused by the uptake of excess carbon dioxide.

These effects risk not just the health of the ocean but the entire planet.

By absorbing more than 90 percent of the excess heat trapped in the atmosphere by greenhouse gases, “oceans are warming faster and faster,” said Angelique Melet, an oceanographer at the European Mercator Ocean monitor.

The UN’s IPCC climate expert panel has said the rate of ocean warming — and therefore its heat uptake — has more than doubled since 1993.

Average sea surface temperatures reached new records in 2023 and 2024.

Despite a respite at the start of 2025, temperatures remain at historic highs, according to data from the Europe Union’s Copernicus climate monitor.

The Mediterranean has set a new temperature record in each of the past three years and is one of the basins most affected, along with the North Atlantic and Arctic oceans, said Thibault Guinaldo, of France’s CEMS research center.

Marine heatwaves have doubled in frequency, become longer lasting and more intense, and affect a wider area, the IPCC said in its special oceans report.

Warmer seas can make storms more violent, feeding them with heat and evaporated water.

The heating water can also be devastating for species, especially corals and seagrass beds, which are unable to migrate.

For corals, between 70 percent and 90 percent are expected to be lost this century if the world reaches 1.5 degrees Celsius (2.7 degrees Fahrenheit) of warming compared to pre-industrial levels.

Scientists expect that threshold — the more ambitious goal of the Paris climate deal — to be breached in the early 2030s or even before.