In this article
The oceans cover about 71 percent of Earth’s surface. They store and move enormous amounts of heat and carbon, which makes them one of the most important parts of the climate system, and one of the least visible.
Key facts
- More than 90% of the extra heat trapped by greenhouse gases since the 1970s has gone into the oceans, according to the IPCC.
- 29% of human carbon dioxide emissions over the past decade were absorbed by the ocean (Global Carbon Budget 2025).
- About 30% more acidic: surface ocean pH has fallen by about 0.1 since the Industrial Revolution.
- About 10 cm: the rise in global average sea level since satellite records began in 1993.
A giant store of heat
Water can absorb far more heat than air. The oceans have taken up more than 90 percent of the extra heat trapped by greenhouse gases since the 1970s, according to the Intergovernmental Panel on Climate Change. Without this, air temperatures would have risen much faster.
The heat does not disappear. Warmer seas fuel stronger storms, stress coral reefs through bleaching, and cause marine heatwaves that harm fish and other wildlife.
Absorbing carbon, and becoming more acidic
The ocean also absorbs carbon dioxide from the air. The Global Carbon Budget 2025 estimates that it took up 29 percent of human emissions over the past decade. That slows the build-up of carbon dioxide in the atmosphere.
When carbon dioxide dissolves in seawater it forms a weak acid. Surface ocean pH has fallen by about 0.1 units since the Industrial Revolution, which on the logarithmic pH scale means the water is about 30 percent more acidic, according to NOAA. That makes it harder for shellfish, corals and some plankton to build their shells and skeletons.
Moving heat around the world
Ocean currents act like a conveyor belt, carrying warm water toward the poles and cold water toward the equator. They help explain why western Europe is much milder than parts of Canada at the same latitude. Scientists are watching whether warming and melting ice could slow some of these currents, which would change regional climates.
Rising seas
Sea level rises for two main reasons: warmer water expands, and melting glaciers and ice sheets add water. Since satellite measurements began in 1993, global average sea level has risen by about 10 centimeters, and the yearly rate roughly doubled over that period. The pace varies from year to year: NASA reported that sea level rose about 0.59 cm in 2024 but only about 0.08 cm in 2025, when a La Nina pattern temporarily moved water onto land as rain.
| Indicator | Change |
|---|---|
| Share of excess heat absorbed by oceans | More than 90% |
| Share of CO2 emissions absorbed (past decade) | 29% |
| Surface ocean pH since pre-industrial times | Down about 0.1 (about 30% more acidic) |
| Global sea level since 1993 | Up about 10 cm |
How scientists watch the oceans
About 4,000 robotic Argo floats drift through the world’s oceans, sinking to 2,000 meters and rising again every ten days to measure temperature and salinity. Together with satellites, ships and moored buoys, they give scientists a continuous picture of how the ocean is changing.
The Science Behind Weather Forecasts
Why it matters to you
Changes at sea affect food supplies, coastal homes, weather and even trade routes; our guide to shipping chokepoints shows how much of the world economy travels by sea. Ocean conditions also shape the forecasts described in the science behind weather forecasts.
Marine heatwaves
Just as land has heatwaves, parts of the ocean can stay unusually warm for weeks or months. Marine heatwaves have become more frequent as the oceans warm. They can bleach coral reefs, drive fish away from traditional fishing grounds and kill seabirds and marine mammals that depend on them. The record-warm northern oceans of 2023 and 2024 are among the most striking recent examples, and they also affected how much carbon dioxide the ocean absorbed.
Frequently asked questions
Is the ocean running out of room for carbon?
The ocean will keep absorbing carbon dioxide, but warmer water holds less gas and changes in circulation can slow uptake. The Global Carbon Budget notes that the ocean sink has been roughly flat since 2016.
How much could sea levels rise this century?
It depends mainly on future emissions and on how quickly the Greenland and Antarctic ice sheets respond. Scientific assessments range from tens of centimeters to more than a meter by 2100 under high emissions.
