Multi - century warming could exceed expectations from past carbon emissions 1

Some earth system modeling studies indicate that the warming due to past carbon emission is 6 proportional to that emission 1 and relatively stable on the multi-centennial to millennial 7 timescale 2,3. This claim implies that future warming will be due to future emissions and offers 8 the possibility of using the observed ratio of CO 2-attributable warming to cumulative carbon 9 emissions to estimate the future emissions cap needed to enforce a specific cap on global 10 warming. The IPCC 5th report likely-range for this transient climate response to cumulative 11 emissions (TCRE) is 0.8-2.5 K/Eg-C – too large a range to be very useful for this purpose 4. 12 However, even if the historical TCRE were known precisely, its predictive accuracy would still 13 be a key concern. 14 15 Most of the studies finding stability of the long-term response to past emissions have been 16 performed with earth system models of intermediate complexity (EMICs) 5. For the most part 17 these models do not simulate cloud feedback which is mainly responsible for the broad range of 18 equilibrium climate sensitivities (ECSs) in atmosphere-ocean global climate models 19 (AOGCMs) 6. Another sensitivity parameter, the transient climate response (TCR), characterizes 20 the warming that occurs during emissions 7 , incorporating the cooling impacts of ocean heat 21 uptake magnitude and pattern. The cooling impact of the heat uptake pattern, known as ocean 22 2 heat uptake efficacy, is also influenced by cloud feedback 8 and so may not be well-represented in 23 current EMICs. 24 25 The ECS/TCR ratio is a driver of long-term warming and it is important to assess the stability of 26 the TCRE with models that produce the full range of possible ECS/TCR ratios. In Fig. 1 we 27 show that the EMICs used in the IPCC 5th assessment have generally smaller ratios than the 28 AOGCMs used in that and earlier assessments. Fig. 1 also shows an estimate of the ECS/TCR 29 ratio based on 1971-2011 heat uptake observations, the IPCC 5th assessment estimate of 30 radiative forcing over this period and AOGCM heat uptake efficacies (Supplemental Section S1). 31 The median ECS/TCR estimate is just above 1.5 but the 90% confidence interval is significantly 32 right-skewed. 33 34 To demonstrate the influence of a high ECS/TCR ratio on TCRE stability, we use the GFDL 35 ESM2M model 9 which has an ECS/TCR ratio …