As Andrew Orlowski reports in The Register, the issue is the use of tree rings as a temperature proxy in order to ‘reconstruct’ past temperatures. The papers in question  incorporated data from trees at the Yamal Peninsula in Siberia:

This dataset gained favour, curiously superseding a newer and larger data set from nearby. The older Yamal trees indicated pronounced and dramatic uptick in temperatures.

How could this be? Scientists have ensured much of the measurement data used in the reconstructions remains a secret -- failing to fulfill procedures to archive the raw data. Without the raw data, other scientists could not reproduce the results. The most prestigious peer reviewed journals, including NatureScience, were reluctant to demand the data from contributors. Until now, that is. and

At the insistence of editors of the Royal Society's Philosophical Transactions B the data has leaked into the open -- and Yamal's mystery is no more. From this we know that the Yamal data set uses just 12 trees from a larger set to produce its dramatic recent trend. Yet many more were cored, and a larger data set (of 34) from the vicinity shows no dramatic recent warming, and warmer temperatures in the middle ages.

In all there are 252 cores in the CRU Yamal data set, of which ten were alive 1990. All 12 cores selected show strong growth since the mid-19th century. The implication is clear: the dozen were cherry-picked.

A small ‘but closely knit’ number of scientists all used the misleading Yamal data to claim that today’s temperatures were unprecedentedly hot. Orlowski notes:

Controversy has been raging since 1995, when an explosive paper by Keith Briffa at the Climate Research Unit at the University of East Anglia asserted that that the medieval warm period was actually really cold, and recent warming is unusually warm. Both archaeology and the historical accounts, Briffa was declaring, were bunk. Briffa relied on just three cores from Siberia to demonstrate this.

Three years later Nature published a paper by Mann, Bradley and Hughes based on temperature reconstructions which showed something similar: warmer now, cooler then. With Briffa and Mann as chapter editors of the UN's Intergovernmental Panel on Climate Change (IPCC), this distinctive pattern became emblematic - the ‘Logo of Global Warming’.

But as the heroic mathematician Steve McIntyre – who helped demonstrate the fraudulence of the even more seminal AGW claim of the ‘hockey-stick curve’ of historic global temperatures – has finally managed to winkle out, their premise was false and their claim was untrue.

Told you so...

The Spectator