Oceans have long been a critical shield for the planet, absorbing the vast majority of the warming caused by burning fossil fuels. By soaking up excess heat and carbon dioxide, they have protected societies from the full impact of greenhouse gas emissions.

But now, this vital ally shows signs of serious stress. Rising ocean temperatures, loss of marine life, increasing sea levels, falling oxygen levels, and acidification are among the alarming changes scientists are observing. These changes not only threaten the health of the oceans but the entire Earth’s climate system.

Heating up faster than ever

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.

According to the United Nations’ Intergovernmental Panel on Climate Change (IPCC), the rate of ocean warming has more than doubled since 1993.

Sea surface temperatures set new records in 2023 and 2024. Although there was a slight cool down early in 2025, temperatures remain historically high, based on data from the European Union’s Copernicus climate monitor.

The Mediterranean Sea has broken its temperature record for three consecutive years and is among the most affected areas, along with the North Atlantic and Arctic oceans, explained Thibault Guinaldo of France’s CEMS research center.

Marine heatwaves are becoming more frequent, longer lasting, and more intense while spreading over larger areas, the IPCC highlighted in its special oceans report.

Warmer seas also fuel more powerful storms by providing them with heat and moisture. This warming can devastate marine species such as corals and seagrass beds that cannot relocate.

Scientists warn that if the world reaches 1.5 degrees Celsius of warming compared to pre-industrial levels, which is the target of the Paris climate agreement, between 70 percent and 90 percent of corals could be lost this century. That threshold may be crossed in the early 2030s or sooner.

Rising seas threaten millions

When water warms, it expands and takes up more space. This thermal expansion, combined with the melting of ice caps and glaciers worldwide, is causing sea levels to rise.

The pace of ocean rise has doubled over the past 30 years and could double again by 2100, reaching about one centimetee per year, recent research shows.

About 230 million people live less than one meter above sea level and face growing risks from flooding and storms.

"Ocean warming, like sea-level rise, has become an inescapable process on the scale of our lives, but also over several centuries," said Melet.

She added, "But if we reduce greenhouse gas emissions, we will reduce the rate and magnitude of the damage, and gain time for adaptation."

Acidification and oxygen loss threaten marine life

The ocean has absorbed 20 to 30 percent of human carbon dioxide emissions since the 1980s. This has caused the waters to become more acidic, weakening corals and making it harder for shellfish, crustaceans, and certain plankton to build their skeletons, according to the IPCC.

"Another key indicator is oxygen concentration, which is obviously important for marine life," Melet said.

Oxygen levels are dropping due to multiple causes, including the warming waters themselves.

Sea ice decline accelerates warming

Sea ice covering the Arctic and Antarctic oceans dropped to a record low in mid-February, shrinking more than a million square miles below the pre-2010 average.

Less sea ice means more solar energy is absorbed by the ocean instead of being reflected back into space. This warms the water further, causing even more ice to melt.

This process contributes to “polar amplification,” a phenomenon that makes global warming more intense and rapid at the poles, Guinaldo explained.