IPCC estimates of climate sensitivity to CO2 overestimated ~80%

Abstract here

Nature 463, 527-530 (28 January 2010) Ensemble reconstruction constraints on the global carbon cycle sensitivity to climate

The processes controlling the carbon flux and carbon storage of the atmosphere, ocean and terrestrial biosphere are temperature sensitive and are likely to provide a positive feedback leading to amplified anthropogenic warming. Owing to this feedback, at timescales ranging from interannual to the 20–100-kyr cycles of Earth's orbital variations, warming of the climate system causes a net release of CO₂ into the atmosphere; this in turn amplifies warming. But the magnitude of the climate sensitivity of the global carbon cycle (termed γ), and thus of its positive feedback strength, is under debate, giving rise to large uncertainties in global warming projections. Here we quantify the median γ as 7.7 p.p.m.v. CO₂ per °C warming, with a likely range of 1.7–21.4 p.p.m.v. CO₂ per °C. Sensitivity experiments exclude significant influence of pre-industrial land-use change on these estimates. Our results, based on the coupling of a probabilistic approach with an ensemble of proxy-based temperature reconstructions and pre-industrial CO₂ data from three ice cores, provide robust constraints for γ on the policy-relevant multi-decadal to centennial timescales... Our results are incompatibly lower (P  0.05) than recent pre-industrial empirical estimates of ~40 p.p.m.v. CO₂ per °C, and correspondingly suggest ~80% less potential amplification of ongoing global warming.

lay explanation of article here.  and here. 
Also see Man made global warming explained - closing the blinds estimating about 1/2 the effect of CO2 on climate as the IPCC