Much attention is paid to the various measures of global surface temperatures which has become the primary observable of global warming and a test of climate models and predictions. However, it is often pointed out that of all the excess energy that the Earth is absorbing because of the Greenhouse Effect the greater part of it by far is absorbed in the oceans.
It was back in the 1950s that it was suggested that the ocean heat content was the major factor in the Earth’s heat balance. In the 1960s the suggestion was made that ocean warming due to anthropogenic global warming was apparent and in the 1970s the US National Research Council said that the atmosphere lags the ocean in warming.
A new paper concerning oceanic heat content by Levitus et al (2012) has attracted some comment. It looked at depths of 0 – 700 m and 0 – 2000 m for 1955 – 2010. The 0 – 2000 m heat content increased by 2.4 x 10E23 Joules, corresponding to a volume mean warming of 0.09 deg C and forcing factor of 0.39 W per sq m. The 0 – 700 m layer increased its heat content by 1.6 X 10E23 J, corresponding to 0.27 W per sq m.
The paper was an updating of previous papers along the same line, taking into account new calibration factors, new historical data and improved quality control.
In 2005 Levitus at al wrote that between 1955 -1998 (0 – 3000 m) the ocean heat content had increased by 1.4 x E23 J. This equates to a mean temperature increase of 0.037 deg C and forcing of 0.20 W per sq m.
In 2000 Levitis et al looked at the oceans between 1948 – 98 ( 0 – 3000 m) and concluded that their heat content had increased by 2 x 10E23 Joules. This corresponded to a volume mean warming of 0.06 deg C and a forcing of 0.3 W per sq m. Shallower water ( 0 – 300 m) increased by 10E23 J.
In the 2000 paper the authors state: “We cannot partition the observed warming to an anthropogenic component or a component associated with natural variability. However, our results support the findings of Hansen et al. (19), who concluded that a planetary radiative disequilibrium of about 0.5 to 0.7 W m−2 existed for the period 1979 to 1996 (with the Earth system gaining heat) and suggested that the “excess heat must primarily be accumulating in the ocean.”
Looking at the estimates of the amount of ocean heat content increase; 2 in 2000, 1.4 in 2005 and 2.4 in 2012 (all in units of 10E23 J) the passing of the past decade doesn’t seem to have altered such values very much. Nonetheless it seems clear there has been substantial ocean warming in the past 50 years: about 3 x 10E21 joules a year added on average, and a few x 10E23 J in total.
The mass of the atmosphere 5 x 10E18 Kg. The mass of hydrosphere; 1.4 x 10E21 kg. The oceans are substantially more massive, and with a larger heat capacity, which means there is a great deal of heat energy in the oceans that cannot easily get out except over geologic timescales.
This difference in energy storage means that in terms of energy, if it was taken from the oceans to warm the atmosphere by 4 deg C, it would only cool the oceans by an imperceptible 10E-3 deg C.
A Question Of Balance
It is interesting to put this energy into context.
The total heat content of the ocean has been estimated as about 10E27 J; so the additional energy placed into it over the past 50 years is, in relative terms tiny, though that is not decrying its absolute effect on the Earth’s system.
Over the past 50 years the energy from the Earth’s interior leaking to the surface is about a few times 10E22 J (two third of which would come up under the ocean; note that mid-ocean spreading centres are concentrated regions of heat emergence) .
The energy associated with stress and pressures due to wind induced surface movement over 50 years, is about a few x 10E23 J.
In terms of influence on sea-level rise, the influx of this extra energy is a minor part of the measurements of sea level we have over the past hundred years or so. Levitus et al (2012) estimates that the influx of a few x 10E23 J over the past 50 years produces a sea-level rise of 0.54 +/- .05 mm per year or about 25 mm since the 1950s. Sea-level has increased 100 mm since then.
Curiously, the temperature of the top layer of ocean has shown no increase in the past decade or so (like the Earth’s surface temperature data). It was once thought that this region had actually cooled somewhat, but that turned out to be an instrumental and selection effect.
The Met Office says it can explain the post 2003 pause in 0 – 700 m Ocean Heat Content as warm water seeping below the 700 m level. Click on image to enlarge.
Looking at Fig 1 in Levitus et al (2012), showing the heat content of the ocean over time, the deeper layers (700 – 2000m) does increase post 2000 as does, at a greater rate, the 0 – 700m layer. However I would not say the gradient, or relative proportion of shallow and deep water heat partition, had changed post 2000. The data suggest that most of the heat has gone into the shallow regions (as to be expected) with the deeper regions warming slowly at a lower and unchanging rate since the start of the data in the mid-1950s. The plateau seen between 1980 – 90 is unexplained.
A major question is, of course, what happened before the mid-1950s? Only with such data can the modern increases in heat content be put into it proper context and anthropogenic influences detected from natural ones.
Prof Hubert H. Lamb, the premier European climatologist of the 20th century, wrote in 1977 that “there has been a general warming of sea temperatures, by 0.5–1.0 deg C, from 1880 to 1965, defined from widely scattered points around the oceans of the world.”