Ocean heat content

Ocean heat content (OHC) or ocean heat uptake (OHU) is the energy absorbed and stored by oceans. To calculate the ocean heat content, it is necessary to measure ocean temperature at many different locations and depths. Integrating the areal density of a change in enthalpic energy over an ocean basin or entire ocean gives the total ocean heat uptake.^[2] Between 1971 and 2018, the rise in ocean heat content accounted for over 90% of Earth's excess energy from global heating.^[3][4] The main driver of this increase was caused by humans via their rising greenhouse gas emissions.^[5]: 1228 By 2020, about one third of the added energy had propagated to depths below 700 meters.^[6][7]

In 2023, the world's oceans were again the hottest in the historical record and exceeded the previous 2022 record maximum.^[8] The five highest ocean heat observations to a depth of 2000 meters occurred in the period 2019–2023. The North Pacific, North Atlantic, the Mediterranean, and the Southern Ocean all recorded their highest heat observations for more than sixty years of global measurements.^[9] Ocean heat content and sea level rise are important indicators of climate change.^[10]

Ocean water can absorb a lot of solar energy because water has far greater heat capacity than atmospheric gases.^[6] As a result, the top few meters of the ocean contain more energy than the entire Earth's atmosphere.^[11] 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.^[12][13][14] 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). It is difficult to measure temperatures accurately over long periods while at the same time covering enough areas and depths. This explains the uncertainty in the figures.^[10]

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.^[15]