06 February 2008

1500 Years of Global Cooling

Careful temperature reconstruction from Swedish tree rings points to a cooling of -0.3 degrees C between the years 500 CE (AD) and 2000 CE.
The article is forthcoming in Climate Dynamics, and the work was conducted by Håkan Grudd of Stockholm University’s Department of Physical Geography and Quaternary Geology, and despite the results, the research was not funded by industry. The focus here is the Torneträsk area in northern Sweden near 68.5°N (within the Arctic Circle) where Scots pines have been growing for millennia. Grudd not only sampled living trees, but he also collected subfossil samples found as dead wood on dry ground and from submerged logs retrieved from small mountain lakes. Many other studies have shown that the pines are sensitive to summer temperatures, so in theory, the tree samples should allow a very long term and relatively accurate reconstruction of past thermal conditions.

Grudd not only measured the width of each tree ring, he also measured the density of the wood in each ring using an Itrax WoodScanner from Cox Analytical Systems (the perfect gift for the man who thinks he has everything). The obvious trick here is to link the width and density time series to the climate in the growth area. Fortunately, Grudd was able to assemble records from “Abisko, a local record (AD 1913–2004) provided by Abisko Scientific Research Station, which is located within the Torneträsk area; (2) Tornedalen, a long composite record (AD 1802–2002) based on a combination of historical data and synoptic station data from Haparanda approx. 350 km south-east of Torneträsk; and (3) Bottenviken, a regional record (AD 1860–2004) provided by the Swedish Meteorological and Hydrological Institute and based on data from six synoptic stations in northern Sweden.” He used fairly standard multivariate statistical methods to link the climate records to the width and density measurements, and just like magic, response functions are developed to estimate summer temperatures from the tree ring data that extend back 1,500 years.

... [Grudd says:]“The late-twentieth century is not exceptionally warm in the new Torneträsk record: On decadal-to-century timescales, periods around AD 750, 1000, 1400, and 1750 were all equally warm, or warmer. The warmest summers in this new reconstruction occur in a 200-year period centred on AD 1000. A ‘Medieval Warm Period’ is supported by other paleoclimate evidence from northern Fennoscandia, although the new tree-ring evidence from Tornetraäsk suggests that this period was much warmer than previously recognised.” (emphasis added)

We will leave you with this very interesting conclusion from Grudd’s research:

[Grudd:] “The new Torneträsk summer temperature reconstruction shows a trend of -0.3°C over the last 1,500 years.”____WorldClimateReport

In the figure above, panel "a" shows the tree ring width-only reconstruction. Panel "b" shows the multi-proxy reconstruction. Panel "c" shows the difference in the two reconstructions.

It is important to realize that weather and climate are typically regional--not global. What was happening in Sweden at this period of time could easily be different from what was happening in Patagonia. Still, this tree ring + other proxies study brings the reconstructions all the way through to the present time--unlike the famous discredited Mann "hockey-stick" that is helping Al Gore become so rich and famous.

H/T Lubos


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