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Volume 11, issue 6 | Copyright
Geosci. Model Dev., 11, 2273-2297, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Model description paper 18 Jun 2018

Model description paper | 18 Jun 2018

FAIR v1.3: a simple emissions-based impulse response and carbon cycle model

Christopher J. Smith1, Piers M. Forster1, Myles Allen2, Nicholas Leach2, Richard J. Millar3,4, Giovanni A. Passerello1, and Leighton A. Regayre1 Christopher J. Smith et al.
  • 1School of Earth and Environment, University of Leeds, Leeds, UK
  • 2Atmospheric Physics Department, University of Oxford, Oxford, UK
  • 3College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter, UK
  • 4Environmental Change Institute, University of Oxford, Oxford, UK

Abstract. Simple climate models can be valuable if they are able to replicate aspects of complex fully coupled earth system models. Larger ensembles can be produced, enabling a probabilistic view of future climate change. A simple emissions-based climate model, FAIR, is presented, which calculates atmospheric concentrations of greenhouse gases and effective radiative forcing (ERF) from greenhouse gases, aerosols, ozone and other agents. Model runs are constrained to observed temperature change from 1880 to 2016 and produce a range of future projections under the Representative Concentration Pathway (RCP) scenarios. The constrained estimates of equilibrium climate sensitivity (ECS), transient climate response (TCR) and transient climate response to cumulative CO2 emissions (TCRE) are 2.86 (2.01 to 4.22)K, 1.53 (1.05 to 2.41)K and 1.40 (0.96 to 2.23)K (1000GtC)−1 (median and 5–95% credible intervals). These are in good agreement with the likely Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) range, noting that AR5 estimates were derived from a combination of climate models, observations and expert judgement. The ranges of future projections of temperature and ranges of estimates of ECS, TCR and TCRE are somewhat sensitive to the prior distributions of ECS∕TCR parameters but less sensitive to the ERF from a doubling of CO2 or the observational temperature dataset used to constrain the ensemble. Taking these sensitivities into account, there is no evidence to suggest that the median and credible range of observationally constrained TCR or ECS differ from climate model-derived estimates. The range of temperature projections under RCP8.5 for 2081–2100 in the constrained FAIR model ensemble is lower than the emissions-based estimate reported in AR5 by half a degree, owing to differences in forcing assumptions and ECS∕TCR distributions.

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Short summary
FAIR v1.3 is a simple Python-based climate model emulator. It takes emissions of greenhouse gases and aerosol and ozone precursors to calculate radiative forcing and temperature change. It includes a simple representation of carbon cycle feedbacks due to temperature and accumulated carbon uptake. Large ensembles can be run with minimal computational expense for any user-specified emissions pathway. We produce such an ensemble using the RCP emissions datasets.
FAIR v1.3 is a simple Python-based climate model emulator. It takes emissions of greenhouse...