A new estimate of the feedback between temperature and atmospheric carbon dioxide (CO 2 ) concentration has been derived from a comprehensive comparison of temperature and CO 2 records spanning the past millennium.

The result, which is based on more than 200,000 individual comparisons, implies that the amplification of current global warming by carbon-cycle feedback will be significantly less than recent work has suggested.

Climate warming causes many changes in the global carbon cycle, with the net effect generally considered to be an increase in atmospheric CO 2 with increasing temperature -- in other words, a positive feedback between temperature and CO 2 . Uncertainty in the magnitude of this feedback has led to a wide range in projections of current global warming: about 40% of the uncertainty in these projections comes from this source.

Recent attempts to quantify the feedback by examining the co-variation of pre-industrial climate and CO 2 records yielded estimates of about 40 parts per million by volume (p.p.m.v.) CO 2 per degree Celsius, which would imply significant amplification of current warming trends.

In this week's Nature, David Frank and colleagues extend this empirical approach by comparing nine global-scale temperature reconstructions with CO 2 data from three Antarctic ice cores over the period ad 1050-1800. The authors derive a likely range for the feedback strength of 1.7-21.4 p.p.m.v. CO 2 per degree Celsius, with a median value of 7.7.