Ocean heat content

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.

OHC changes are thus made more readily comparable to seawater heat exchanges with ice, freshwater, and humid air.

In practice, the integral can be approximated by summation using a smooth and otherwise well-behaved sequence of in-situ data; including temperature (t), pressure (p), salinity (s) and their corresponding density (ρ).

Sunlight penetrates to a maximum depth of about 200 m; the top 80 m of which is the habitable zone for photosynthetic marine life covering over 70% of Earth's surface.

The thermocline is the transition between upper and deep layers in terms of temperature, nutrient flows, abundance of life, and other properties.

At the start of each 10-day measurement cycle, a float descends to a depth of 1000 meters and drifts with the current there for nine days.

It then descends to 2000 meters and measures temperature, salinity (conductivity), and depth (pressure) over a final day of ascent to the surface.

At the surface the float transmits the depth profile and horizontal position data through satellite relays before repeating the cycle.

[28] Starting 1992, the TOPEX/Poseidon and subsequent Jason satellite series altimeters have observed vertically integrated OHC, which is a major component of sea level rise.

[30] The partnership between Argo and satellite measurements has thereby yielded ongoing improvements to estimates of OHC and other global ocean properties.

[11][31] This high percentage is because waters at and below the ocean surface - especially the turbulent upper mixed layer - exhibit a thermal inertia much larger than the planet's exposed continental crust, ice-covered polar regions, or atmospheric components themselves.

[3][36][37] Achieving complete and accurate results from either accounting method is challenging, but in different ways that are viewed by researchers as being mostly independent of each other.

[36] Increases in planetary heat content for the well-observed 2005–2019 period are thought to exceed measurement uncertainties.

[31] From the ocean perspective, the more abundant equatorial solar irradiance is directly absorbed by Earth's tropical surface waters and drives the overall poleward propagation of heat.

Over time, a sustained imbalance in Earth's energy budget enables a net flow of heat either into or out of greater ocean depth via thermal conduction, downwelling, and upwelling.

[9][41] Altogether these processes enable the ocean to be Earth's largest thermal reservoir which functions to regulate the planet's climate; acting as both a sink and a source of energy.

[32] From the perspective of land and ice covered regions, their portion of heat uptake is reduced and delayed by the dominant thermal inertia of the ocean.

Although the average rise in land surface temperature has exceeded the ocean surface due to the lower inertia (smaller heat-transfer coefficient) of solid land and ice, temperatures would rise more rapidly and by a greater amount without the full ocean.

[5]: 1228  The heat uptake results from a persistent warming imbalance in Earth's energy budget that is most fundamentally caused by the anthropogenic increase in atmospheric greenhouse gases.

[43]: 41  There is very high confidence that increased ocean heat content in response to anthropogenic carbon dioxide emissions is essentially irreversible on human time scales.

[45] This results in changes among ocean currents, and an increase of the subtropical overturning, which is also related to the El Niño and La Niña phenomenon.

[46] Model studies indicate that ocean currents transport more heat into deeper layers during La Niña years, following changes in wind circulation.

[47][48] Years with increased ocean heat uptake have been associated with negative phases of the interdecadal Pacific oscillation (IPO).

The ice loss reduces polar albedo, amplifying both the regional and global energy imbalances.

[72] Warming of the deep ocean has the further potential to melt and release some of the vast store of frozen methane hydrate deposits that have naturally accumulated there.

The ocean heat content (OHC) has been increasing for decades as the ocean has been absorbing most of the excess heat resulting from greenhouse gas emissions from human activities. [ 1 ] The graph shows OHC calculated to a water depth of 700 and to 2000 meters.
Graph of different thermoclines (depth versus ocean temperature ) based on seasons and latitude
The global distribution of active floats in the Argo array [ 26 ]
Oceanographer Josh Willis discusses the heat capacity of water , performs an experiment to demonstrate heat capacity using a water balloon and describes how water's ability to store heat affects Earth's climate.
Earth heat inventory (energy accumulation) in ZJ for the components of the Earth's climate system relative to 1960 and from 1960 to 2018. The upper ocean (0–300 m, light blue line, and 0–700 m, light blue shading) accounts for the largest amount of heat gain. [ 3 ]
Surface air temperatures over land masses have been increasing faster than the sea surface temperature .
Map of the ocean heat anomaly in the upper 700 meters for year 2020 versus the 1993–2020 average. [ 44 ] Some regions accumulated more energy than others due to transport drivers such as winds and currents.