YAD06

YAD06 is a tree growing in the Yamal Peninsula of Siberia. A core sample from this tree, YADO61, provided data used to support Michael Mann's hockey stick interpretation of global climate history. Mann's graph was first published in 1998. It shows global temperatures as stable for 900 years until the 20th century, when carbon dioxide emissions supposedly caused a rapid increase in global temperature. Mann created the graph to address reconstructions made by earlier climatologists that show a Medieval Warm Period between 950 and 1150. These reconstructions suggest that the climate was somewhat warmer at that time than it is at present. Although Mann claimed to have used a dozen proxies, analysis showed that only the data from YAD06 had the hockey stick pattern that Mann reported as his result.

Compared to other samples taken at Yamal, the YAD061 core was an outliner by a factor of eight sigma. The hockey stick pattern of growth may be explained if YAD06 was in the shade of other trees during its early life. After these older trees died, YAD06 could have received more sunlight, allowing it to grow more quickly. Older trees tend to grow more slowly, quite apart from sunlight and climate issues. This factor has so far not been taken into account in climate reconstructions.

The Yamal samples were collected by two Russian scientists, Hantemirov and Shiyatov. However, their findings did not show anything resembling a hockey stick pattern. The Yamal data was rejiggered by Steven Briffa, whose findings were published in 2000. Briffa's paper was seized upon by Mann and other researchers to support the hockey stick interpretation. Inconveniently for Mann, YAD06's growth has slowed since 1960, possibly due to the tree's age. The trend was disguised with further manipulation. For the post-1960 period, "Mike [Mann]'s Nature trick of adding in the real temps" was used to "hide the decline," according to the Climategate emails.

Mann's graph appeared prominently in the report of the UN's Intergovernmental Panel on Climate Change, used a basis for international policy on climate change. YAD06 has been dubbed, "the most influential tree in the world." In 2013, the Climate Research Unit of the University of East Anglia quietly revised its Yamal data set. The current reconstruction, based on ring data gathered from dozens of trees, shows that the growth rate for trees in Yamal has been relatively stable for centuries, with no outstanding acceleration or deacceleration in the 20th century.