The Past Global Changes (PAGES) project has released a progress article that has just appeared in Nature Geoscience. The article’s title is Continental-scale temperature variability during the past two millennia (this article is paywalled unfortunately). Watts Up With That (WUWT) has had a couple of posts about this already, but noone of them have said anything particularly controversial. If Marcott et al. (2013) is anything to go by, however, there are likely to be a number of other posts to follow in the coming days.
Why is this study so interesting? Well, it uses proxies from many different locations around the world to determine the temperature history of the Earth for the last 2000 years. As can be seen in the figure below (which I took from Skeptical Science), it largely reproduces the “Hockey Stick” shape first presented in a paper by Mann, Bradley and Hughes (1999). The curves in the figure are from previous studies, while the data points (with errors) are from the PAGES study. The name Hockey Stick refers to the uptick in the temperature anomalies starting in the late 1800s and has been extremely controversial, with many skeptics claiming that there is a major problem with this work (often making accusations of fraud and deceit). Despite this, it has been reproduced in many studies including in the work presented by the PAGES project. One of the conclusions of this study was that the 20th century was the warmest or nearly the warmest in all regions except Antarctica.
Another interesting conclusion was that although the Medieval Warm Period and the Little Ice Age are clearly present in the temperature constructions, these appear not to have been global events. For example, the Northern Hemisphere had a sustained warm period from about 830 AD to 1100 CE, while Australasia and South American had a warm period from 1160 to 1300 CE. This is fairly evident from the figure below which shows 30-year mean temperatures for the 7 continental scale regions with red being warmer and blue being cooler.
I think the study is very interesting, but there has already been some criticism on the WUWT site and there are a few posts by Steven McIntyre on his Climate Audit site. Let me make a few basic comments. The PAGES project consists of 78 researchers from 24 different countries and from 60 different institutions. It uses 511 different time series, most of which were tree rings, but does include glacier ice, speleothems, and sediment from ocean and lake bottoms. It also considers all 7 continental-scale regions on the planet. According to their FAQ, 360 of their 511 records were not used in the Mann, Bradley and Hughes (1999) study, so it has some independence.
Admittedly, I’m getting most of my information about this study from their own FAQ. I’m therefore assuming here that they’re being honest, but I have no reason to suspect otherwise. I also think that their website is a great example of how to present this kind of work to the public. It seems very thorough and contains lots of information that attempts to explain what they did and the significant of the work, and includes discussion of the uncertainties. Essentially this seems to be a very large project with numerous researchers from around the world who have produced one of the most detailed temperature reconstructions ever and that appears to be largely consistent with much of the work that has been done in the last 14 years or so. That doesn’t make it correct, but science does require some judgement of the strength of the evidence. There seem to be – in some sense – two options. Evidence provided by a large group of professional scientists (plus all the previous work that seems consistent with this study) against evidence provided by a group of people who may or may not have any scientific training lead by a mathematician who (according to my Google search) is a semi-retired mining consultant. Admittedly, there hasn’t been all that much criticism of this particular study from those who are typically skeptical, so maybe they are starting to give some credence to the published science. On the other hand, if they do ratchet up their criticism (as has been the norm so far) it’s going to have to be pretty convincing to persuade me that they somehow know more about temperature reconstructions than a large (and global) group of climate scientists.