Estimating Carbon Budgets for Ambitious Climate Targets

Abstract

Carbon budgets, which define the total allowable CO2 emissions associated with a given global climate target, are a useful way of framing the climate mitigation challenge. In this paper, we review the geophysical basis for the idea of a carbon budget, showing how this concept emerges from a linear climate response to cumulative CO2 emissions. We then discuss the difference between a CO2-only carbon budget associated with a given level of CO2-induced warming, and an effective carbon budget associated with a given level of warming caused by all human emissions. We present estimates for the CO2-only and effective carbon budgets for 1.5 and 2 °C, based on both model simulations and updated observational data. Finally, we discuss the key contributors to uncertainty in carbon budget estimates, and suggest some implications of this uncertainty for decisionmaking. Based on the analysis presented here, we argue that while the CO2-only carbon budget is a robust upper bound on allowable emissions for a given climate target, the size of the effective carbon budget is dependent on the how quickly we are able to mitigate non-CO2 greenhouse gas and aerosols emissions. This suggests that climate mitigation efforts could benefit from being responsive to a changing effective carbon budget over time, as well as to potential new information that could narrow uncertainty associated with the climate response to CO2 emissions.

The impact of aerosol emissions on the 1.5 °C pathways

“…After correcting for this bias, they estimated that the 66% carbon budget for 1.5 • C may be as large as 240 GtC (880 GtCO 2 ). This new analysis introduces considerable new uncertainty into estimates of remaining carbon budgets, though it is also generally consistent with another recent independent estimate of the remaining budget for 1.5 • C [16].…”

“…Millar et al [14] assumed that non-CO 2 forcings would follow RCP2.6 (Representative Concentration Pathway with a radiative forcing value of ±2.6 W m −2 in the year 2100 relative to pre-industrial values), and their carbon budget results are therefore contingent on an arbitrary single scenario of non-CO 2 emissions. Uncertainty associated with both the choice of emissions scenario and the magnitude of non-CO 2 forcing is a large contributor to carbon budget uncertainty [17] given that increased (decreased) non-CO 2 contributions to future warming will decrease (increase) the allowable CO 2 emissions [16]. Given the large uncertainty associated with aerosol forcing in particular, it is important to better quantify the effect of aerosol uncertainty in the context of meeting ambitious climate targets.…”

UKESM1: Description and Evaluation of the U.K. Earth System Model

“…These additional Earth system components, as well as couplings across components and coupling with the physical climate model, allow a number of important policy and science questions to be addressed that would not be possible with GC3.1. Primary among these are estimates of available carbon emission budgets commensurate with limiting global warming below specific targets (Matthews et al, 2017). Introducing prognostic atmospheric chemistry into GC3.1 allows an assessment of the mitigation potential from reducing short lived climate forcers such as methane, black carbon, and tropospheric ozone (Collins et al, 2018;Stohl et al, 2015), while coupling atmospheric chemistry to terrestrial biogeochemistry opens up the potential to study the risk of permafrost thaw, methane release, and their impacts on Arctic warming.…”

The influence of non-CO 2 forcings on cumulative carbon emissions budgets

“…The results presented here are sensitive to uncertain future scenarios of non-CO 2 greenhouse gases and aerosol forcing [24][25][26]. However, we would not expect significant differences in the carbon budgets consistent with 1.5 • C and 2.0 • C levels of warming calculated from other RCP scenarios, as CO 2 is the dominant forcing, and the ratio of CO 2 to total forcing is approximately constant across the RCP scenarios (supplementary figure S1).…”

“…Using a consistent approach, we find that inclusion of non-CO 2 forcings has a net-warming effect [17][18][19], leading to carbon budget reductions compared with the CO 2 -only simulations, by 257 PgC and 418 PgC, for 1.5 • C and 2.0 • C temperature targets, respectively, and results in a narrower range of the ALL-forcing carbon budgets spread, compared to the spread of CO 2 -only carbon budgets. Understanding the influence of non-CO 2 forcings on carbon budgets and their uncertainties is crucial for international climate policies regarding mitigation of both CO 2 and non-CO 2 agents [24,26,29].…”