The temperature graphs produced at Tornetrask show "pronounced variability on all timescales, from year-on-year variations right up to century-on-century," says Briffa. On the longer timescales, for instance, they show 20 major cooling periods during the past two millenia, including long spells between 500 and 850, between 1100 and 1350 and between 1580 and 1750, the little ice age. There were also long warm spells between 900 and 1100, known as the medieval warm period, and 1360 to 1560. [ed: show graph from NERC paper].
Further back, early results suggest a strong warm era from 4000 to 3300 BC, and a cool period ending around 5070 BC. But there are intriguing gaps, for which no tree rings can be found. These, says Briffa, "suggest some major calamity that destroyed trees. Volcanoes, perhaps, or a rapid rise in the water tables." A 19-year gap between 1130 and 1111 BC, for instance, coincides with volcanic ash showing up in Greenland ice.
"What all this means," says Briffa, "is that the old image of the 10 000 years since the end of the last ice age -- the Holocene era -- as climatically tranquil looks increasingly inaccurate."
...Under the IPCC umbrella, [Tim Barnett of Scripps] and Phil Jones of the CRU have formed a small "detections group", to look for these tell-tale patterns. "We are systematically looking at the patterns, past and present, of all the main forcings on climate," Barnett says. They will investigate how the world's climate systems respond to volcanoes, to changes in the ocean circulation, to solar cycles and so on. "Then we will compare those patterns with what we are seeing today. What we hope is that the current patterns of temperature change prove distinctive, quite different from the patterns of natural variability in the past." And if that turns out to be the case, he says, "we will be able to close down this issue of attribution, perhaps within three to five years."