Ocean heat content tells us how much heat energy there is in the oceans. It is
measured by autonomous Argo floats and
devices lowered from research vessels and other ships.
90% of the excess energy trapped in the earth system by anthropogenic greenhouse
gas emissions goes into the oceans. Ocean heat content is a direct measure of
the accumulation of that energy. Unlike surface temperature, which can change a
lot from one year to the next because of El Niño and La Niña, ocean heat content
rises more steadily.
As the oceans warm, the water in the oceans expands raising sea level. We can
calculate how much sea level should rise based on how much the oceans have warmed.
The calculated sea-level change can be combined with other sources of sea level
rise (like melting glaciers and storage of water on land) and compared to actual
measurements of sea-level change.
Changes in the temperature of the oceans can affect how much oxygen and carbon
dioxide can dissolve in the seawater, which in turn can change the amounts of these
gases in the atmosphere. All else being equal, less gas will dissolve in warmer water.
Overall ocean heat content has increased since the 1950s. The increase has generally
been steadier than that of surface temperature. There are slight drops after large
volcanic eruptions such as that of Pinatubo in 1991 or El Chichon in 1984.
Although ocean heat content is less sensitive to year to year changes associated
with El Niño and La Niña, it does vary a little as El Niño and La Niña affect the
flows of energy from the ocean to atmosphere and vice versa.
The world's oceans have a huge volume so even small changes in temperature can
correspond to large changes in energy. Therefore, it is important to be able to measure
very small changes in temperature in order to get an accurate measure of ocean heat
change. In order to measure heat content we have to measure temperatures through the
whole volume of the upper oceans, so we need measurements at different depths as well
as at different locations.
Several different kinds of instruments have been used to measure temperature
“profiles”, which are a series of temperature measurements at different depths. CTD
(Conductivity-Temperature-Density) instruments make very precise measurements of water
temperature and depth. They are usually deployed from research vessels, so the number
of measurements available is rather small. Since the early 2000s, autonomous robot
floats, known as Argo floats, have measured large numbers of profiles in the upper
2000m of the ocean. From the 1970s, eXpendable BathyThermographs (XBTs) were used. An
XBT drops through the water, unspooling a long wire behind it. The XBT measures
temperature as it descends and relays that back to the observer via the wire. The
depth of the measurement can be calculated from how long the XBT has been falling.
XBT measurements are less precise than CTD or Argo measurements and the equations used
to work out the depths of the measurements sometimes give inaccurate answers which need
to be corrected using the more precise CTD measurements.
According to the IPCC AR5, “it is very likely that anthropogenic forcings have made a
substantial contribution to upper ocean warming (above 700m) observed since the 1970s.
This anthropogenic warming has contributed to global sea level rise over this period
through thermal expansion.”