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.
Highlights
The IPCC’s Special Report on Global Warming of 1.5 °C1 (SR1.5) concluded: ‘Reaching and sustaining net zero global anthropogenic CO2 emissions and declining net non-CO2 radiative forcing would halt anthropogenic global warming on multi-decadal timescales
Proposed by Tom Wigley in 1998 under the name of a ‘Forcing-Equivalent Index’, CO2-fe emissions[27] express an emissions time series of any climate pollutant in terms of the time series of CO2 emissions that would have an identical impact on effective radiative forcing (ERF), and global mean surface temperature (GMST) on all timescales. They are obtained by converting the ERF associated with that pollutant to a time series of change in CO2-equivalent concentrations, and computing impact of covarying physical climate uncertainty implicit in the choice of to cumulative carbon Emissions (TCRE)
Refs. 1,2,24 all remove a quantity of warming the CO2 emissions required to produce that CO2 concentration perturbation using a carbon cycle model[25]
Summary
The IPCC’s Special Report on Global Warming of 1.5 °C1 (SR1.5) concluded: ‘Reaching and sustaining net zero global anthropogenic CO2 emissions and declining net non-CO2 radiative forcing would halt anthropogenic global warming on multi-decadal timescales. The maximum temperature reached is determined by cumulative net global anthropogenic CO2 emissions up to the time of net zero CO2 emissions and the level of non-CO2 radiative forcing prior to the time that maximum temperatures are reached.’. This highlights the importance of future cumulative CO2 emissions, often termed the ‘remaining carbon budget’[2,3,4,5], together with the increasingly important role of non-CO2 climate drivers as peak warming is approached. There are, complications[1,2,9,12] in the use of TCRE to derive the remaining carbon budget, including the precise definition and estimated current level of global warming; committed warming due to past CO2 emissions, or the zero emissions commitment (ZEC); possible contributions of Earth
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