Guest Post by Craig Loehle (cloehle -at- ncasi.org)
When I published my 2000 year reconstruction (Loehle, C. 2007. A 2000 Year Global Temperature Reconstruction based on Non-Treering Proxy Data. Energy & Environment 18:1049-1058) and the subsequent correction with better confidence intervals (Loehle, C. and Hu McCulloch. 2008. Correction to: A 2000 Year Global Temperature Reconstruction based on Non-Treering Proxy Data. Energy & Environment 19:93-100) it was an immediate hit in the blogosphere.
At the same time, I have been repeatedly insulted about it on the web. It is claimed that it has been debunked, is junk, that E&E is not a “real” journal, that I’m a hack, that I “only” used 18 series (though 2 were composites covering China & North America), etc. In the ClimateGate emails, Mann called it “awful” (which I’ll take as a compliment!). Lot’s of fun. In this post I demonstrate perhaps a little vindication.
A new reconstruction is now out (Ljungqvist, F.C. 2010. A new reconstruction of temperature variability in the extra-tropical northern hemisphere during the last two millenia. Geografiska Annaler 92A(3):339-351). Many past reconstructions have used bristlecones and similar trees with strange growth forms, and then weighted these heavily. Ljungqvist drops these data from the US Southwest and from Mongolia. He uses more data for the older periods and only uses long records (though not all 2000 yrs, as in my reconstruction). Some of his data overlap with mine, but not too much. He uses the CPS (Composite Plus Scale) method favored by dendro paleo Team members. Here is the abstract.
ABSTRACT. A new temperature reconstruction with decadal resolution, covering the last two millennia, is presented for the extratropical Northern Hemisphere (90–30°N), utilizing many palaeotemperature proxy records never previously included in any largescale temperature reconstruction. The amplitude of the reconstructed temperature variability on centennial time-scales exceeds 0.6°C. This reconstruction is the first to show a distinct Roman Warm Period c. AD 1–300, reaching up to the 1961–1990 mean temperature level, followed by the Dark Age Cold Period c. AD 300–800. The Medieval Warm Period is seen c. AD 800–1300 and the Little Ice Age is clearly visible c. AD 1300–1900, followed by a rapid temperature increase in the twentieth century. The highest average temperatures in the reconstruction are encountered in the mid to late tenth century and the lowest in the late seventeenth century. Decadal mean temperatures seem to have reached or exceeded the 1961–1990 mean temperature level during substantial parts of the Roman Warm Period and the Medieval Warm Period. The temperature of the last two decades, however, is possibly higher than during any previous time in the past two millennia, although this is only seen in the instrumental temperature data and not in the multi-proxy reconstruction itself. Our temperature reconstruction agrees well with the reconstructions by Moberg et al. (2005) and Mann et al. (2008) with regard to the amplitude of the variability as well as the timing of warm and cold periods, except for the period c. AD 300–800, despite significant differences in both data coverage and methodology. reprint available from author: firstname.lastname@example.org
Fig. 3. Estimations of extra-tropical Northern Hemisphere (90–30°N) decadal mean temperature variations (dark grey line) AD 1–1999 relative to the 1961–1990 mean instrumental temperature from the variance adjusted CRUTEM3+HadSST2 90–30°N record (black dotted line showing decadal mean values AD 1850–1999) with 2 standard deviation error bars (light grey shading). data available at ftp://ftp.ncdc.noaa.gov/pub/data/paleo/contributions_by_author/ljungqvist2010/ljungqvist2010.txt
Gee, this looks kind of familiar. Let’s see how this looks overlaid with my graph. I centered both on their respective long-term mean values (I did NOT rescale) and got the following.
x-axis is Calendar date AD and y-axis is anomaly temperature deg C.
There is excellent agreement over the past 1100 years (r = 0.86) with less agreement prior to that (r = 0.6 for entire record). My peak temperature occurs about 100 years earlier, but I agree with the new reconstruction which has a peak closer to what Craig Idso has identified from his MWP project. The two graphs agree on the warming trend from AD 250 to AD 900, though differ in the rate.
The new recon shows the Roman Warm Period, which I agree is probably more correct than in my graph. So, a new paper with much more data, using the “approved” CPS method gets essentially the same result as I got.
I would also note that Moberg et al. 2005 in Nature (the low-frequency signal component) looks very similar (but he is an approved person, so no one gives him grief about it). No unprecedented warming in recent decades, just a repeat of what looks to me like a periodic pattern of warming and cooling.
The MWP looks real.