Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
doi:10.5194/acp-2016-405
© Author(s) 2016. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
17 May 2016
Review status
A revision of this discussion paper is under review for the journal Atmospheric Chemistry and Physics (ACP).
A modified impulse-response representation of the global response to carbon dioxide emissions
Richard J. Millar1,2, Zebedee R. Nicholls1, Pierre Friedlingstein3, and Myles R. Allen1,2,4 1Department of Physics, University of Oxford, Oxford, UK
2Oxford Martin Net Zero Carbon Investment Initiative, Oxford Martin School, University of Oxford, Oxford, UK
3Department of Mathematics, University of Exeter, Exeter, UK
4Environmental Change Institute, University of Oxford, Oxford, UK
Abstract. Projections of the response to anthropogenic emission scenarios, evaluation of some greenhouse gas metrics and estimates of the social cost of carbon, often require a simple model that links emissions of carbon dioxide CO2 to atmospheric concentrations and global temperature changes. An essential requirement of such a model is to reproduce the behaviour of more complex models as well as an ability to sample their range of response in a transparent, accessible and reproducible form. Here we adapt the simple model of the Intergovernmental Panel on Climate Change 5th Assessment Report (IPCC-AR5) to explicitly represent the state-dependence of the CO2 airborne fraction and reproduce several idealised experiments performed with more complex models. We find that a simple linear increase in 100-year integrated airborne fraction with cumulative carbon uptake and global temperature change is both necessary and sufficient to represent the response of the climate system to CO2 on a range of timescales and under a range of experimental designs. Quantified ranges of uncertainty (analogous to current assessed ranges in Equilibrium Climate Sensitivity and Transient Climate Response) in integrated airborne fraction over the 21st century under a representative mitigation scenario, and an assessed range in how much this quantity may have changed relative to pre-industrial conditions, would be valuable in future scientific assessments.

Citation: Millar, R. J., Nicholls, Z. R., Friedlingstein, P., and Allen, M. R.: A modified impulse-response representation of the global response to carbon dioxide emissions, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-405, in review, 2016.
Richard J. Millar et al.
Richard J. Millar et al.
Richard J. Millar et al.

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Short summary
Simple representations of the global coupled climate-carbon-cycle system are required for climate policy analysis. Existing models have often failed to capture important physical dependencies of the climate response to carbon dioxide emissions. In this paper we propose a simple but novel modification to impulse-response climate-carbon-cycle models to capture these physical dependencies. This simple model creates an important tool for both climate policy and climate science analysis.
Simple representations of the global coupled climate-carbon-cycle system are required for...
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