In a letter published in the journal Nature (Nature 463, 527-530 (28 January 2010)) entitled “Ensemble reconstruction constraints on the global carbon cycle sensitivity to climate” the authors discuss the processes controlling the carbon flux and carbon storage of the atmosphere, ocean and terrestrial biosphere. These processes are likely to provide a positive feedback leading to amplified anthropogenic (i.e., human caused) warming. But the magnitude of the climate sensitivity of the global carbon cycle and thus of its positive feedback strength, is under debate, giving rise to large uncertainties in global warming projections. The paper describes a study designed to quantitatively estimate the feedback parameter, γ, based on pre-industrial CO2 estimates based on “proxies” such as ice cores.

The authors conclusion:

“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.”

In short, the carbon cycle feedback effect is weaker than formerly thought by climate researchers. This will require a revision of the simulation models used to forecast climate change and will, in all likelihood, lower the projected impact of human activity on the climate. An amplification reduction in the 80% range could result in dramatically lower projected impact.

All models are wrong, some models are useful. Corollary: apply models with care and always temper their interpretation with sound judgment.

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