February 4, 2008

1,500 Years of Cooling in the Arctic

The Arctic is melting, right? There is simply no questioning this pillar of the greenhouse scare, and images of ice melting, polar bears struggling, and indigenous people crying the blues are all part of any self-respecting presentation of global warming. Imagine a study published in a major journal showing that a location in the Arctic has “a trend of -0.3°C over the last 1,500 years.” Of course, you would never have learned of such a result had you not discovered World Climate Report.

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.

As seen in Figure 1, the region has definitely been warming over the past century, and a case could be made for a warming of nearly 2°C over the past 100 years. Come to think of it, that could be a great headline “Arctic Warming Confirmed!” However, Grudd explains:

“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:

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


Figure 1. Reconstructed summer (April–August) temperature for the period AD 500–2004. Panel “a” shows the reconstruction from ring widths exclusively. Panel “b” shows the ‘‘multi-proxy’’ reconstruction from widths and densities. Both series are expressed as anomalies (in °C) from their 1951–1970 mean. The annual data (grey) has been filtered to emphasize climatic variability on 30- and 100-year timescales (black curves). Panel “c” shows the difference on multi-decadal (dark grey) and centennial (light grey) timescales between the two reconstructions (expressed as an in-between fill of the curves), with the ‘‘multiproxy’’ reconstruction showing on average 0.2°C lower temperature estimates (from Grudd, 2008).

Reference:

Grudd, H. 2008. Torneträsk tree-ring width and density AD 500–2004: a test of climatic sensitivity and a new 1500-year reconstruction of north Fennoscandian summers. Climate Dynamics, DOI 10.1007/s00382-007-0358-2.




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