The ocean has shielded humanity from the worst effects of climate change, but it is now running a fever. In 2025, the number of days of marine heatwaves—prolonged periods when sea temperatures become abnormally and dangerously warm—was more than triple that of the early 1990s.
These are not abstract statistics. A severe and persistent marine heatwave can bleach coral reefs, strip away kelp forests that shelter young fish, empty fishing grounds, and, if frequent enough, push entire ecosystems past the point of recovery. It disrupts the ocean's chemistry—its acidity, oxygen levels, and carbon exchange with the air—and can fuel fiercer weather on land. For coastal communities dependent on the sea for food and livelihoods, the damage is immediate and personal.
The Ocean as a Buffer
Karina Von Schuckmann, an oceanographer who has spent her career studying where the heat from climate change actually goes, notes that the answer is overwhelmingly the sea. The ocean has absorbed more than 90% of the excess heat trapped by human activities, quietly buffering those on land from the full force of warming. For decades, this made it our greatest ally. However, ocean warming and more frequent, intense marine heatwaves signal that this buffer is straining. The heat we have poured into the ocean is beginning to surface as harm.
Earth's Energy Imbalance
The Indicators of Global Climate Change (IGCC) report, an annual health check compiled by over 70 researchers from more than 50 institutions, highlights a key metric: Earth's energy imbalance. This is the gap between the energy received from the sun and the energy radiated back into space. In a stable climate, these are roughly equal, but human activities have widened this gap. Greenhouse gases thicken the atmosphere's insulating blanket, trapping heat. Additionally, cleaning up air pollution has reduced the reflective haze that once deflected sunlight, while feedback loops—such as melting ice exposing dark ocean water—amplify warming. The imbalance has more than doubled since the late 20th century, with far less energy leaving than coming in.
This imbalance drives nearly everything else recorded in the report: rising temperatures, melting ice, fiercer extremes, and intensifying marine heatwaves. Human-induced warming has reached approximately 1.37°C above pre-industrial levels. Sea-level rise has more than doubled in recent decades, accelerating to a new record of 23 cm since 1901, pushing floodwaters further into low-lying coastlines.
Threats to Monitoring
Von Schuckmann expresses deep concern: the ability of scientists and policymakers to track these changes is now under threat. The knowledge about the ocean and Earth's energy imbalance relies on a sophisticated network of sensors in global waters and satellites. Recently, the scaling back of this work was announced—four of five monitoring sites across the Pacific and Atlantic are set to close, with equipment already being pulled from the water. Other funding faces similar pressures. At the moment we most need to see clearly, we are turning off the lights.
However, paying attention is not the same as looking on helplessly. The IGCC report provides an unprecedented understanding of how human activity is pulling the climate further out of balance, and it lies within our power to ease it. Nearly every indicator is flashing red, but citizens, businesses, and policymakers still hold the tools to bring the planet back into balance.
