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

Christopher J Smith,Myles Allen,Leighton A Regayre,Giovanni A Passerello,Piers M Forster,Nicholas Leach,Richard J Millar

doi:10.5194/gmd-11-2273-2018

Abstract

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 (1000 GtC)−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.

Highlights

Most multi-model studies, such as the Coupled Model Intercomparison Project (CMIP), which produces headline climate projections for the Intergovernmental Panel on Climate Change (IPCC) assessment reports, compare atmosphere– ocean general circulation models that are run with prescribed concentrations of greenhouse gases
The ratio of transient climate response (TCR) to equilibrium climate sensitivity (ECS), the realised warming fraction (RWF), is approximately independent of TCR in CMIP5 models (Millar et al, 2015), and the prior distribution could alternatively be defined in terms of the TCR and RWF joint distribution, which is explored in Sect
We present a simple model, Finite Amplitude Impulse Response (FAIR) v1.3, that calculates global temperature change, effective radiative forcing from a variety of drivers and concentrations of greenhouse gases

Summary

Introduction

Most multi-model studies, such as the Coupled Model Intercomparison Project (CMIP), which produces headline climate projections for the Intergovernmental Panel on Climate Change (IPCC) assessment reports, compare atmosphere– ocean general circulation models that are run with prescribed concentrations of greenhouse gases. Greenhouse gas and aerosol emissions time series are provided by integrated assessment modelling groups based on socio-economic narratives (Moss et al, 2010; Meinshausen et al, 2011b), which are converted to atmospheric concentrations by simple climate–carbon-cycle models such as MAGICC6 (Meinshausen et al, 2011a). Earth system models can be run in emissions mode, where emissions of carbon dioxide are used as a starting point and the atmospheric CO2 concentrations are calculated interactively in the model, with atmospheric concentration changes being the residual of emissions minus absorption by land and ocean sinks. While many models include the functionality to be run in CO2 emissions-driven mode, these integrations were not the main focus of CMIP5 (the fifth phase of CMIP; Taylor et al, 2012). Smith et al.: FAIR: a simple emissions-based impulse response and carbon cycle model

Methods

Results

Conclusion