What ocean heating reveals about global warming

The heat content of the oceans is growing and growing.  That means that the greenhouse effect has not taken a pause and the cold sun is not noticeably slowing global warming.

NOAA posts regularly updated measurements of the amount of heat stored in the bulk of the oceans.  For the upper 2000 m (deeper than that not much happens) it looks like this:

The amount of heat stored in the oceans is one of the most important diagnostics for global warming, because about 90% of the additional heat is stored there (you can read more about this in the last IPCC report from 2007).  The atmosphere stores only about 2% because of its small heat capacity.  The surface (including the continental ice masses) can only absorb heat slowly because it is a poor heat conductor.  Thus, heat absorbed by the oceans accounts for almost all of the planet’s radiative imbalance.

If the oceans are warming up, this implies that the Earth must absorb more solar energy than it emits longwave radiation into space. This is the only possible heat source. That’s simply the first law of thermodynamics, conservation of energy. This conservation law is why physicists are so interested in looking at the energy balance of anything. Because we understand the energy balance of our Earth, we also know that global warming is caused by greenhouse gases – which have caused the largest imbalance in the radiative energy budget over the last century.

If the greenhouse effect (that checks the exit of longwave radiation from Earth into space) or the amount of absorbed sunlight diminished, one would see a slowing in the heat uptake of the oceans. The measurements show that this is not the case.

The increase in the amount of heat in the oceans amounts to 17 x 10²² Joules over the last 30 years.  That is so much energy it is equivalent to exploding a Hiroshima bomb every second in the ocean for thirty years.

The data in the graphs comes from the World Ocean Database.  Wikipedia has a fine overview of this database.  The data set includes nine million measured temperature profiles from all of the world’s oceans.  One of my personal heroes, the oceanographer Syd Levitus, has dedicated much of his life to making these oceanographic data freely available to everyone.  During the Cold war that even landed him in a Russian jail for espionage for a while, as he was visiting Russia on his quest for oceanographic data (he once told me of that adventure over breakfast in a Beijing hotel).

How to deny data

Ideologically motivated “climate skeptics” know that these data contradict their claims, and respond … by rejecting the measurements. Millions of stations are dismissed as “negligible” – the work of generations of oceanographers vanish with a journalist’s stroke of a pen because what should not exist, cannot be. “Climate skeptics’” web sites even claim that the measurement uncertainty in the average of 3000 Argo probes is the same as that from each individual one.  Thus not only are the results of climate research called into question, but even the elementary rules of uncertainty calculus that every science student learns in their first semester.  Anything goes when you have to deny global warming.  Even more bizarre is the Star Trek argument – but let me save that for later.

Slowdown in the upper ocean

Let us look at the upper ocean (for historic reasons defined as the upper 700 m):

And here is the direct comparison since 1980:

We see two very interesting things.

First:  Roughly two thirds of the warming since 1980 occurred in the upper ocean.  The heat content of the upper layer has gone up twice as much as in the lower layer (700 – 2000 m).  The average temperature of the upper layer has increased more than three times as much as the lower (because the upper layer is only 700 m thick, and the lower one 1300 m).  That is not surprising, as after all the ocean is heated from above and it takes time for the heat to penetrate deeper.

Second:  In the last ten years the upper layer has warmed more slowly than before.  In spite of this the temperature still is changing as rapidly there as in the lower layer.  This recent slower warming in the upper ocean is closely related to the slower warming of the global surface temperature, because the temperature of the overlaying atmosphere is strongly coupled to the temperature of the ocean surface.

That the heat absorption of the ocean as a whole (at least to 2000 m) has not significantly slowed makes it clear that the reduced warming of the upper layer is not (at least not much) due to decreasing heating from above, but rather mostly due to greater heat loss to lower down:  through the 700 m level, from the upper to the lower layer.  (The transition from solar maximum to solar minimum probably also contributed a small part as planetary heat absorption decreased by about 15%, Abraham, et al., 2013).  It is difficult to establish the exact mechanism for this stronger heat flux to deeper water, given the diverse internal variability in the oceans.

Association with El Niño

Completely independently of this oceanographic data, a simple correlation analysis (Foster and Rahmstorf ERL 2011) showed that the flatter warming trend of the last 10 years was mostly a result of natural variability, namely the recently more frequent appearance of cold La Niña events in the tropical Pacific and a small contribution from decreasing solar activity.  The  effect of La Niña can be seen directly in the following figure, without any statistical analysis.  It shows the annual values of the global temperature with El Niño periods highlighted in red and La Niña periods in blue.  (Weekly updates on the current El Niño situation can be found here.)

One finds that both the red El Niño years and the blue La Niña years are getting warmer, but given that we have lately experienced a cluster of La Niña years the overall warming trend over the last ten years is slower.  This can be thought of as the “noise” associated with natural variability, not a change in the “signal” of global warming (as discussed many times before here at RealClimate).

This is consistent with the finding that reduced warming is not mainly a result of a change in radiation balance but due to oceanic heat storage. During La Niña events (with cold ocean surface) the ocean absorbs additional heat that it releases during El Niño events (when the ocean surface is warm). The next El Niño event (whenever it comes – that is a stochastic process) is likely to produce a new global mean temperature record (as happened in 2010).

Kevin Trenberth, who has recently published a paper on this topic, explains the increased heat uptake in the deep ocean:

The reason for the change is a specific change in the winds, especially in the subtropical Pacific, where the trade winds have become noticeably stronger.  That altered ocean currents, strengthening the subtropical sea water circulation thus providing a mechanism to transport heat into the deeper ocean.  This is related to the decadal weather pattern in the Pacific associated with the La Niña phase of the El Niño phenomenon. 

New results from climate modelling

A study by Kosaka and Xie recently published in Nature confirms that the slowing rise in global temperatures during recent years has been a result of prevalent La Niña periods in the tropical Pacific.  The authors write in the abstract:

Our results show that the current hiatus is part of natural climate variability tied specifically to a La Niña like decadal cooling. 

They show this with an elegant experiment, in which they “force” their global climate model to follow the observed history of sea surface temperatures in the eastern tropical Pacific. With this trick the model is made to replay the actual sequence of El Niño and La Niña events found in the real world, rather than producing its own events by chance.  The result is that the model then also reproduces the observed global average temperature history with great accuracy.

There are then at least three independent lines of evidence that confirm we are not dealing with a slowdown in the global warming trend, but rather with progressive global warming with superimposed natural variability:

1.  Our correlation analysis between global temperature and the El Niño Index.

2.  The measurements of oceanic heat uptake.

3.  The new model calculation of Kosaka and Xie.

Beam me up Scotty!

Now to the most amusing attempt of “climate skeptics” to wish these scientific results away.  Their argument goes like this:  It is not possible that warming of the deep ocean accelerates at the same time as warming of the upper ocean slows down, because the heat must pass through the upper layer to reach the depths. A German journalist put it this way:

Winds can do a lot, but can they beam warm surface waters heated by carbon dioxide 700 meters further down? 

This argument reveals once again the shocking lack of understanding of basic physics in “climate skeptic” circles.  First the alleged problem is lacking any factual basis – after all, in the last decades the upper layer of the oceans has warmed faster than the deeper (even if recently not quite as fast as before).  What is the problem with the heat first warming the upper layer before it penetrates deeper? That is entirely as expected.

Second, physically there is absolutely no problem for wind changes to cool the upper ocean at the same time as they warm the deeper layers.  The following figure shows a simple example of how this can happen (there are also other possible mechanisms).

The ocean is known to be thermally stratified, with a warm layer, some hundreds of meters thick, lying on top of a cold deep ocean (a).  In the real world the transition is more gradual, not a sharp boundary as in the simplified diagram.  Panel (b) shows what happens if the wind is turned on. The surface layer (above the dashed depth level) becomes on average colder (less red), the deep layer warmer.  The average temperature changes are not the same (because of the different thickness of the layers), but the changes in heat content are – what the upper layer loses in heat, the lower gains. The First Law of Thermodynamics sends greetings.

Incidentally, that is the well-known mechanism of El Niño: (a) corresponds roughly to El Niño (with a warm eastern tropical Pacific) while (b) is like La Niña (cold eastern tropical Pacific). The winds are the trade winds.  The figure greatly exaggerates the slope of the layer interface, because in reality the ocean is paper thin.  Even a difference of 1000 m across the width of the Pacific (let’s say 10,000 km) leads to a slope of only 1:10,000 – which no one could distinguish from a perfectly horizontal line without massive vertical exaggeration.

Now if during the transition from (a) to (b) the upper layer is heated by the greenhouse effect, its temperature could remain constant while that of the lower one warmed. Simple classical physics without beaming.

Beam me up Scotty!  There is no intelligent life on this planet.

Links

Tamino provides his usual detailed analysis of the new study by Kosaka and Xie.

Dana Nuccitelli in the Guardian on the same paper with some further interesting aspects that I have not talked about here.

Another important point that is often forgotten in the discussion:  The data hole in the Arctic that explains part of the reduced warming trend (maybe even more than previously thought).

And a reminder:  The warming trend of the 15-year period up to 2006 was almost twice as fast as expected (0.3°C per decade, see Fig. 4 here), and (rightly) nobody cared. We published a paper in Science in 2007 where we noted this large trend, and as the first explanation for it we named “intrinsic variability within the climate system”. Which it turned out to be.

Recent Literature:

Levitus et al. (Geophysical Research Letters 2012). Documentation of the heat increase in the world’s oceans since 1955.  Included are uncertainty analyses, maps of the measurement coverage and many illustrations of the regional and vertical distribution of the warming.

Balmaseda et al. (Geophysical Research Letters 2013) shows among other things that El Niño events are associated with a strong loss of heat from the oceans.  As discussed above, during an El Niño the ocean loses heat to the surface because the surface of the ocean (see Fig. (a) above) is unusually warm.  Further, during volcanic eruptions the ocean cools but for another reason: because volcanic aerosols shade the sun and thus the oceans are heated less than normal.

Guemas et al. (Nature Climate Change 2013) shows that the slower warming of the last ten years cannot be explained by a change in the radiative balance of our Earth, but rather by a change in the heat storage of the oceans, and that this can be at least partially reproduced by climate models, if one accounts for the natural fluctuations associated with El Niño in the initialization of the models.

Abraham et al. (Reviews of Geophysics 2013). Very recent, wide ranging review of temperature measurements in the oceans with a detailed discussion of the accuracy of the data, planetary energy balance and the effect of the warming on sea levels.