Climate change is reshaping the ocean in nearly every measurable way. The ocean absorbs more than 90% of the excess heat trapped by greenhouse gases, and it has taken in so much carbon dioxide that its chemistry is shifting. The result is a cascade of changes: warmer water, rising seas, less oxygen, more acidic conditions, and disrupted ecosystems from the surface to the deep.
The Ocean Is Heating Up Fast
Because seawater is so effective at absorbing heat, the ocean has quietly shouldered the vast majority of global warming. In 2025, the ocean absorbed an additional 23 zettajoules of heat energy, breaking the ocean heat content record for the ninth consecutive year. That’s the longest streak of consecutive records ever documented. To put a zettajoule in perspective, one zettajoule is roughly the amount of energy the entire United States uses in a year. The ocean absorbed 23 times that in a single year on top of what it already held.
This warming isn’t evenly distributed. Surface waters heat up first, but the excess energy is gradually mixing into deeper layers, meaning the effects will persist for centuries even if emissions stopped today. Warmer surface temperatures also fuel stronger hurricanes, alter rainfall patterns over land, and create the conditions for marine heatwaves.
Marine Heatwaves Are More Common
Marine heatwaves are periods when ocean temperatures in a region spike well above normal for days to months at a time. They’ve become dramatically more frequent. Between 1925 and 2016, average marine heatwave frequency increased by 34% and their duration grew by 17%, resulting in a 54% increase in total marine heatwave days globally. Satellite data from 1982 onward shows this trend is accelerating.
These events are devastating for marine life. A prolonged marine heatwave can wipe out kelp forests, trigger mass coral bleaching, force fish populations to migrate to cooler waters, and kill seabirds that depend on specific prey species. The 2014-2016 “Blob” in the northeastern Pacific, for example, collapsed fisheries, starved sea lions, and spread toxic algal blooms along the U.S. West Coast.
Sea Levels Are Rising and Accelerating
Global sea level is rising at about 3.3 millimeters per year on average, but that number masks a sharp acceleration. The rate was about 2.1 millimeters per year in 1993. By 2024, it had more than doubled to 4.5 millimeters per year. Total sea level rise from 1993 to 2023 was 11.1 centimeters, roughly 4.4 inches.
Two forces drive this rise. First, warming water expands, a process called thermal expansion that accounts for roughly a third of recent sea level rise. Second, melting glaciers and ice sheets in Greenland and Antarctica are adding enormous volumes of freshwater to the ocean. For coastal communities, the practical consequence is that storm surges reach further inland, flooding happens more often during high tides, and saltwater intrudes into freshwater supplies. Even a few centimeters of rise can transform a once-in-a-century flood into a once-in-a-decade event.
Ocean Acidification
The ocean absorbs roughly a quarter of all the carbon dioxide humans emit. That sounds helpful, and it does slow atmospheric warming, but it comes at a steep cost. When CO₂ dissolves in seawater, it forms carbonic acid. Since the start of the industrial era, the pH of surface ocean waters has dropped by 0.1 units. Because the pH scale is logarithmic, that small-sounding number translates to a 30% increase in acidity.
This shift is especially harmful to creatures that build shells or skeletons from calcium carbonate: oysters, mussels, sea urchins, some plankton, and corals. More acidic water makes it harder for these organisms to form and maintain their structures. Young shellfish are particularly vulnerable. In the Pacific Northwest, oyster hatcheries have already experienced large-scale die-offs of larvae linked to more acidic water reaching the coast. At the base of the food web, tiny shell-building plankton called pteropods are showing signs of shell dissolution in the Southern Ocean, with potential ripple effects up the food chain.
The Ocean Is Losing Oxygen
Warmer water holds less dissolved oxygen. Since the 1960s, the global ocean has lost about 2% of its oxygen content. That may sound modest, but it translates to enormous volumes, and the losses are concentrated in specific regions where the effects are severe. Over 500 low-oxygen sites have been identified in coastal waters and estuaries worldwide.
These oxygen-depleted zones, sometimes called dead zones, can suffocate fish, crabs, and other marine animals that can’t escape quickly enough. Even outside true dead zones, declining oxygen levels force fish into shallower, narrower bands of habitable water, compressing their habitat and making them more vulnerable to predators and fishing pressure. Deep-ocean species that are adapted to stable conditions are especially at risk because they have no shallower refuge to move to.
Coral Reefs Under Extreme Stress
Coral reefs support roughly a quarter of all marine species despite covering less than 1% of the ocean floor. They are also among the most temperature-sensitive ecosystems on Earth. Corals bleach when sea surface temperatures rise just 1°C above their normal seasonal maximum. This doesn’t immediately kill the coral, but it expels the symbiotic algae that provide corals with food and color. If temperatures exceed 2°C above the seasonal maximum or if bleaching is prolonged, the coral dies.
The years 2023 and 2024 saw the most widespread mass bleaching event ever recorded, affecting reefs across the Caribbean, the Pacific, and the Indian Ocean simultaneously. Reefs can recover from mild bleaching if water temperatures drop within a few weeks, but the gap between bleaching events is shrinking, giving corals less time to bounce back. Acidification compounds the problem by weakening the reef structures themselves, making them more susceptible to storm damage and erosion.
Ocean Currents Are Slowing
One of the most closely watched consequences of ocean warming involves the Atlantic Meridional Overturning Circulation, or AMOC. This system of currents carries warm water northward near the surface and cold, dense water southward at depth. It plays a major role in regulating climate across Europe, West Africa, and the Americas.
As Greenland’s ice sheet melts, it pours freshwater into the North Atlantic, making the surface water less dense and less likely to sink. This disrupts the engine that drives the circulation. Research from Caltech projects the AMOC will weaken by 18 to 43 percent by the end of this century. The consequences of that weakening would include changes in regional sea level rise, colder conditions in northern Europe, and drier weather in parts of the Amazon and West Africa. While some earlier projections warned of a near-collapse, more recent modeling suggests a limited but still significant decline is more likely.
The Ocean’s Ability to Help Is Fading
The ocean has been acting as a massive carbon sink, absorbing CO₂ and buffering the climate system. But there are signs this service is weakening. Between 1994 and 2004, the global ocean stored an estimated 29 billion metric tons of human-caused carbon. In the following decade, from 2004 to 2014, that number dropped to 27 billion metric tons. The difference suggests the ocean’s capacity to keep absorbing carbon at the same pace is diminishing.
This matters because if the ocean absorbs less CO₂, more stays in the atmosphere, accelerating warming. It’s a feedback loop: warming reduces the ocean’s ability to mitigate warming. The warmer the surface water gets, the less CO₂ it can dissolve, similar to how a warm soda goes flat faster than a cold one.
What This Means for Fisheries and Food
All of these changes converge on the ocean’s ability to support marine life, including the fish and shellfish that billions of people depend on for protein and income. Under a high-emissions scenario, global maximum catch potential is projected to decrease by about 7.7% by 2050 compared to the year 2000. That’s a global average; tropical regions face steeper losses because warm-water species are already near their thermal limits and are migrating toward the poles.
Countries near the equator, many of which are among the world’s poorest and most dependent on seafood, stand to lose the most. Meanwhile, some higher-latitude fisheries may see temporary gains as species shift poleward, but these gains come with their own disruptions to established ecosystems and fishing economies. The combination of warming, acidification, and oxygen loss doesn’t just reduce fish populations. It reshapes where fish live, what they eat, and how fast they grow, creating uncertainty for an industry that feeds over 3 billion people worldwide.