Ocean heat and acidification hit record levels in 2025
The world's oceans absorbed a record 23 zettajoules of heat in 2025, marking the ninth straight year of rising ocean heat content.
The world’s oceans absorbed a record amount of heat in 2025, marking the ninth consecutive year of rising ocean heat content and reinforcing long-term warming trends. A staggering 23 zettajoules of thermal energy was taken up by the seas last year, a quantity roughly 40 times greater than total global human energy consumption for the same period. The rate of marine warming has doubled over the past two decades, with the ocean capturing approximately 90 percent of the excess energy trapped by greenhouse gas emissions.
Widespread marine heatwaves stress ecosystems
Marine heatwaves have intensified, particularly in recent years. Between January 2023 and September 2025, 84% of the world’s coral reefs experienced bleaching-level heat stress. Nearly 5% of the ocean has seen a persistent shift in phytoplankton growth patterns over the past 30 years, indicating a fundamental change at the base of the marine food web.
Sea level rise accelerates due to ocean warming
Thermal expansion from this absorbed heat is a primary driver of rising seas. Global average sea levels have risen by roughly 3.8 millimeters annually since 1999, accumulating to nearly 10 centimeters in total. The rate of increase accelerated to 4.2 millimeters per year between 2012 and 2025.
| Period | Average Annual Sea Level Rise |
|---|---|
| Since 1999 | 3.8 mm |
| 2012 to 2025 | 4.2 mm |
Ocean acidification and warming threaten marine life and coasts
The global ocean has become approximately 17 percent more acidic since 1985, having absorbed about one-third of anthropogenic carbon dioxide emissions. This acidification impairs the ability of calcifying organisms like corals, oysters, and lobsters to build and maintain shells, threatening commercial fisheries and the coastal protection barriers they form. In the western Mediterranean, sea level rise is accelerating alongside localized temperature spikes exceeding 4.6 degrees Celsius above historical averages. Record-low Antarctic sea ice extents have already triggered breeding failures among ice-dependent species like Emperor penguins.
Regional hotspots amplify global risks
Regional disparities are stark. Western Europe and the Mediterranean are heating twice as fast as the global average, with temperatures in parts of Spain and Portugal reaching 40C in September 2026. These hot coastal waters exacerbate land-based heatwaves by preventing nighttime cooling. The combined problems of rising sea levels and extreme events are actively eroding the Mediterranean coast. Storm Louis in 2024 generated 26-foot waves along the Spanish coastline, causing infrastructure damage, widespread power outages and a confirmed fatality. In northern Europe, intense and prolonged winds over the Bothnian Bay pushed so much water out that sea levels temporarily plunged by 60 inches, forcing suspension of ferry operations for 18 hours.
Arctic sea surface temperatures have climbed by roughly 4.2 degrees Celsius over the past four decades, with monthly average sea ice loss equivalent to the landmass of Spain every decade. Pierre Bahurel observed that "the ocean is transforming before our eyes. Warming, rising seas and ecosystem degradation are interconnected signs of significant and long-term change." Without coordinated global action on climate change, air quality, and ocean health, these interconnected challenges will intensify, affecting ecosystems and human communities worldwide.
