Wednesday, January 23, 2013

New paper notes long-term climate change may be 'inherently unpredictable'

A paper published today in the Journal of the Atmospheric Sciences reveals the abject failure of climate models to project either the transient or long-term ["equilibrium"] climate change from greenhouse gases. The paper notes, "The Equilibrium Climate Sensitivity (ECS) has a large uncertainty among models participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) and has recently been presented as “inherently unpredictable”One way to circumvent this problem is to consider the Transient Climate Response (TCR)[short-term response to greenhouse gases]. However, the TCR among AR4 models also differ by more than a factor of 2." Furthermore, the authors find that the projections vary by 30% even within the same model with the same equilibrium climate sensitivity."

Prior posts on the abject failure of climate models


Transient Climate Response in Coupled Atmospheric-Ocean General Circulation Models

Mao-Chang Liang,1,2,* Li-Ching Lin,3 Ka-Kit Tung,4 Yuk L. Yung,5 and Shan Sun6
1 Research Center for Environmental Changes, and Institute of Astronomy and Astrophysics, Academia Sinica, Taiwan
2 Graduate Institute of Astronomy, National Central University, Taiwan
3 Institute of Earth Sciences, Academia Sinica, Taiwan
4 Department of Applied Mathematics, University of Washington, USA
5 Division of Geological and Planetary Sciences, California Institute of Technology, USA
6 Earth System Research Laboratory, National Oceanic and Atmospheric Administration, USA
Abstract

The Equilibrium Climate Sensitivity (ECS) has a large uncertainty among models participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) and has recently been presented as “inherently unpredictable”. One way to circumvent this problem is to consider the Transient Climate Response (TCR). However, the TCR among AR4 models also differ by more than a factor of 2. We argue that the situation may not necessarily be so pessimistic, because much of the inter-model difference may be due the fact that the models were run with their oceans at various stages of flux adjustment with their atmosphere. This is shown by comparing multi-millennium long runs of the Goddard Institute for Space Studies coupled model (GISS-EH) and the Community Climate System Model (CCSM4) with what were reported to AR4. The long model runs here reveal the range of variability (~30%) in their TCR within the same model with the same ECS. The commonly adopted remedy of subtracting the “climate drift” is ineffective and adds to the variability. The culprit is the natural variability of the control runs, which exists even at quasi-equilibration. Fortunately, for simulations with multi-decadal time horizon, robust solutions can be obtained by branching off thousand-year-long control runs that reach “quasi-equilibration” using a new protocol, which takes advantage of the fact that forced solutions to radiative forcing forget their initial condition after 30-40 years and instead depends mostly on the trajectory of the radiative forcing.

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