Mike Mann will have a twitfest on Twitter trying to knock this one down. Data from 91 Northern Hemisphere proxies was used to reconstruct temperature. See reconstruction graph (figure 5) below.
Via The GWPF:
A new paper, looking back at the climate of the past two thousand years, published in the journal “Climate of the Past,” will either cause something of a stir, or provide confirmation of what some regard as having already emerged from the peer-reviewed scientific literature. The title of the paper is, “The extra-tropical Northern Hemisphere temperature in the last two millennia: reconstructions of low-frequency variability,” by B Christiansen of the Danish Meteorological Institute and F C Ljungqvist of Stockholm University.
The climate of the past few hundred years is of clear importance because it allows scientists to put today’s warm period into context, and provides some evidence of the influence of the quantity of greenhouse gasses that mankind has injected into the atmosphere. In much literature and during many debates statements to the effect that it is warmer now than it has been for thousands of years are frequently used.
As the authors point out the major problem with reconstructing the climate of the past few thousand years is that the so-called instrumental period – for which we have direct measurements – only stretches back as far as the middle of the 19th century. To overcome this researchers in this paper compile an impressive number of temperature proxies situated in the extra-tropical Northern Hemisphere. There are 91 in total, comprising ice-cores, tree-rings (density and width), lake and sea sediments, historical records, speleotherms, and pollen. All of them go back to 1500 AD and 32 go back as far as 1 AD.
The reconstruction of past climate has improved significantly in the past few years due to the availability of more proxies and better statistical analysis. The authors acknowledge this and point out the differences that are emerging from the reconstructions conducted about a decade ago. They mention two such reconstructions performed by Michael Mann that they say, perhaps typically for the period, show little variability. They add they display, “little evidence for previous temperature anomalies comparable to those of the 20th century.” The authors conclude that previous climate reconstructions “seriously underestimate” variability and trends in the climate record of the past two millennia.
This new analysis shows that the warming we have seen in the late-20th century is not unprecedented, as can be seen in figure 5 (from the paper). Seen in the reconstruction is a well-defined peak of temperature between 950–1050 AD. They also find that the first millennium is warmer than the second.
The researchers conclude:
“The level of warmth during the peak of the MWP (Medieval Warm Period) in the second half of the 10th century, equaling or slightly exceeding the mid-20th century warming, is in agreement with the results from other more recent large-scale multi-proxy temperature reconstructions.”
Ljungqvist et al. also show that, “on centennial time-scales, the MWP is no less homogeneous than the Little Ice Age if all available proxy evidence, including low-resolution records are taken into consideration in order to give a better spatial data coverage.”
In conclusion this impressive piece of research makes a significant contribution to a growing body of evidence that both the global extent of the MWP, and the temperature was similar, or even greater than the Current Warm Period, even though the atmospheric CO2 concentrations today are some 40% greater than they were during the MWP.
Some argue that without anthropogenic greenhouse gasses the world would have cooled in the past few decades. That might be the case, but the statement that it is warmer now than it has been for thousands of years is untrue. The rate of warming seen recently is also not unprecedented.
In the context of climate sensitivity – the real world climatic reaction to increasing greenhouse gasses – and climate model uncertainty, it is an interesting question to ask: if Nature alone in the past can produce temperatures like those we see today, why can’t she do so again?
=============================================================
The link to the journal is here. Abstract below.
The extra-tropical Northern Hemisphere temperature in the last two millennia: reconstructions of low-frequency variability
B. Christiansen1 and F. C. Ljungqvist2
1Danish Meteorological Institute, Copenhagen, Denmark
2Department of History, Stockholm University, Stockholm, Sweden
Abstract. We present two new multi-proxy reconstructions of the extra-tropical Northern Hemisphere (30–90° N) mean temperature: a two-millennia long reconstruction reaching back to 1 AD and a 500-yr long reconstruction reaching back to 1500 AD. The reconstructions are based on compilations of 32 and 91 proxies, respectively, of which only little more than half pass a screening procedure and are included in the actual reconstructions. The proxies are of different types and of different resolutions (annual, annual-to-decadal, and decadal) but all have previously been shown to relate to local or regional temperature. We use a reconstruction method, LOCal (LOC), that recently has been shown to confidently reproduce low-frequency variability. Confidence intervals are obtained by an ensemble pseudo-proxy method that both estimates the variance and the bias of the reconstructions. The two-millennia long reconstruction shows a well defined Medieval Warm Period, with a peak warming ca. 950–1050 AD reaching 0.6 °C relative to the reference period 1880–1960 AD. The 500-yr long reconstruction confirms previous results obtained with the LOC method applied to a smaller proxy compilation; in particular it shows the Little Ice Age cumulating in 1580–1720 AD with a temperature minimum of −1.0 °C below the reference period. The reconstructed local temperatures, the magnitude of which are subject to wide confidence intervals, show a rather geographically homogeneous Little Ice Age, while more geographical inhomogeneities are found for the Medieval Warm Period. Reconstructions based on different subsets of proxies show only small differences, suggesting that LOC reconstructs 50-yr smoothed extra-tropical NH mean temperatures well and that low-frequency noise in the proxies is a relatively small problem.
The paper is not paywalled and be read in its entirety here. (PDF)
One century before and after the maximum of the MWP it was much colder, showing how relatively quick these natural oscillations may happen.
The amazing thing about this paper is that it includes a lot of flawed proxies familiar to Climate Audit readers. There is Yamal, Avam-Taimyr and Tornetrask. Yet despite this the global warming period re-emerges. How much more significant will natural fluctuations in climate appear if the biases in the individual proxy studies could be eliminated?
[snip. — mod.]
I highly recommend reading this post from the author of this new paper, written a couple years ago. He asks the questions he set out to answer in this paper, and this is interested if for nothing else. The really interesting part is where he criticizes previous reconstructions including Mann’s Hockey Stick.
http://klimazwiebel.blogspot.com/2010/05/guest-post-by-bo-christiansen-on.html
After more consideration, I think this paper is even more significant that I first realized. A tenet in the scientific method is to reject the null hypothesis. In other words, for a new theory such as AGW to be accepted one must eliminate all natural causes as an explanation for the observations.
This new paper shows our current temperatures are within naturally occurring phenomena. Even if we can’t explain the cause, we can still say it’s not unusual. Therefore the AGW theory cannot be accepted, purely based on the scientific method.
http://en.wikipedia.org/wiki/Null-hypothesis
Unless someone can find a problem with this new paper, the real deniers seem to be those that refused to accept the scientific method. Maybe CO2 can warm the planet, but this new paper undermines their theory. They need something more than the correlations and something causal that cannot be refuted.
Jim;
Not to be too snarky, Jim, but was this really your first clue? It has been clear from early days that the off-handed dismissal of natural variation, while simultaneously acknowledging little or no understanding of its major components, was outrageously wrong. Then Kevin Trenberth tried to assert that the AGW speculation should be taken as the Null Hypothesis unless some other idea were first proven.
Various ploys, like trying to make the late 20th C warming look steeper than earlier ones were attempted, but really just depended on post-facto selective comparisons. This paper is no surprise.
Brian, *I* didn’t need a clue. I’ve been a “denier” for more than ten year. I’m aware of Trenberth’s claims. I’m equally aware of the “ploys” to make the current temperature records biased to support the AGW theory. My question is directed to those consenting scientists. This “paper is no surprise” but this is also the first solid 2000 year reconstruction of temperatures, is it not? This paper, more than any other data I’m aware of, contradicts the claim the current warming is unpredicted or even unusual. If you have other such data, please mention it. You don’t have to convince me. I’m saying we have the best evidence yet to convince them!
Jim;
To convince them? Not possible. After investing so much in their scientific-politico scam, and receiving such munificent rewards from their efforts, they are quite impervious to arguments. They will have to be crushed, not convinced.