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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
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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. |