Estimating Remaining Carbon Budgets Using Temperature Responses Informed by CMIP6

Martin Rypdal,Kristoffer Rypdal,Rune G Graversen,Hege-Beate Fredriksen,Niklas Boers,Endre Falck Mentzoni,Andreas Johansen,Kai-Uwe Eiselt,Andreas Martinsen

doi:10.3389/fclim.2021.686058

Abstract

A remaining carbon budget (RCB) estimates how much CO2 we can emit and still reach a specific temperature target. The RCB concept is attractive since it easily communicates to the public and policymakers, but RCBs are also subject to uncertainties. The expected warming levels for a given carbon budget has a wide uncertainty range, which increases with less ambitious targets, i.e., with higher CO2 emissions and temperatures. Leading causes of RCB uncertainty are the future non-CO2 emissions, Earth system feedbacks, and the spread in the climate sensitivity among climate models. The latter is investigated in this paper, using a simple carbon cycle model and emulators of the temperature responses of the Earth System Models in the Coupled Model Intercomparison Project Phase 6 (CMIP6) ensemble. Driving 41 CMIP6 emulators with 127 different emission scenarios for the 21st century, we find almost perfect linear relationship between maximum global surface air temperature and cumulative carbon emissions, allowing unambiguous estimates of RCB for each CMIP6 model. The range of these estimates over the model ensemble is a measure of the uncertainty in the RCB arising from the range in climate sensitivity over this ensemble, and it is suggested that observational constraints imposed on the transient climate response in the model ensemble can reduce uncertainty in RCB estimates.

Highlights

The concept of remaining carbon budgets (RCBs) is appealing and highly applicable to climate mitigation policy (Zickfeld et al, 2009)
Our results show that the linear relationship between total emissions and maximum GSAT is an excellent approximation for each temperature-response model for cumulative emissions up to 5,000 Gt CO2 after 2018, but that the ETCRE varies considerably over the ensemble of different temperature responses (Figure 4)
Over the ensemble we find a mean ETCRE of 0.42◦C per 1,000 Gt CO2, with a 66% confidence range of 0.35–0.47◦C per 1,000 Gt CO2 (Figure 5A)

Summary

INTRODUCTION

The concept of remaining carbon budgets (RCBs) is appealing and highly applicable to climate mitigation policy (Zickfeld et al, 2009). Analyzing 11 CMIP6 models, Arora et al (2020) found CO2-only TCRE in the range 0.33 − 0.58◦C per 1,000 Gt CO2, with a mean of 0.44◦C per 1,000 Gt CO2 The basis of these estimates are scenarios where atmospheric CO2 concentration increases by 1% per year, and not scenarios where we reduce emissions to mitigate climate change. In the CMIP5 ensemble, the multi-model mean ratio of CO2 forcing to total anthropogenic forcing has been estimated to be 0.86 (Meinshausen et al, 2011; Matthews et al, 2017), which yields a multi-model mean ETCRE of 0.51◦C per 1,000 Gt CO2 based on the CO2-only estimate of 0.44◦C per 1,000 Gt CO2 (Gillett et al, 2013) Another approach (Matthews et al, 2017) is to estimate the ETCRE by dividing the observed 1861–2015 GSAT increase of 0.99◦C by the 1870–2015 cumulative CO2 emissions of 2,035 Gt CO2 to obtain ETCRE = 0.49◦C per 1,000 Gt CO2.

MODELING SET-UP

RESULTS

DISCUSSION

DATA AVAILABILITY STATEMENT