Global warming does not slow down

9 04 2013

Julien Emile-Geay

Thanks to the miracles of credit card reward programs, I have been gainfully receiving, for some time now, a weekly copy of the distinguished magazine The Economist. It is usually a fine read, especially since its editors distanced themselves from the laughable flat-Earthing of the Wall Street Journal a long time ago. It was therefore  a bit of a shock to read last week (on the cover, no less): “global warming slows down” (holding the quotation marks with gloves).  Had they been bought by Rupert Murdoch, or were they privy to new data, of which the  isolated climate scientist that I am had remained woefully ignorant? Well, neither, it seems.

Opening its pages with a mix of curiosity and skepticism (yes, we climate scientists are skeptics too – skepticism is not the privilege of deniers but the hallmark of healthy minds), I read the piece “Global Warming: apocalypse perhaps a little later”. It argued that since some recent estimates of climate sensitivity have been revised downward, the world might not scorch as early as we once thought. Still, they wisely argued that this was no excuse for inaction, and that the extra time should be used to devise plans to mitigate, and adapt to, man-made climate change.

Though I  agree with the consequent, I wholeheartedly disagree with the premise.

Digging a little deeper, it appears that they devoted a whole piece on climate sensitivity (“Climate Science: a sensitive matter“).  James Annan was right in pointing out its quality – it is a nuanced piece of science writing for a lay audience,  something all of my colleagues and I know the difficulty of achieving. It wasn’t the science journalist’s job to pick winners, but as a climate scientist I can tell you that not all the evidence presented therein carried equal weight. My issue has to do with the emphasis (here and elsewhere) that just because surface temperature  has been practically flat for the past decade (i.e.  the transient climate response (TCR) may not be as high as one would have concluded from the 1970-2000 warming), this means that equilibrium climate sensitivity (ECS) must also be lower.

Now, the “sensitive matter” piece does take care to distinguish the two concepts (transient vs equilibrium warming), but the message did not reach the editors, who conflate them with flying colors on the front page (“global warming slows down”). So while said editors can apparently hire good science writers, it would be even better if they  read their writings carefully.

My personal take is that TCR  is an ill-suited measure of climate response, because it only considers surface temperature. When energy is added to the climate system (e.g. by increasing the concentration of heat-trapping substances like carbon dioxide), it can go do any of 3 things:

  1. raise surface temperature
  2. help water change phase (from liquid to vapor, or from ice to liquid)
  3. raise temperature somewhere else (e.g. the deep ocean).

Tracking surface warming is certainly of paramount importance, but it’s clearly not the whole story. Focusing exclusively on it misses the other two outcomes. Do we have evidence that this might be a problem? Well, most glaciers in the world are losing mass, and a recent article by Balmaseda et al [2013], shows very clearly that the heat is reaching the deep ocean (see figure below). In fact, the rise in ocean heat content is more intense as depth than it is at the surface, for reasons that are not fully understood. To be fair, nothing involving the tri-dimensional ocean circulation is particularly straighforward when viewed through the lens of a few decades of observations, but the ocean heat content is quite a  robust variable to track, so even if the causes of this unequal warming are nebulous, the unequal warming isn’t.

Image

The Balmaseda et al study concludes that, when considering the total rise in ocean heat content, global warming has not slowed down at all: it has in fact accelerated. I’m not quite sure I see it this way: warming is still decidedly happening, but it does look like this rate of increase has slowed down somewhat (as may be expected of internal climate variability). Therefore, to meet the Economist halfway, I am willing to embrace conservatism on this issue: the climate system is still warming, and let’s agree that the warming has neither slowed down nor accelerated.

In fairness, the Economist’s “sensitive matter” piece did quote the Balmaseda et al. study as part of its rather comprehensive review ; in my opinion, however, this is not just one data point on a complex issue (as their piece implies), but is a real game-changer. It confirms the findings of Meehl et al, [2011], who predicted exactly that: greenhouse energy surpluses need not materialize  instantly as surface warming ; in many instances, their model produced decades of relative “pause” in surface temperature amidst a century-long warming. It did so because the ocean circulation is quite variable, and sometimes kidnaps some of the heat to great depths, so it takes time before the whole of the climate system feels it. This is one reason for the essential distinction between ECS and TCR: the inherent variability of the climate system means that it may take a long time to reach equilibrium. That’s what’s really bothersome about ECS: it’s not observable on any time scale we care about, so it is of limited relevance in discussing reality (it is important to understanding climate models, however).  Perhaps TCR should be based on some measure of ocean heat content? This might already have been done, but I am not aware of it. Actually, sea level might be our best proxy for integrated ocean heat content plus melted ice, and non-surprisingly it is still going up.

So despite the jargon, the basic idea is quite simple: more CO2 means a warmer climate system as a whole, and sooner rather than later.  So it is becoming increasingly urgent to do something about it, as they point out.  Now, what would it take to convince the Wall Street Journal of that?

UPDATE (April 16, 3:30 PST): further accounts of The Economist’s unduly optimistic perspective are given here and here.  Another paper published this week in Nature Climate Change (nicely summarized here)  also emphasizes the important role of the ocean in mediating surface warming.

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