HR: 08:30h
AN: U51B-03 INVITED    [Abstracts]
TI: State dependence of climate sensitivity and its implications
AU: * Allen, M
EM: myles.allen@physics.ox.ac.uk
AF: University of Oxford Department of Physics, Clarendon Laboratory Parks Road, Oxford, OX1 3PU, United Kingdom
AU: Knutti, R
EM: reto.knutti@env.ethz.ch
AF: ETH Zurich Institute for Atmospheric and Climate Science, Universitätstrasse 16 Zürich, Zürich, CH-8092, Switzerland
AU: Stone, D
EM: stoned@atm.ox.ac.uk
AF: University of Oxford Department of Physics, Clarendon Laboratory Parks Road, Oxford, OX1 3PU, United Kingdom
AU: Frame, D
EM: dframe@atm.ox.ac.uk
AF: Oxford University Centre for the Environment, South Parks Road, Oxford, OX1 3QY, United Kingdom
AB: It has long been noted (Senior and Mitchell, 2000; Boer and Yu, 2004) that the climate sensitivity of global climate models evolves over time, changing by up to +/-20% in long runs following stabilization of greenhouse forcing. We explore the magnitude and origins of this effect in the ensemble of global coupled models contributed to the IPCC AR4 ensemble. Two possible parameterizations of this effect are discussed, the first in which the net feedback parameter, λ, depends on the degree to which global temperature is out of equilibrium and the second in which λ simply depends simply on global temperature itself. We show that the second parameterization provides a generally better fit to the GCMs' output and explore its implications for recent attempts to constrain long-term climate sensitivity using recent observed climate change. The possibility of an uncertain temperature- dependence of λ has even more profound implications for efforts to constrain climate sensitivity using the last glacial maximum, but this is not the focus of this talk. The possibility of state-dependent feedbacks is a further nail in the coffin of attempts to provide a useful, objectively constrained, upper bound on the long-term equilibrium response to stabilization of atmospheric greenhouse gases at any currently conceivable level. We note, however, that this does not preclude our use of observations to constrain equally policy-relevant quantities, such as the familiar transient climate response, or TCR, the equivalent carbon dioxide concentration consistent with a two degree warming, or C2K, or the transient warming commitment, or TWC. The TWC is the maximum warming we expect to result from a linear ramp increase in radiative forcing followed by a linear decrease: at current values of the atmospheric airborne fraction for carbon dioxide, this is approximately equivalent to the forcing that would result from a complete cessation of emissions at any given time. Although this is not a feasible emissions scenario, it represents a much more natural (and better constrained) measure of the warming attributable to greenhouse emissions to date than the more familiar stabilization commitment scenario. The support of the modeling groups contributing to the WCRP CMIP-3 model intercomparison is gratefully acknowledged.
DE: 1626 Global climate models (3337, 4928)
SC: Union [U]
MN: 2007 Fall Meeting