CMIP6 climate models imply high committed warming
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CMIP6 climate models imply high committed warming Chris Huntingford 1
& Mark S. Williamson
2,3
& Femke J. M. M. Nijsse
3
Received: 9 April 2020 / Accepted: 25 August 2020/ # The Author(s) 2020
Current climate change is highly transient, and Earth is at a significant distance from thermal equilibrium. This well-established finding is predominantly due to the large flux of thermal energy currently entering the oceans. If globally the sources and sinks of atmospheric GHGs were to become zero, this is identical to stating that their concentrations are subsequently invariant. Such a policy would correspond to a full offset of the remaining emissions by both land and ocean CO2 drawdown, as well as any implementation of carbon capture and storage (CCS) methods. The 5th Intergovernmental Panel on Climate Change (IPCC) report defines this as a “constant composition commitment” (Collins et al., 2013). With fixed atmospheric GHGs, the planet will move to an equilibrium state, at which time land and ocean CO2 offsets will tend to zero. Hence, without any deliberate CCS implementation, gross GHG emissions will then need to be negligible. However, an equilibrium state also implies thermal stability too, with global warming invariant. It is the difference between the observed transient planetary response and equilibrium climate state for the same GHG levels that characterise this extra warming. We analyse this additional warming for historical and current GHG concentrations, utilising ECS values for the new ESMs in the Coupled Model Intercomparison Project v6 (CMIP6). The current lag in global warming behind an equilibrium state has been explored with the CMIP5 ensemble of ESM simulations, following the forcing protocol of Taylor et al. (2012). These calculations raise the prospect that even current atmospheric GHG levels commit to near-surface global warming likely greater than 1.5 °C above pre-industrial levels (and even higher over land; Huntingford and Mercado, 2016). With rising international discussion of minimising global emissions, which in some circumstances could correspond to invariant atmospheric GHG levels, there is renewed interest in the magnitude of stabilised global warming levels. Here, we perform a simplistic but illustrative analysis to define this lag, utilising the ECS values of the recent CMIP6 ESMs derived by established methods (Gregory et al., 2004).
* Chris Huntingford [email protected]
1
Centre for Ecology and Hydrology, Wallingford, Oxfordshire OX10 8BB, UK
2
Global Systems Institute, University of Exeter, Exeter EX4 4QE, UK
3
College of Engineering, Mathematics and Physical Science, University of Exeter, Exeter, UK
Climatic Change
The ECS value for each climate model is the ESM-specific projected warming for a doubling of atmospheric CO2. Other radiatively active atmospheric gases, including nonCO2 greenhouse gases, are compared by their impact on radiative forcing, leading to a single aggregated statistic of carbon dioxide equivalent concentrations, CO2e (ppm). The radiative response to
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