365 Days of Climate Awareness 183 – Changes in Oceanic Heat Content

It is estimated that the ocean stores roughly 90% of the heat added to the biosphere due to increased greenhouse gases in the atmosphere. Because ocean water stores roughly 800 times as much heat by volume as air, the majority of heat on the planet’s surface is both stored and circulated by the ocean. For that reason scientists in recent decades have devoted increasing effort to measuring temperatures, and their changes over time, at greater ocean depths.

Top 2000 m change in heat content vs 1981-2010 average.

In recent decades the technology has existed for a fleet of over 3,000 “Argo” floats, among other designs, to float freely in the water at various depths (as a function of density) recording data such as temperature and salinity. The floats periodically float to the surface, broadcast their position and the data they have gathered, and then descend back to depth. Passive drifters like these have helped researchers gain tremendous understanding of the dynamics and heat content of the intermediate (500-1000 m/1780-3560 ft) and deeper (>1000 m) ocean.

Global change in ocean heat content, top 700 m (shallow and intermediate).

The bulk, but far from all, of the ocean’s heat is in the shallow and intermediate zone, due to direct influx of energy from the sun. Given that much of the ocean (outside of the polar regions) is stratified, events which transport heat to intermediate and deep depths tend to be episodic. ENSO is perhaps the dominant global example. It is estimated that La Niña events, with their associated increase in wind and surface current activity, increase global downward transport of heat in the ocean by as much as 30%. In this way La Niña is a prime mechanism for oceanic storage of the heat gained through global warming.

Ocean heat content change vs 1993 — 2019 average, for various depths.

By every indicator—satellite, ship-based, and drift buoy measurements—the oceans are warming rapidly, at nearly all depths. The energy content of the heat is on a staggering scale: the increases are in Zetajoules (Zj), 10²¹ J (one thousand billion billion joules). One Zetajoule is roughly 18 times the annual energy consumption of the human race.

Global heat content increase since 1960, oceans and other regions.

Tomorrow: oceanic salinity changes.

Be brave, and be well.