Darlings of the denialosphere reaffirm that, underlying natural variability, the man-made global warming signal is real – and say it’s accelerating

OR: DENIALIN’ AIN’T EASY

Monday, like any other day, was a pretty miserable one to be a climate denialist.

As with Intelligent Design proponents (née Creation Science proponents, née Creationists), 9/11 Truthers, the anti-vax loons, et al., in order to give themselves the appearance of credibility, climate denialists have to simultaneously wrap themselves in the vestments of science while rejecting what the science actually says. It is, as you can imagine, an entirely unconvincing act.

The evidence that man-made processes, notably our emissions of greenhouse gases, are warming the planet and altering the climate has grown astoundingly in the 30 years since the National Academies of Science-commissioned Carbon Dioxide and Climate: A Scientific Assessment. Each week, evidence pours in in the form of primary scientific literature documenting the changes to our climate, their already-significant impacts on plants and animals, and so on. Calls to cut GHG emissions are heard from the science academies of nations around the world. There is, to put it mildly, a stunning lack of scientific evidence suggesting we should keep on as we are and expect no fallout.

Hence you get Serious Thinkers like Jonah Goldberg asserting that, despite a study’s lead author’s clear statement that his paper on sunspots has nothing to do with anthropogenic warming, we should in fact believe it has something to do with- and indeed undermines- our understanding of anthropogenic warming. There are only a few avenues open to denialists who seek to keep up the charade that they care about science: attempt to discredit the actual science and substitute their own bloggy version, champion studies that are tremendously flawed, clumsily misrepresent papers that don’t actually challenge the undeniable reality of anthropogenic warming, or some combination thereof.

Enter Kyle Swanson and Anastasios Tsonis.

Swanson and Tsonis have been exploring an apparent series of climatic regime changes (marked by the synchronization, coupling, and decay on multidecadal scales of naturally variable climate features) superimposed on the long term warming trend of the 20th century (e.g. here and here). They think that the existence of these regimes might better explain some features of the temperature record than the mechanisms usually invoked (e.g. mid-century cooling via sulfate production). Naturally, their hypothesis has been seized upon by a denialosphere desperate for anything that seemingly challenges the mainstream on global warming (e.g. here, here, here, etc.) and touted as evidence that “the science isn’t settled”, “nature not humans controls the climate”, and similar rubbish. Thus Swanson and Tsonis became a sort of fig leaf behind which the denialists sought to conceal their anti-science beliefs.

For its part, the reaction from the larger climate science community has been along the lines of, “interesting, but absent any physical explanation, not entirely convincing at the present” and it was repeatedly pointed out that Swanson and Tsonis’s work wasn’t saying anything like what the denialists imagined it to. In the interests of clarifying some misconceptions, RealClimate recently hosted a guest post by Swanson. There, Swanson points out that those attempting to use any findings of increased influence of natural variability over the 20th century as a rebuttal to concerns over enhanced greenhouse warming couldn’t be more wrong; a climate system more sensitive to natural variability is one also that will respond more strongly to anthropogenic forcing:

A climate that is highly sensitive to radiative forcing (i.e., responds very strongly to increasing greenhouse gas forcing) by definition will be unable to quickly dissipate global mean temperature anomalies arising from either purely natural dynamical processes or stochastic radiative forcing, and hence will have significant internal variability. The opposite also holds.

In a new paper published this week in the early edition of the journal PNAS entitled Long-term natural variability and 20th century climate change (or here), Swanson and Tsonis, along with coauthor George Sugihara (from here on “SGT”), have attempted to address more explicitly the mechanism by which this enhanced natural variability hypothesis might work and its impact on the 20th century temperature record.

In the paper, SGT make a point to highlight the same argument made by themselves and others in response to specious (denialist) claims that their findings mean anthropogenic warming is as a result less of a concern:

Viewed in this light, the lack of modeled compared to observed interdecadal variability may indicate that current models underestimate climate sensitivity.

Reiterating that a climate system highly sensitive to natural variability is one also that will respond more strongly to anthropogenic forcing. A higher climate sensitivity would mean, for example, that the IPCC estimates of ~3°C warming in response to doubled CO2 are too optimistic.

The meat of the paper is devoted to attempting to identify and isolate the sources for SGT’s proposed increased  natural variability. Those familiar with the subject of natural variability in the climate system will no doubt guess correctly where SGT believe the enhanced variability comes from: Yes, changes in ocean circulation. SGT isolate what the describe as ” residual anomaly sea surface temperature (RASST) fields”, which they test against sea surface temperatures modeled under control (i.e. non-anthropogenically forced) conditions, and find that natural variability exceeds that captured by the 10 CMIP3 models tested.

As I recently devoted a bit of space to, climate science understands that the real world global average temperature will not, due to the influence of natural variability, warm monotonically, even though the forced component of climate over time will show a virtually monotonic increase. Thus the IPCC SRES projections are not the same as explicit year-to-year predictions and confusing the two is a huge source of confusion about what to expect under global warming. In essence, real world temperatures will vary around the forced component of climate (as reflected by the SRES ensemble average). SGT’s proposal that natural variability is being underestimated by some models should exaggerate this effect- increasing both the amount of variability around the warming trend and the trend itself.

And in fact, SGT find that by isolating the forced component of climate over time, this signal (thin solid line) is not only monotonic, but nearly a quadratic fit for 20th century temps, while temperature over even interdecadally-averaged (21 yr) periods shows significant variability around the underlying, accelerating warming trend.

Like Mojib Latif, they warn- but do not predict- that the existence of natural variability on interannual-to- multidecadal scales could work against political efforts to mitigate against dangerous anthropogenic climate change, due to the appearance of a pause or “cooling” in temperatures despite a clear underlying warming trend.

Finally, the presence of vigorous climate variability presents significant challenges to near-term climate prediction, leaving open the possibility of steady or even declining global mean surface temperatures over the next several decades that could present a significant empirical obstacle to the implementation of policies directed at reducing greenhouse gas emissions.

Obviously, those who claiming that SGT’s findings are evidence that anthropogenic warming is not a serious issue would be deluding themselves. Just in case this isn’t clear, SGT point out early on in the paper and in its conclusion that increased sensitivity is actually a cause for more concern, not less, about our continued forcing the climate:

[O]minously, a climate with large magnitude natural long-term variability in general is a climate very sensitive to imposed forcings, raising concerns about extreme impacts due to future climate change…

…To paraphrase C. S. Lewis, the climate system appears wild, and may continue to hold many surprises if pressed.

That sounds downright alarming, doesn’t it?

This is all not to say that SGT 2009 is necessarily a better explanation of rough patches in our understanding of the 20th century temp record than the current placeholders, much less a declaration that climate sensitivity is being significantly underestimated by the IPCC- the multiple lines of evidence pointing to a value of ~3°C are not easily dismissed [ahem]. I’m sure others with an interest in time series analysis will have their own thoughts [ahem], and it wouldn’t surprise me if RealClimate addresses this paper along with the alleged “prediction” by Mojib Latif.

I am, however, eagerly awaiting to see how the denialosphere treats their former poster boys, as Swanson and Tsonis have given them little room to run.

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One response to “Darlings of the denialosphere reaffirm that, underlying natural variability, the man-made global warming signal is real – and say it’s accelerating

  1. Pingback: About that Daily Mail “mini ice age story” « The Way Things Break

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