As Temperatures Climb, the Ocean Takes the Brunt

From scorching temperatures in South Africa to unprecedented heat waves in Europe, extreme heat is breaking records and transforming people’s lives across the world. While we feel this heat most immediately in our home and workplaces, the reality is that most of it actually ends up in the ocean.

The ocean absorbs about 91% of the excess heat trapped by human-caused greenhouse gas emissions. This growing “energy imbalance” explains why ocean temperatures keep breaking records and why sea levels keep climbing. Those same greenhouse gas emissions are also changing the chemistry of seawater.

The effects of these changes are increasingly being felt on land. But for those working in the ocean space, climate change is already here. The ocean is our clearest indicator of climate change — and the effects of these changes are sobering.

The Rate of Ocean Warming Has More Than Doubled  

Because of its huge volume and water’s high heat capacity, the ocean acts as a giant heat reservoir for the planet, absorbing 91% of the excess heat caused by greenhouse gases emitted over the last century. That’s equivalent to the heat of about 7.6 billion cups of tea being poured into the ocean every second.  

As a result, ocean temperatures have steadily risen, with myriad impacts on people and the planet — from more intense storms and climate patterns like El Niño to species migrating in search of cooler, more suitable waters and growing threats to the crucial fisheries we depend on for food security and livelihoods.

The upper 2,000 meters of the ocean stored more heat than ever before in 2025 — marking the ninth consecutive year of record-breaking heat storage. On top of that, the rate of warming observed between 2005 and 2025 was more than double the rate recorded between 1960 and 2005. Not only is the ocean warming — it’s warming faster.

This trend is one of the clearest indicators we have of climate change, and one that’s particularly visible at the poles. Arctic sea ice reached its lowest annual maximum extent on record in March 2025 and again in 2026 (about 14.3 million square kilometers in both years). In 2025 Antarctic sea ice recorded its third-lowest annual maximum extent (about 17.8 million square kilometers) and second-lowest summer minimum (about 2 million square kilometers). As reflective white sea ice disappears, it exposes the darker ocean water below, which absorbs more heat, creating a feedback loop that further accelerates warming.

Sea-Level Rise Has Almost Doubled

While dwindling sea ice is alarming, it's just the tip of the (melting) iceberg.  

By the end of 2025, the global average sea level was about 11 centimeters higher than it was in January 1993. The rate of this increase has sped up from roughly 2.65 millimeters per year (1993–2011) to about 4.75 millimeters per year (2012–2025). This acceleration is driven by both the thermal expansion of seawater as it warms and the melting of land-based ice from glaciers and ice sheets.

These changes are reshaping ecosystems, altering ocean circulation and contributing to rising sea levels worldwide. Higher sea levels increase the frequency of high-tide flooding, worsen storm surges, accelerate coastal erosion and allow saltwater to intrude into freshwater supplies and farmland. The consequences are not only environmental but also social and economic and will be felt for generations.  

Low-lying nations and coastal communities are already experiencing many of these impacts. In The Gambia, for example, rising seas are pushing saltwater farther up the Gambia River and into surrounding farmland. In some villages, once-productive rice and vegetable fields have been abandoned because the soil has become too salty for crops. As suitable land becomes scarcer, communities are running out of options to grow food.  

In places where relocation is no longer possible, some governments are creating pathways to allow people freedom of movement to move to a new country. One example is Tuvalu in the South Pacific, where an agreement with Australia allows eligible Tuvaluans to migrate as rising seas increasingly flood areas that once remained dry at high tide. Tuvaluans are facing the consequences of a crisis to which their country has contributed very little. Beyond the loss of homes and infrastructure, sea-level rise raises profound questions about identity and sovereignty, such as: what happens to a country if its people are forced to leave?

The Ocean’s Chemistry Is Changing  

As well as absorbing heat, the ocean is also a major carbon sink, absorbing roughly 25% of human-generated carbon dioxide emissions. However, this service comes at a cost.

As carbon dioxide dissolves into seawater, it alters ocean chemistry. While acidification varies across the ocean, observations show that the average acidity of surface ocean waters worldwide has increased by around 30% since the preindustrial era. This spells trouble for marine life and those who rely on it.

Coral reefs, shell-forming species and other marine organisms are especially vulnerable because acidification reduces the availability of carbonate ions needed to build calcium carbonate shells and skeletons. These impacts can ripple across the ocean: Ecosystems like coral reefs support around 25% of life in the ocean, as well as crucial food webs. The decline of these ecosystems alone will affect fisheries, food security and other coastal economies that depend on them.

The effects of more acidic ocean environments have been seen before when oyster hatcheries in Oregon and Washington experienced mass larval die-offs. These were later linked to more acidic, low-aragonite seawater entering hatchery tanks and impeding crucial processes for the larvae to make their shells. This proved to be a huge issue for the region that is significant within the shellfish trade and was valued at about $111 million when it came close to collapse, before growers adopted monitoring and water-treatment systems.

Because acidification is progressing faster in some regions than the global average, some places may face ecological impacts sooner and with greater intensity than global averages suggest. Sustained ocean surface warmth also contributes to deoxygenation and lowers nutrient resupply. Layer onto that marine heatwaves and increased acidity and we have compounded pressures on reefs, kelp forests, shellfish, food webs and, ultimately, coastal livelihoods and national economies.

Climate Change Is Accelerating. So Must Our Response  

The ocean is sending us an unmistakable message: Climate change is accelerating, and our response must accelerate with it. Limiting every fraction of a degree of warming matters, because it means less heat accumulating in the ocean, less sea-level rise locked in for future generations and greater resilience for the communities and ecosystems that depend upon a healthy ocean.

The science is increasingly clear that some degree of ocean change is now unavoidable. Deep ocean warming and interior ocean acidification are effectively irreversible on our timescales. This does not mean giving up. It means doing everything we can now to reduce near-term risks by protecting natural buffers such as mangroves, reefs, wetlands and seagrasses; planning coastal infrastructure and services so that communities stay safe when floods, storms or heatwaves overwhelm them; and acting earlier, using forecasts and ocean-observation systems to protect people, livelihoods and ecosystems before impacts hit — all while continuing to cut emissions. Achieving that depends on three key actions happening together:

• Viewing adaptation as essential. For coasts, the acceleration in sea‑level rise is the metric that should be resetting adaptation timetables. The impacts of a warming, acidifying ocean must be factored into infrastructure investment, urban planning, disaster preparedness and national development strategies today, rather than treated as future challenges.
• Investing in healthy ocean ecosystems. Mangroves, saltmarshes, seagrasses, coral reefs and other coastal ecosystems help buffer shorelines, store carbon and sustain livelihoods. Protecting and restoring these systems will not stop ocean warming, but it can help societies absorb its impacts while delivering multiple economic, social and environmental benefits.
• Making ocean data visible. Ocean metrics should be presented alongside emissions so leaders have a clearer picture of climate progress and risk, as well as how they directly affect ecosystems, food security and coastal safety.

The encouraging news is that many of the solutions already exist. Sustainable Ocean Plans, ecosystem protection and restoration, climate-resilient fisheries management, resilient coastal infrastructure and accelerated decarbonization can all help reduce risk while supporting long-term prosperity. The challenge is now delivering at the pace and scale that ecosystems and communities demand.