Even as scientific knowledge advances and sharpens, one conclusion has remained steady: the largest unknown in the climate system is human behavior. The future still depends on whether societies reduce emissions, prepare for unavoidable impacts, and act together. If they do, a livable climate — one that can sustain both human communities and ecosystems — remains within reach. Meanwhile, however, the perils keep staring us in the face and the response is … inadequate.
The world’s oceans are heating in a way that feels less warning than verdict, absorbing the overwhelming majority of the damage inflicted by an economic system that treats the atmosphere as a free sewer and the sea as an infinite sponge.
Impacts on the atmosphere get the most public attention, but more than 90% of the excess heat trapped by greenhouse gases ends up in seawater, making ocean heat content (OHC) — rather than surface air temperature — the gold standard for measuring the climate crisis. And despite the assertions of science deniers like the entire Trump regime, there is a crisis, an emergency, as ever more research shows.
According to an international study published last week in Advances in Atmospheric Sciences, the oceans are hotter than they have been for at least 1,000 years. They are heating faster than they have in the past 2,000 years. The average ocean surface temperature last year is expected to have tied with 2023 as the second-hottest year since 1850, with 2024 being the hottest.
“Global OHC is the single best indicator that the planet is warming,” said Kevin Trenberth, a co-author of the study and a scholar at the National Center for Atmospheric Research, a 66-year-old institution Donald Trump has announced he wants to shutter.
“Global warming is ocean warming,” said Prof. John Abraham, a member of the study team at the University of St. Thomas in Minnesota. “Each year the planet is warming — setting a new record has become a broken record. If you want to know how much the Earth has warmed or how fast we will warm into the future, the answer is in the oceans.”
Although the planet moved in 2025 into the cooler La Niña phase of the Pacific Ocean’s El Niño-La Niña, cycle, the upper 2,000 meters (1.2 miles) of the global ocean absorbed 23 zettajoules of heat last year — an amount of energy equivalent to more than 200 times the world’s annual electricity use, or, as Johnny Sturgeon at Inside Climate News starkly put it, 12 Hiroshima-sized atomic bombs every second.
What the oceans absorb does not stay neatly contained. Hotter seas evaporate faster, loading the atmosphere with moisture and energy that storms then weaponize. Hurricanes grow stronger. Rain falls harder. Floods spread wider. As Trenberth explained, warming oceans create weather on steroids.
The fingerprints of ocean heat appeared everywhere in 2025: lethal monsoon rains across South and Southeast Asia; catastrophic flash floods in Texas; wildfire seasons intensified by heat-driven drought; and marine heatwaves that pushed coral reefs past physiological limits. Lijing Cheng, an oceanographer at the Chinese Academy of Sciences and a lead author of the study, warned that when seawater exceeds corals’ thermal tolerance, “the symbiotic system collapses,” triggering mass bleaching events that ripple through entire marine food webs.
The warming is global, but not evenly shared. Scientists recorded exceptional heat accumulation last year in the Southern Ocean, the South Atlantic, the North Pacific, and enclosed seas like the Mediterranean, which is becoming hotter, saltier, more acidic, and less oxygenated all at once. Around Antarctica, researchers are deeply alarmed by repeated collapses in winter sea ice, a development that threatens to accelerate ice-sheet loss on land and thus long-term sea-level rise.
This matters because sea-level rise is not just about water creeping up a beach. It’s about forced migration. It’s about whether low-lying island nations, coastal megacities, and delta communities—home to hundreds of millions of people—will be allowed a future at all.
Nations and corporations that profited most from fossil fuels outsourced the heat to the oceans, while the costs are borne disproportionately by the Global South. World Weather Attribution researchers emphasized in their 2025 assessment that climate impacts consistently hit marginalized communities hardest, and even the science meant to document those impacts is skewed by data gaps rooted in colonial inequality.
A small but persistent cohort of commentators has argued that ocean warming is overstated, or that the transition from El Niño to La Niña in 2025 proves natural variability is the key factor. But the data cuts through that fog. Even during La Niña conditions, global ocean heat content reached an all-time high, marking the ninth consecutive year of record-breaking warming.
Heat that sinks to depths of 2,000 meters takes decades to arrive and centuries to dissipate. On human timescales, damage is irreversible — locked in by physics and political delay.
The 2025 United Nations climate talks ended, once again, with lofty language and minimal binding commitments, even as governments continued to approve new oil and gas projects scientists say are incompatible with a livable future. In the United States. Trump and congressional Republicans spent the year rolling back climate action and undermining or trying to demolish agencies tasked with disaster preparedness, a “polluters-first agenda,” as Margie Alt of the Climate Action Campaign put it, that made denial functionally official policy.
Despite this and a plethora of year-end bad news about the climate, there’s reason for optimism as renewable energy installations soar.
Co-author Abraham said, “As long as the Earth’s heat continues to increase, ocean heat content will continue to rise and records will continue to fall. The biggest climate uncertainty is what humans decide to do. Together, we can reduce emissions and help safeguard a future climate where humans can thrive.”
Likewise, Michael E. Mann, a climatologist and director of the University of Pennsylvania’s Center for Science, Sustainability and the Media, noted that while deep-ocean warming is permanent on a human timescale, surface temperatures could stabilize if emissions stopped. The uncertainty, he said, is not technological but human. We know what to do. Retreating on our unprecedented but still inadequate efforts to cut emissions by transforming the global energy system is not it.
Shrugging off the crisis isn’t either. But ongoing monitoring by the Media and Climate Change Observatory, led by the University of Colorado, Boulder, has revealed a steep worldwide drop in climate coverage over the past few years.
