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GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L10703, doi:10.1029/2005GL025540, 2006

Missing feedbacks, asymmetric uncertainties, and the underestimation of future warming

Margaret S. Torn

Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA


John Harte

Energy and Resources Group, University of California, Berkeley, California, USA


Abstract

Historical evidence shows that atmospheric greenhouse gas (GhG) concentrations increase during periods of warming, implying a positive feedback to future climate change. We quantified this feedback for CO2 and CH4 by combining the mathematics of feedback with empirical ice-core information and general circulation model (GCM) climate sensitivity, finding that the warming of 1.5–4.5°C associated with anthropogenic doubling of CO2 is amplified to 1.6–6.0°C warming, with the uncertainty range deriving from GCM simulations and paleo temperature records. Thus, anthropogenic emissions result in higher final GhG concentrations, and therefore more warming, than would be predicted in the absence of this feedback. Moreover, a symmetrical uncertainty in any component of feedback, whether positive or negative, produces an asymmetrical distribution of expected temperatures skewed toward higher temperature. For both reasons, the omission of key positive feedbacks and asymmetrical uncertainty from feedbacks, it is likely that the future will be hotter than we think.

Received 19 December 2005; accepted 24 March 2006; published 26 May 2006.

Index Terms: 0426 Biogeosciences: Biosphere/atmosphere interactions (0315); 0428 Biogeosciences: Carbon cycling (4806); 1615 Global Change: Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 4805, 4912); 1622 Global Change: Earth system modeling (1225); 1631 Global Change: Land/atmosphere interactions (1218, 1843, 3322).

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Citation: Torn, M. S., and J. Harte (2006), Missing feedbacks, asymmetric uncertainties, and the underestimation of future warming, Geophys. Res. Lett., 33, L10703, doi:10.1029/2005GL025540.