Global warming? Where is the heat?

14 05 2013

Another guest post from Mark Cochrane…….

Much has been made of the meme that global warming has stopped or ‘paused’ for the past X years, where the value of X depends on how you play the climate change denial game.

However, we humans are rather fixated on our own experience as central to everything. We live on land and experience weather of the atmosphere, so, understandably, we consider the conditions in these areas to be the important ones. The observations are still within that shown by model simulations but the rate of surface air warming has slowed over the last decade. (Note – a slowing rate of temperature increase is not the same thing as a decrease in temperature) The planet is 70% covered in water though, and the first 150 metres of the ocean have 100 times the heat capacity of the atmosphere.  The ocean, however, is a lot deeper than that, averaging 3,790 metres (12,430 ft).

While we worry about our 0.8 C of atmospheric warming, to date, the fact of the matter is that >95% of all of the excess heat energy has actually gone into the oceans (either directly or through melted ice). Believe it or not, scientists have not been blind to this fact and have literally thousands of sophisticated thermometers that rise and sink between 0 and 2000 meters as they float through the ocean. Whenever they break the surface these ARGO floats transmit their data via satellite to a master database (here they all are) (link).

If you look at the temperature of the ocean waters, not too surprisingly they are relatively warm at the surface, with an even temperature in the ‘mixed layer’ (usually <150 metres) where wind and weather keep things stirred up and they get colder as you go deeper from there. It shouldn’t be too surprising that there are large scale dynamics in the ocean, somewhat analogous to long term weather patterns in the atmosphere. This is important for the climate we experience because at times lots of warm water piles up on the surface and at other times the water turns over and we get colder surface waters. The most famous example of this phenomenon is the El Nino Southern Oscillation (ENSO) – generally known as El Niño (when hot) and La Niña (when cold).

For the last three years we have been experiencing a slightly unusual double-dip La Niña. Currently we are at or near neutral conditions. Have you ever been near the coast or on the water early in the spring when it was supposed to be a warm day? On or near the water it is still cold because of that giant heat sink of cold water. La Niña is sort of like that globally, it should be relatively cold and yet 2012 was the warmest recorded year in the United States and just ask Australians what they’ve been experiencing of late! The point being that within the next few years we are likely to have our next El Niño and things will truly get warm.

ENSO is not the whole story of ocean dynamics. We’ve known for a while that the ocean temperature uptake varies over decadal time periods (e.g. see Meehl et al 2011). Both modeling and actual measurements (see the Argo float data!) show that heat is going into both the near surface (about 70% in the top 700m = >2,300ft) and deeper depths (about 30% in water deeper than 700m). ‘Hiatus’ decades experience little surface warming but relative rapid levels of deep ocean warming while other decades show the opposite pattern.

This has been shown several times now (Levitus et al 2012, Nuccitelli et al 2012, Balmaseda et al 2013)

In any case, Levitus et al provide some context for the amount of ocean heat uptake as reported (here)

Putting Ocean Heating Into Perspective

The amount of global warming which has gone into the oceans over the past 55 years is quite impressive.

“The global linear trend of OHC2000 is 0.43×1022 J yr-1 for 1955-2010 which corresponds to a total increase in heat content of 24.0±1.9×1022 J”

This is an immense amount of energy being added to the oceans which Levitus et al. put into perspective (emphasis added):

“We have estimated an increase of 24×1022 J representing a volume mean warming of 0.09°C of the 0-2000m layer of the World Ocean.  If this heat were instantly transferred to the lower 10 km of the global atmosphere it would result in a volume mean warming of this atmospheric layer by approximately 36°C (65°F).”

Levitus et al. note that of course this heat won’t be instantly transferred to the atmosphere (fortunately!), and that this comparison is simply intended to illustrate the immense amount of energy being stored by the oceans.

This heating amounts to 136 trillion Joules per second (Watts), which is the equivalent of more than two Hiroshima “Little Boy” atomic bomb detonations per second, every second over a 55-year period.  And Levitus et al. note that this immense ocean heating has not slowed in recent years – more of it has simply gone into the deeper ocean layers.

The kicker comes from Balsameda et al (2013) though, who show that, if anything, global warming has accelerated in recent years as heat gets pumped into the deep oceans (below). Does anyone wonder what effect this might have?




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