In 1999 climate scientist Michael E. Mann hand his colleagues published a reconstruction of average temperatures last millenium-round on northern hemisphere come to be known as “hockeystick”-curve (M.E.Mann,R.S.Bradley, M.K.Hughes: “Northern Hemisphere Temperatures During the Past Millennium: Inferences, Uncertainties, and Limitations”, Geophysical Research Letters, 1999). This scientific paper was really a pioneering feat! It was the first reliable reconstruction of temperatures in global scale over such a long time.
Prof. Michael E. Mann holding a tree grate with tree rings Source: Homepage Michael E. Mann
The team of scientists used proxy data from tree rings, corals and ice cores on the one hand and instrumental temperature data on the other hand such as it was. The “hockeystick” demonstrated at first how extraordinary (human-made) global warming during 20th century really was! For that reason all climate skeptics dislike this curve up to the present day.
The Hockeystick-Curve. The graph resembles a hockeystick in shape: with a long part of curve declining slightly as the shaft and with a short part of curve upturning suddenly as the blade. Source: Mann, Bradley und Hughes 1999
Assembling and evaluation of all data for “hockeystick” happened in a very sophisticated way. The proxy-data, mainly tree rings extending to 1980 were validated by instrumental data back to 1854, occasional to 18th century yet. Long proxy record and shorter instrumental record overlapped a good way. Using only instrumental data from time intervall 1902 to 1980 for calibration of proxy data, Mann and his collegues were able to use leftover intervall from 1854 to 1901 (and the rare earlier instrumental data from 18th century) to verify this calibration. Calibration and validation of proxy data worked accurately, meaning that „hockeystick“ is correct in this regard.
CO2 is a greenhouse gas but it also acts as a fertilizer, especially for trees in high elevations. More CO2 let tree ring width and tree ring density (of summerly late wood) mimic an additional rise in temperature that does not really exist. Thus the tree ring data were readjusted.
Above: Records of two tree ring data series (ITRDB Millenial, North American Treeline), which diverged from 19th century on due to influence of CO2. ITRDB Millenial includes trees in high elevations much sensitive to dunging effect of CO2. Below: Variations in atmospheric CO2 and residual between the two data series, used for readjustment. Source: Mann, Bradley und Hughes, 1999
Two examples how carefully Mann and his team had gone about it! All that and then some you can read in original paper, downloadable on Mann´s homepage (see sources below). It´s worth it! Michael E. Mann was also a lead author of 3rd. IPCC-Report about global warming in 2001.
In the course of General Assembly of European Geosciences Union (EGU) in April 2012 Michael E. Mann received the Oeschger-Medal for his research! Hans Oeschger (1927) was a Swiss scientist, famous because of his ice core research. Together with Willi Dansgaard he discovered a series of abrupt climate changes (Dansgaard-Oeschger Events) during the last glacial period by analysis of Greenland ice cores. Also he measured first the glacial-interglacial change of atmospheric CO2 encased in ice core´s tiny air bubbles.
But also some climate sceptics tried again and again to discredit Mann scientifically and personally. Fortunately they failed yet. I hope it stays that way!
Jens Christian Heuer