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Global temp trends may have paused, but hold off on the champagne awhile

The behavior of oceans may hold the key to the puzzle of a recent apparent plateau in global temperatures.

Seekers after good news on climate change will have to look beyond The Economist’s commentary reprinted in Saturday’s Strib, reporting that “some scientists are arguing that man-made climate change is not quite so bad a threat as it appeared to be a few years ago” and has, in fact, “plateaued.”

I admit those conclusions grabbed my attention, all the more so because I admire The Economist’s analytical approach to climate issues and to environmental subjects in general.

You can still find publications willing to report that “some scientists” think global warming is a fiction, a conspiracy, a hoax. But The Economist prefers real scientists and real data, and so I began to dig into the devilish details behind these conclusions, both in the full-length analysis from which the commentary was derived and elsewhere.

Here’s what I found:

Unlike the commentary, the analysis reaches no conclusion that “climate change has plateaued.” It does report, and discuss, two important but brief and recent temperature trends:

  • “The five-year mean global temperature has been flat for a decade.” That’s a key measure, to be sure, and the quote is from NASA’s James Hansen, perhaps the most influential single scientist in this field.
  • Ed Hawkins, of the University of Reading, in Britain, points out that surface temperatures since 2005 are already at the low end of the range of projections derived from 20 climate models. If they remain flat, they will fall outside the models’ range within a few years.”

A puzzle, not a reversal

Hawkins is a real climatologist, too, and some climate deniers have used his finding — over his objections — to argue that global warming either has halted or never even happened.

That’s not The Economist’s take, not by a mile:

The mismatch between rising greenhouse-gas emissions and not-rising temperatures is among the biggest puzzles in climate science just now. It does not mean global warming is a delusion. Flat though they are, temperatures in the first decade of the 21st century remain almost 1°C above their level in the first decade of the 20th. But the puzzle does need explaining.

One possible explanation is that something out of the ordinary has been happening in this relatively short time frame. Another is that “climate sensitivity” — the degree to which greenhouse-gas levels drive global temperatures upward — is being offset by factors we still don’t fully understand.

Heat absorption by seawater is once such factor, and it may be more potent than was thought. While the oceans’  surface temperatures have also leveled off in recent years, recent research shows that the deep ocean — waters more than two kilometers down — has been warming more than we noticed, and may greatly increase this offset.

A second offset is reflective “aerosols” like volcanic dust and sulfates. But here the research trends seem to be moving in an unhelpful direction.

Based on a leaked draft of the Intergovernmental Panel on Climate Change’s next report, due out in about six months, The Economist says aerosols appear to be helping less than was thought and some of them  — like carbon soot — are actually making warming worse.

‘Faustian bargain’ with carbon

Hansen himself, in a recent blog, refers to aerosol-induced optimism as a “Faustian bargain” in which we hope that atmospheric carbon loads lasting for millennia will somehow be canceled by aerosols that “fall out after about five days.”

A third factor is natural variability, a sort of background level of warming and cooling that occurs independent of human-caused atmospheric changes. Without going anywhere near the claims of some deniers that all recently observed warming is natural, The Economist reports that

There is some evidence that the natural variability of temperatures may be somewhat greater than the IPCC has thought. A recent paper by Ka-Kit Tung and Jiansong Zhou in the Proceedings of the National Academy of Sciences links temperature changes from 1750 to natural changes (such as sea temperatures in the Atlantic Ocean) and suggests that “the anthropogenic global-warming trends might have been overestimated by a factor of two in the second half of the 20th century.”

 It is possible, therefore, that both the rise in temperatures in the 1990s and the flattening in the 2000s have been caused in part by natural variability.

How long this plateau may last is of course a matter of much controversy, but it’s important to note that every scientist and study referenced in The Economist’s analysis predicts that global temperatures will continue to rise.

Their disagreements are over the amount of that increase, arising primarily from differences in how they approach modeling:

One type of model — general-circulation models, or GCMs — use a bottom-up approach. These divide the Earth and its atmosphere into a grid which generates an enormous number of calculations in order to imitate the climate system and the multiple influences upon it. The advantage of such complex models is that they are extremely detailed. Their disadvantage is that they do not respond to new temperature readings. They simulate the way the climate works over the long run, without taking account of what current observations are. Their sensitivity is based upon how accurately they describe the processes and feedbacks in the climate system.

 The other type — energy-balance models — are simpler. They are top-down, treating the Earth as a single unit or as two hemispheres, and representing the whole climate with a few equations reflecting things such as changes in greenhouse gases, volcanic aerosols and global temperatures. Such models do not try to describe the complexities of the climate. That is a drawback. But they have an advantage, too: unlike the GCMs, they explicitly use temperature data to estimate the sensitivity of the climate system, so they respond to actual climate observations.

Different models, different numbers

The closest we have to consensus projections on climate come from the IPCC, which uses the first type of modeling. Based on the leaked draft, The Economist says the panel’s 2013 report will retain its 2007 assessment that the most likely warming scenario is for an increase of 3º C —”extremely damaging” — while raising the upper, worst-case limit from 4.5º C  to 6º C  or even 7º C .

Other recent projections —almost all of which use the second method—put the most likely extent of warming in the range of 1.6º C  to 1.9º C  and the upper limits at 3º C to 4.5º C.

Taking all this evidence together, The Economist analysis concludes that

a small reduction in estimates of climate sensitivity would seem to be justified: a downwards nudge on various best estimates from 3°C to 2.5°C, perhaps; a lower ceiling (around 4.5°C), certainly. If climate scientists were credit-rating agencies, climate sensitivity would be on negative watch. But it would not yet be downgraded.

If that counts as good news with you, then you are more susceptible to soothing than I.

Comments (8)

  1. Submitted by Michael Kelberer on 04/09/2013 - 09:27 am.

    In fact, don’t bother to buy the champagne

    In fact, the recent (and very short by ecosystem timeframes) leveling of the mean global air temperature is likely a case of paying Peter now, only to have to pay Paul back later. According to a paper in the American Geophysical Union’s Geophysical Research Letters, the reason the air temps aren’t rising is that the global warming energy has gone into heating the deeper ocean waters. Normally, that doesn’t happen – the surface water’s warm first and since warm water floats over cold water, the deeper ocean is slow to heat. But – enter the Pacific Decadal Oscillation (PDO), which during the past decade has been pushing colder water up from the depths, and circulating the warm water down. With the cold water on top, the ocean has been absorbing more of the heat energy of global warming, leaving the surface air temperatures unchanged.
    Trouble is, when the next PDO cycle starts, that pattern will reverse, and much of that absorbed energy will re-enter the atmosphere, and we’ll pay for this decade of “flat” temperatures with a decade of rapidly increasing temperatures. In fact, say the scientists, this is exactly what happened in the late 80s and 90s (1998 remaining one of the hottest years on record).
    That’s the trouble with looking at one number (mean global AIR temperature) as a proxy for the complex phenomenon of climate change. We’re coasting now, but hang on to your hat!

  2. Submitted by Ray Schoch on 04/09/2013 - 10:38 am.

    Horticultural zones

    According to most of the gardening and horticultural sources I regularly use as an enthusiastic xeric gardener, Denver and Minneapolis are both in Zone 4. Having lived in both places for several years, I can now testify truthfully that horticultural zones definitely do not tell the whole story. Even if the climate-change sky is not falling, a gradual change from the Minnesota climate of yesteryear to one more like Omaha will have rather important effects on everything from tourism to agriculture. When I arrived here, I assumed most of what I’d learned about xeric gardening on the Colorado Front Range would be irrelevant to Minnesota. It appears more and more that perhaps that’s not the case.

    Indeed, there’s little reason to splurge on champagne at the moment.

  3. Submitted by Lance Groth on 04/09/2013 - 12:49 pm.

    Feedback loops

    I haven’t read the Economist article yet, but I see no mention in this article of feedback loops. The release of large amounts of methane (far more potent a greenhouse gas than CO2, but fortunately also fairly short-lived in the atmosphere) from melting tundra, and potentially even melting methane hydrates on the sea bottom, can change the picture dramatically. The disruption of ocean circulation currents due to warming and the influx of fresh water can also dramatically alter the ocean’s heat exchange mechanisms.

    Dumping vast amounts of greenhouse gases into the atmosphere is a case of playing with fire. It is a planetary scale uncontrolled experiment, the outcome of which cannot be known with precision, but which is certain to change the world we know in ways that we won’t like. Once the consequences become clear, the problem cannot be easily undone. Arguing about whether the models need to be adjusted a degree or two this way or that seems to me to be rather like trying to decide whether to take one aspirin or two, when the pain in your head is caused by a malignant tumor.

    • Submitted by rolf westgard on 04/10/2013 - 09:23 am.

      Feedback loops

      There are indeed vast amounts of GHGs going into the atmosphere. But the main feedback loops seem to be preventing any rise in atmospheric temperatures. As Stanford’s Nobel physicist put it recently, “carbon dioxide is a non-poisonous gas, essential to life, and better yet, destined for capture and recycling by plants.”

      • Submitted by Lance Groth on 04/10/2013 - 12:53 pm.

        Straw man

        Rolf – c’mon, you know better. Water is good for you too, unless you drink too much of it. Yes, plants need CO2. The climate system needs a certain amount too, or the planet would be an iceball. But too much overheats the planet, which is abundantly clear from paleo records of various types. With 7+ billion people dependent on our agricultural systems, screwing with the climate system without knowing how it will really play out is just plain dumb.

        I tend to view the current “pause” in escalating temperatures as a brief respite during which we ought, if we are as intelligent as we like to think we are, to get our house in order, and get a serious program underway to develop clean alternatives to fossil fuels. Nibbling at the edges won’t do it, it requires something on the scale of the Manhattan project, or the Marshall Plan.

        This is not to even mention the tremendous damage that extraction, processing, and spilling does. There are plenty of good reasons to change our ways. Only inertia and greed stand in the way.

        I know you’re also old enough, as I am, to remember how things were 40 or 50 years ago. The climate has already changed dramatically. The ice isn’t melting because it’s getting colder. Let’s get real, here.

  4. Submitted by rolf westgard on 04/09/2013 - 07:48 pm.

    Actual data

    Global temperatures from January 2011 through February 2013, as reported by the Climate Data Center, are below the lowest estimates from each of the four Assessment Reports issued by the IPCC since its founding. Temperatures from 1997 to the present are warmer than in the preceding decades, but have not risen since 1997 despite a continuing rise in atmosphere green house gases. I don’t know why, and nobody else does either.

  5. Submitted by Eric Ptak on 04/12/2013 - 09:42 pm.

    Funny Thing

    I read the article in the Economist, and I cannot help but chuckle at the fact that James Hansen, the NASA climate scientist who was prominently mentioned in the article, retired from NASA to become a climate activist and has been arrested at protests. I can only guess that his data were adulterated for the article in question.

    Another scientist mentioned in the article, Dr Ed Hawkins of the University of Reading’s Department of Meteorology, was among several leading climate scientists to denounce the misrepresentation of a Met Office report on global temperatures as proof that ‘global warming is at a standstill’.

    I can only gather that the Economist article is a load of flimflam, and wish I had not wasted my time reading it.

  6. Submitted by rolf westgard on 04/14/2013 - 07:03 pm.

    The numbers don’t lie

    Global temps are at a standstill.

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