Quantifying non-CO2 contributions to remaining carbon budgets

Abstract

The IPCC Special Report on 1.5 °C concluded that anthropogenic global warming is determined by cumulative anthropogenic CO2 emissions and the non-CO2 radiative forcing level in the decades prior to peak warming. We quantify this using CO2-forcing-equivalent (CO2-fe) emissions. We produce an observationally constrained estimate of the Transient Climate Response to cumulative carbon Emissions (TCRE), giving a 90% confidence interval of 0.26–0.78 °C/TtCO2, implying a remaining total CO2-fe budget from 2020 to 1.5 °C of 350–1040 GtCO2-fe, where non-CO2 forcing changes take up 50 to 300 GtCO2-fe. Using a central non-CO2 forcing estimate, the remaining CO2 budgets are 640, 545, 455 GtCO2 for a 33, 50 or 66% chance of limiting warming to 1.5 °C. We discuss the impact of GMST revisions and the contribution of non-CO2 mitigation to remaining budgets, determining that reporting budgets in CO2-fe for alternative definitions of GMST, displaying CO2 and non-CO2 contributions using a two-dimensional presentation, offers the most transparent approach.

The Multi‐Decadal Response to Net Zero CO2 Emissions and Implications for Emissions Policy

“…The RAZE appears weakly scenario dependent, although this is less pronounced than in the ZEC which, as expected, scales with cumulative emissions prior to the date of net zero (MacDougall et al., 2020). In ESMs, a negative average RAZE is observed for the 3670 GtCO 2 experiment, but positive average RAZE for 7330 GtCO 2 , both based on small samples and due to a mixture of strong positive carbon‐climate feedbacks and slower carbon sinks in ACCESS, UKESM and GFDL‐ESM2M (such feedbacks are observed in other large perturbation experiments (Jenkins et al., 2021; Leach et al., 2021; Meinshausen et al., 2011; Millar & Friedlingstein, 2018; Nicholls et al., 2020)). In the EMICs the RAZE is consistently negative, but still weakens as the perturbation size increases.…”

“…Using this framework, Seshadri (2017) then derives an expression (Jenkins et al., 2021; Seshadri, 2017) for the warming response to a RF timeseries F ( t ) over a multi‐decadal time interval $${\Delta}t$$: $$${\Delta}T={\kappa }_{F}\left({\Delta}F+\rho \overline{F}{\Delta}t\right)$$$ where $${\Delta}T$$ is the temperature change and $${\Delta}\mathrm{F}\left(\mathrm{t}-\frac{{\mathrm{c}}_{1}{d}_{1}}{{\mathrm{c}}_{2}}\right)\,\approx {\Delta}\mathrm{F}$$ is the change in forcing over the period $${\Delta}t$$ (for a thermal cycle with sub‐decadal, $${d}_{1}$$ and $${c}_{1}$$, or multi‐century, $${d}_{2}$$ and $${c}_{2}$$, timescales and associated efficiacies). $$\overline{F}$$ is the average forcing over the period $${\Delta}t$$ compared to preindustrial, $$\rho $$ is the fractional rate of adjustment to constant forcing (RACF, in units per year) (Cain et al., 2019), and …”

Methane and the Paris Agreement temperature goals

“…The maximum ambition scenario (pink) reduces CH 4 emissions by 88% and is an outlier scenario which was specifically designed to maximize non-CO 2 reductions [42]. The models are not independent of each other, and this consistency does not represent a 'most likely' future scenario [39]. It is simply the most cost-efficient mechanism for these models to avoid exceeding the 1.5°C [38,42], and the experimental design of many studies (end-of-century targets) may give an unintentional preference for the use of CO 2 removal over non-CO 2 mitigation which has a shorter term effect on temperature [45].…”

“…Scenarios modelled in figure 1 are taken from the IPCC's Special Report on the Global Warming of 1.5°C (SR15) [38] scenario database [36]. The CO 2 , CH 4 and N 2 O emissions and RF are the 1.5°Ccompatible model-scenario combinations which are consistent with present-day RF levels in each pollutant (using the method of [39]). The emissions of CO 2 , CH 4 and N 2 O in these 22 scenarios are shown in figure 2.…”

Net Zero: Science, Origins, and Implications

“…The quantity in square brackets in Equation 19provides an estimate of aggregate CO 2warming-equivalent (CO 2 -we) emissions (20,(147)(148)(149), so called because it would have the same warming impact over this time interval as the emission of that amount of CO 2 (147). More precise methods of computing warming-equivalent emissions have been proposed, but as illustrated in Figure 4, the use of simple trailing 10-and 20-year averages to calculate E S in Equation 19is already remarkably accurate at capturing the global temperature response to various forcing agents for both rising and falling emissions.…”