How large is Uncertainty in Calibrated Perturbed Physics Ensembles?
Abstract
Estimates of the response of the climate system to greenhouse gas forcing are still highly uncertain as exemplified by the 2013 IPCC report estimate for the equilibrium warming to doubling CO2 of 1.5-4.5K. Here, using the third Hadley Centre Coupled Model (HadCM3) we calibrate atmospheric parameters known to be important in feedback strength. We produced several atmospheric model configurations that simulate large scale observations as well as the standard model does and for which the coupled model configurations require no flux correction. The parameters sets produced are all very different from one another. The coupled models have a range of equilibrium climate sensitivities in response to doubling CO2 of 2.9 - 3.4K. Equilibrium warming in response to quadrupling CO2 concentrations is twice that for doubling. We also computed the transient climate response (TCR) finding a range of 2-2.2 K which increases to 4.6-5 K at quadrupling CO2. TCR at high CO2 levels appears to be related to the simulated pre-industrial temperatures and arises from changes in the Atlantic Overturning circulation which is very sensitive to the pre-industrial temperatures. Across the perturbed ensemble changes in other variables shows little scatter once normalised by the global-mean change. Our results show, for HadCM4, that uncertainty in perturbed physics when parameters are calibrated against current large scale observations is small suggesting that uncertainties from Perturbed Physics Ensemble are too large. Thus uncertainty in feedback strength in models likely arises from the functional form of parameterisation or the observational targets used to tune models.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFM.A33C2370T
- Keywords:
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- 3305 Climate change and variability;
- ATMOSPHERIC PROCESSES;
- 3310 Clouds and cloud feedbacks;
- ATMOSPHERIC PROCESSES;
- 3337 Global climate models;
- ATMOSPHERIC PROCESSES;
- 3354 Precipitation;
- ATMOSPHERIC PROCESSES