The ocean is hotter and more acidic than ever

The ocean is the planet's great heat sink, storing roughly 90% of the excess energy that greenhouse gases trap. That makes ocean heat content one of the clearest and least noisy indicators of global warming — and it keeps breaking records.

A ninth straight record

2025 set another record for upper-ocean heat, the ninth consecutive record year, with the top 2000 m gaining energy measured in tens of zettajoules. Because water has enormous heat capacity, this stored energy fuels stronger marine heatwaves, more intense storms, and further sea-level rise through thermal expansion.

The other CO₂ problem: acidification

The ocean also absorbs roughly 29% of human CO₂ emissions. When CO₂ dissolves in seawater it forms carbonic acid, and surface pH has already fallen about 0.1 units since the industrial era — equivalent to a 30–40% increase in acidity.

Because pH is logarithmic, a shift of 0.1 units is a very large chemical change.

That change makes it harder for corals, shellfish and plankton to build their calcium-carbonate skeletons and shells, threatening food webs that billions of people depend on. Warming and acidification together are why coral reefs have suffered repeated mass-bleaching events.

A slow-motion shift

Neither indicator swings much from year to year, which is exactly why scientists trust them. Both point steadily in the same direction, and both will keep moving until CO₂ emissions stop.