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Ensemble reconstruction constraints on the global carbon cycle sensitivity to climate


Fig. 2. Long-term temperature variation, amplitude and uncertainty. Ensemble reconstruction constraints on the global carbon cycle sensitivity to climate

Nature
Vol. 463, pp.527-532, 28 January 2010
doi:10.1038/nature08769

David C. Frank1,2, Jan Esper3, Christoph C. Raible2,4 Ulf Büntgen1, Valerie Trouet1, Benjamin Stocker2,4, and Fortunat Joos2,4
1 Swiss Federal Research Institute WSL, Zürcherstrasse 111, CH-8903 Birmensdorf, Switzerland
2 Oeschger Centre for Climate Change Research, University of Bern, Zähringerstrasse 25, CH-3012 Bern, Switzerland
3 Department of Geography, Johannes Gutenberg University, Becherweg 21, 55099 Mainz, Germany
4 Climate and Environmental Physics, Physics Institute, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland
ABSTRACT:
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. By using an ensemble of >200,000 members, quantification of γ is not only improved, but also likelihoods can be assigned, thereby providing a benchmark for future model simulations. Although uncertainties do not at present allow exclusion of γ calculated from any of ten coupled carbon-climate models, we find that γ is about twice as likely to fall in the lowermost than in the uppermost quartile of their range. 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.
Download data from the WDC Paleo archive:
1,000 Year Ensemble Reconstructions of Temperature and CO2:
Data Description
Unsmoothed reconstructions
10-year smoothed reconstructions (figure 2a)
50-year smoothed reconstructions (anaylsis)
Temperature percentiles based on 10-year smoothed data
Variably smoothed records of atm. CO2 concentrations

To read or view the full study, please visit the Nature website.
It was published in Nature, Vol. 463, pp.527-532, 28 January 2010
doi:10.1038/nature08769
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