Imran - this is important because past temperatures (1000 AD to 1850) have been determined by assuming a positive linear relationship between tree rings (growth), and temperature. This work has shown that historical temperatures have been almost completely stable (at least when compared to temperature fluctuations seen currently). If past temperatures have been stable, and we're only seeing significant temperature increases now, then the increasing CO2 concentrations must be to blame.
The problem is, the initial assumption has never been validated. Everybody has just continued along with this assumption as the foundation to their work, even though most logical people would ask “how do you separate temperature signals from precipitation signals in tree rings?”.
What this recent work is showing, is that tree rings are not a good proxy for temperature, but also that the relationship, is in fact, negative. This seriously calls into question the historical temperature estimates and the idea that temperatures were stable prior to the industrial revolution.
