New research shows oceans warming to depths of 2,000 metres, human-driven land subsidence intensifying sea level risks in China, and pandemic lockdowns altering bird evolution in Los Angeles.

Warmer oceans spell danger for humans and ecosystems

The oceans are a giant reservoir not only for water but also for CO2 and heat. They have absorbed about half of all the additional CO2 that humans have pumped into the air over the last 250 years and more than 90 per cent of all the additional solar energy (as heat) that the Earth has retained as a result of the higher concentration of CO2 in the atmosphere. Consequently, the oceans now hold more total energy (Ocean Heat Content or OHC) than in pre-industrial times and the water is warmer.

The increased heat content and temperature were initially mainly limited to the oceans’ surface layers but they are now extending down to 2,000 metres. OHC is one of the most robust measures of long-term climate change because it is less sensitive to short term fluctuations than, for instance, the commonly used global average surface temperature.

The OHC of the top 2,000m has been increasing steadily since the 1950s, with the rate of change increasing four-fold from 2.9 Zettajoules per year (ZJ/yr) during 1958-85 to 11.4 ZJ/yr during 2007-25. In 2025 the OHC increased by 23 ZJ. The largest changes in 2025 occurred in the North Atlantic and Southern Oceans, Mediterranean Sea and an area of the Indo-Pacific. The North Atlantic and Mediterranean are also hotspots of salinisation, deoxygenation and acidification which all pose significant threats to ocean ecosystems.

The histogram below displays the annual variation of the OHC from 1958-2025 compared with the average for 1981-2010.

The global mean Sea Surface Temperature (SST) has also increased markedly since the 1950s, at an average rate of 0.12oC/decade. In 2025, the SST was 0.12oC lower than in 2024 (mainly due to a weak La Niña) but even so the SST in 2025 was 0.49oC above the 1981-2010 baseline and the third highest on record.

Hotter oceans speed up sea level rise via thermal expansion of the water. They also increase the frequency and intensity of marine heatwaves, with disastrous consequences for coral reefs. Warmer water also increases evaporation. Warmer, wetter air contains more energy, leading to more extreme and more unpredictable weather, particularly stronger rainfalls and more rapid intensification of........

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