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December 17, 2004

When Climatologists Attack!!

The climatologists are angry, and are on the warpath against the industry-funded Tech Central Station:

RealClimate » Temperature Variations in Past Centuries and the so-called "Hockey Stick": ...coined by the former head of NOAA’s Geophysical Fluid Dynamics Laboratory, Jerry Mahlman, to describe the pattern common to numerous proxy and model-based estimates of Northern Hemisphere mean temperature changes over the past millennium... a long-term cooling trend from the so-called "Medieval Warm Period" (broadly speaking, the 10th-mid 14th centuries) through the "Little Ice Age" (broadly speaking, the mid 15th-19th centuries), followed by a rapid warming during the 20th century that culminates in anomalous late 20th century warmth.... Numerous myths.. can be found on various non-peer reviewed websites.... Estimates of Northern Hemisphere average temperature changes from climate model simulations employing estimates of long-term natural (e.g. volcanic and solar) and modern anthropogenic (greenhouse gas and sulphate aerosol) radiative forcings of climate agree well, in large part, with the empirical, proxy-based reconstructions. One notable exception is a study by Gonzalez-Rouco et al (2003) that makes use of a dramatically larger estimate of past natural (solar and volcanic) radiative forcing than is accepted in most studies, and exhibits greater variability than other models. Yet, as in all of the other simulations, even in this case unprecedented warmth is indicated for the late 20th century.

The simulations all show that it is not possible to explain the anomalous late 20th century warmth without including the contribution from anthropogenic forcing factors, and, in particular, modern greenhouse gas concentration increases. A healthy, vigorous debate can be found in the legitimate peer-reviewed climate research literature with regard to the precise details of empirically and model-based estimates of climate changes in past centuries... it nonetheless remains a widespread view among paleoclimate researchers that late 20th century hemispheric-scale warmth is anomalous in a long-term (at least millennial) context, and that anthropogenic factors likely play an important role in explaining the anomalous recent warmth.

RealClimate » Myth vs. Fact Regarding the "Hockey Stick": Numerous myths regarding the so-called "hockey stick" reconstruction of past temperatures, can be found on various non-peer reviewed websites, internet newsgroups and other non-scientific venues. The most widespread of these myths are debunked below: MYTH #1: The "Hockey Stick" Reconstruction of Northern Hemisphere mean temperatures which indicates anomalous late 20th century warmth, is based solely on two publications by climate scientist Michael Mann and colleagues (Mann et al, 1998;1999). This is patently false. Nearly a dozen model-based and proxy-based reconstructions of Northern Hemisphere mean temperature by different groups all suggest that late 20th century warmth is anomalous in a long-term (multi-century to millennial) context.... Some proxy-based reconstructions suggest greater variability than others. This greater variability may be attributable to different emphases in seasonal and spatial emphasis (see Jones and Mann, 2004; Rutherford et al, 2004; Cook et al, 2004). However, even for those reconstructions which suggest... greater variability in general in past centuries... late 20th century hemispheric warmth is still found to be anomalous in the context of the reconstruction (see Cook et al, 2004).

RealClimate » False Claims by McIntyre and McKitrick regarding the Mann et al. (1998) reconstruction : Given that each of the criticisms of MBH98 raised by MM are demonstrably false, one might well be led to wonder how MM, using the MBH98 method and their putative "corrected" version of the MBH98 proxy dataset, were able to obtain a reconstruction so at odds with the MBH98 reconstruction and virtually all existing reconstructions (in particular, in its apparent indication of anomalous 15th century warmth). Rather than "correcting" the MBH98 proxy data set, we demonstrate that the reconstruction of MM resulted, instead, from their selective censoring of key indicators from the MBH98 proxy dataset. Indeed, we are able to reproduce the MM reconstruction of anomalous 15th century warmth when the entire ITRDB North American data set (and the "Queen Anne" series) are censored from the proxy network (Figure 4).

RealClimate » Rutherford et al 2004 highlights: The claims of McIntyre and McKitrick regarding the Mann et al (1998) temperature reconstruction have recently been discredited by the following peer-reviewed article... Rutherford, S., Mann, M.E., Osborn, T.J., Bradley, R.S., Briffa, K.R., Hughes, M.K., Jones, P.D., Proxy-based Northern Hemisphere Surface Temperature Reconstructions: Sensitivity to Methodology, Predictor Network, Target Season and Target Domain, Journal of Climate, in press (2004): "It should be noted that some falsely reported putative errors in the Mann et al.(1998) proxy data claimed by McIntyre and McKitrick (2003) are an artifact of (a) the use by these latter authors of an incorrect version of the Mann et al. (1998) proxy indicator dataset, and (b) their misunderstanding of the methodology used by Mann et al. (1998) to calculate PC series of proxy networks over progressively longer time intervals. In the Mann et al. (1998) implementation, the PCs are computed over different time steps so that the maximum amount of data can be used in the reconstruction.

"For example, if a tree-ring network comprises 50 individual chronologies that extend back to AD 1600 and only 10 of those 50 extend to AD 1400 then calculating one set of PCs from 1400 to 1980 (the end of the Mann et al. (1998) calibration period) would require the elimination of 40 of the 50 chronologies available back to AD 1600. By calculating PCs for two different intervals in this example (1400-1980 and 1600-1980) and performing the reconstruction in a stepwise fashion, PCs of all 50 series that extend back to AD 1600 can be used in the reconstruction back to AD 1600 with PCs of the remaining 10 chronologies used to reconstruct the period from 1400-1600. The latter misunderstanding led McIntyre and McKitrick (2003) to eliminate roughly 70% of the proxy data used by Mann et al. (1998) prior to AD 1600, including 77 of the 95 proxy series used by Mann et al. (1998) prior to AD 1500. This elimination of data gave rise to spurious, anomalous warmth during the 15th century in their reconstruction, sharply at odds with virtually all other empirical and model-based estimates of hemispheric temperature trends in past centuries (see e.g. Jones and Mann, 2004).

As I understand things (and I may be wrong), if there is any science in McIntyre and McKitrick at all (and I am not convinced that there is), it is an observation that North America during the "Medieval warm period" was a lot colder than North America today, while Europe during the "Medieval warm period" looks almost as warm as Europe today. Hence if you regard America as irrelevant and throw out the American data, late 20th-century warming does not look as anomalous as in studies that look at the entire northern hemisphere.

Posted by DeLong at December 17, 2004 11:05 AM

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...while if you instead arbitrarily regard Europe as irrelevant (kind of a hip thing to do on the right), suddenly global warming looks really scary. Oops.

This kind of analysis is fun!

Posted by: Ben V-L at December 17, 2004 12:35 PM

America is irrelevant? Is this the new rallying cry of Tech Central Station?

Posted by: Kuas at December 17, 2004 01:17 PM

Climate change can't be real and if it is appening it's not caused by humans. Common sense says that dumping billions of tons of poison into the air each year won't cause any ill effects whatsoever. Thus smog days are a hoax, no one has ever gotten skin cancer and it's completely normal that the 10 hottest years in the past 150 occured since 1990. Nothing to see here folks, move along ! In fact no one has ever gotten sick from pollution because pollution doesn't exist, it's another lie by those radical lefties to steal your hard earned money! Life is perfect, keep buying trinkets, close your eyes, we'll take care of everything, you won't be happy without the trinkets, focus on the trinkets people!

Posted by: zzz at December 17, 2004 02:22 PM

My only question about RealClimate is: Why did it take them so long? The models have existed for years: the rationalist "skeptic movement" has done an admirable job of debunking paranormal nonsense, and evolutionary biologists twigged years ago to the fact that they needed public resources to teach open-minded but confused laypeople about evolution. (They haven't managed to swing the Gallup numbers, but they at least make it possible for people who want the truth to find it.)

But on climate science there really hasn't been this level of defensive effort until now; whereas the anti-environmentalists have had well-funded efforts to muddy the perception of the scientific debate going for a long time, and environmentalist advocacy groups have been passionate about policy but not particularly interested in pushing the details of the science.

The "climate change skeptics" have even successfully infiltrated the skeptic movement to some degree (via the catchphrase "junk science"), and made it fashionable for people who really ought to know better to believe conspiracy theories about climatologists all colluding to run an enormous, politically motivated scam. They rightly find the same sorts of claims silly when they come up about relativity, evolution or the moon landings, but somehow this one gets traction, possibly because the people promulgating it portray themselves as pro-technology rationalists.

Posted by: Matt McIrvin at December 18, 2004 03:14 PM

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