Ocean heat content (OHC) or ocean heat uptake (OHU) is the energy absorbed and stored by oceans.[2] It is an important indicator of global warming.[3] Ocean heat content is calculated by measuring ocean temperature at many different locations and depths, and integrating the areal density of a change in enthalpic energy over an ocean basin or entire ocean.[4]
Between 1971 and 2018, a steady upward trend[5] in ocean heat content accounted for over 90% of Earth's excess energy from global warming.[6][7] Scientists estimate a 1961–2022 warming trend of 0.43 ± 0.08 W/m², accelerating at about 0.15 ± 0.04 W/m² per decade.[8] By 2020, about one third of the added energy had propagated to depths below 700 meters.[9][10] The five highest ocean heat observations to a depth of 2000 meters all occurred in the period 2020–2024.[5] The main driver of this increase has been human-caused greenhouse gas emissions.[11]: 1228
Ocean water can absorb a lot of solar energy because water has far greater heat capacity than atmospheric gases.[9] As a result, the top few meters of the ocean contain more energy than the entire Earth's atmosphere.[12] Since before 1960, research vessels and stations have sampled sea surface temperatures and temperatures at greater depth all over the world. Since 2000, an expanding network of nearly 4000 Argo robotic floats has measured temperature anomalies, or the change in ocean heat content. With improving observation in recent decades, the heat content of the upper ocean has been analyzed to have increased at an accelerating rate.[13][14][15] The net rate of change in the top 2000 meters from 2003 to 2018 was +0.58±0.08 W/m2 (or annual mean energy gain of 9.3 zettajoules).[3]
Changes in ocean temperature greatly affect ecosystems in oceans and on land. For example, there are multiple impacts on coastal ecosystems and communities relying on their ecosystem services. Direct effects include variations in sea level and sea ice, changes to the intensity of the water cycle, and the migration of marine life.[16]
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