Real Skepticism About the New Marcott 'Hockey Stick'

A new global temperature reconstruction over the past 11,300 years by Marcott et al. (2013) has been described as 'the new hockey stick,' and adopted into 'the wheelchair' by Jos Hagelaars by including temperatures further in the past and projected for the future (Figure 1).


Figure 1: The temperature reconstruction of Shakun et al (green – shifted manually by 0.25 degrees), of Marcott et al (blue), combined with the instrumental period data from HadCRUT4 (red) and the model average of IPCC projections for the A1B scenario up to 2100 (orange).

The Marcott paper has been subjected to an immense amount of scrutiny, particularly in the climate contrarian blogosphere, with criticisms about everything from the wording of its press release to the timing of its Frequently Asked Questions (FAQ) publication.  Unfortunately climate contrarians have been so noisy in their generally invalid criticisms that the media has begun to echo them, for example in this Washington Post blog.

With all the hubub, it's easy to lose sight of the important conclusions of this paper.  The bottom line is that the rate of warming over the past century is very rapid and probably unprecedented for the past 11,000 years.  That's actually both good and bad news.

Why Climate Contrarians Should Love the Hockey Sticks

The last, best hope for climate contrarians is for climate sensitivity (the total global surface warming in response to the increased greenhouse effect from doubled atmospheric CO2) to be low.  We know the planet is warming due to humans increasing the greenhouse effect, and the only remaining plausible argument against taking action to do something about it is the hope that future climate change will be relatively minimal.

This is where climate contrarians lose the plot.  It's understandable to look at 'hockey stick' graphs and be alarmed at the unnaturally fast rate of current global warming.  But in reality, the more unnatural it is, the better.  If wild temperature swings were the norm, it would mean the climate is very sensitive to changes in factors like the increased greenhouse effect, whereas the 'hockey stick' graphs suggest the Earth's climate is normally quite stable.

On the one hand, these graphs do suggest that current climate change is unnatural – but we already knew that.  We know that humans are causing global warming by rapidly burning large quantities of fossil fuels.  On the other hand, the past climate stability suggests that climate sensitivity is probably not terribly high, which would mean we're not yet doomed to catastrophic climate change.  See, good news!

In their efforts to deny that the current warming is unprecedented and human-caused, climate contrarians are actually scoring a hockey stick own-goal because they're also arguing that the climate is more sensitive than the IPCC believes.  For those who oppose taking major steps to reduce greenhouse gas emissions, that's the worst possible argument to make

The good news for climate contrarians is that the current rate of global warming appears to be unprecedented over the past 11,000 years.  During that timeframe, the difference between the hottest and coldest average global surface temperature is around 0.7°C, with the cooling between those temperatures happening slowly, over about 5,000 years. 

Over the past 100 years, we've seen about 0.8°C global surface warming (Figure 2).  While the time resolution in the Marcott reconstruction is relatively low, there is simply no evidence of a similarly rapid or large natural climate change in the past 11,000 years.  As Tamino at the Open Mind blog shows, any similar rapid and large warming event to the current one would likely have shown up in the Marcott analysis, despite its low resolution.  Tamino concludes,

"the Marcott et al. reconstruction is powerful evidence that the warming we’ve witnessed in the last 100 years is unlike anything that happened in the previous 11,300 years."

While it may seem counter-intuitive, that's a good thing, because it means the climate is not highly unstable.

regem + hadcrut4

Figure 2: Regularized expectation maximization (RegEM) Marcott reconstructions (black), plus the HadCRUT4 series in 20-year averages centered on the times of the Marcott reconstruction (blue).  Created by Tamino.

The Hockey Stick 'Blade' is Real

Much of the manufactured controversy about the Marcott paper is in regards to the 'blade' or 'uptick' – the rapid warming at the end of the graph over the past century.  While their reconstruction does identify an approximately 0.6°C warming between 1890 and 1950, the authors note in the paper that this result is probably not "robust."  Tamino notes that this uptick appears to largely be a result of proxies dropping out (although a smaller uptick seems to be a real feature), as many individual proxies do not extend all the way to the year 1950.  If proxies with colder temperatures drop out, the remaining reconstruction can show an artificial warming toward the end.

In the paper, where they talk about temperatures over the past decade, the authors reference the instrumental temperature record rather than the proxies.  As Tamino notes in another excellent analysis of the paper,

"for the Marcott et al. reconstruction data coverage shrinks as one gets closer to the present. But that’s not such a problem because we already know how temperature changed in the 20th century."

Certain parties have complained that the press release and subsequent media coverage of the paper have not made it sufficiently clear that the 'blade' of this hockey stick comes from the instrumental temperature data.  This is the focus of the Washington Post article, for example.  However, the authors were clear on this point in the paper, and in several interviews and subsequent discussions, like the FAQ.

In reality, the 'blade' of the 'hockey stick' – the instrumental temperature record – is our most accurate temperature data set.   As noted in the FAQ on RealClimate, the instrumental temperature record is also consistent with proxies from other studies.  For example Anderson et al. (2012) compares their study's natural proxy temperature reconstruction (Paleo; solid line in Figure 3) to the instrumental surface temperature record (MLOST; dashed line in Figure 3) and finds a strong correlation (of 0.76) between the two.  Reanalysis data, as in Compo et al. (2013), has also independently confirmed the instrumental global surface temperature record accuracy (correlations between 0.84 and 0.92), as of course did the Koch-funded Berkeley Earth study.

Fig 1

Figure 3: Paleo Index (solid) and the merged land-ocean surface temperature anomalies (MLOST, dashed) relative to 1901-2000. The range of the paleo trends index values is coincidentally nearly the same as the instrumental temperature record although the quantities are different (index values versus temperature anomalies °C)From  Anderson et al. (2012).

There may be some valid criticism that the press release and some media discussions were not clear that the comments about recent unprecedented warming are based on comparing the instrumental temperature record to the Marcott reconstruction, but that is a very minor criticism that has no bearing on the scientific validity of the discussion or the Marcott paper.  Unfortunately the media has begun amplifying this minor and scientifically irrelevant point. 

Real Skepticism

It's worth taking a moment here to reflect on real skepticism.  Spending literally dozens of blog posts attacking a study because its results seem inconvenient is not real skepticism.  Comparing climate scientists to the mafia is not real skepticism.  Nitpicking minor details in press releases and media articles while ignoring the discussion in the paper itself is certainly not real skepticism.

If you want an example of real skepticism, look no further than Tamino's Open Mind blog.  Tamino read the Marcott paper, noted they had expressed doubt about the robustness of the final uptick in their proxies, looked at the data, identified the proxy dropout issue, tried some new analyses, and found that the proxy uptick is probably real but probably smaller than it appears in the paper.  Also see similar efforts by Nick Stokes.  These are the approaches of real skeptics.  At least the manufactured controversy over the Marcott paper has served to show who the real skeptics and "honest brokers" are.

The irony is that the climate contrarians are being their own worst enemies here.  A 'hockey stick' shape means less past natural variability in the climate system, which suggests that climate is relatively stable.  It's revealing that in their zealotry to deny that the current global warming could possibly be unnatural and unprecedented, the contrarians are actively trying to undermine their only potentially valid remaining argument against serious climate mitigation. 

Nevertheless, all signs indicate that the current rate of warming is very rapid, probably unprecedented in the past 11,000 years; that if we're not at the highest temperatures during that timeframe, we will be soon; and that despite the contrarians' best efforts to argue otherwise, we're not yet doomed to catastrophic climate change.

Also see this good post on the subject by John Timmer at Ars Technica, and this one by Climate Science Watch.

Posted by dana1981 on Wednesday, 10 April, 2013

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