Equivalence of GHG emissions under the 2oC limit - Steve smith et al
Equivalence of GHG emissionsunder the 2°C limitSteve Smith1, 2Jason Lowe2Laila Gohar21UKCommittee on Climate Change2Met Office Hadley Centre
Motivation• Internationally agreed policy goal: ΔTmax < 2°C• Climate policies are multi-gas, but only weakly linked to ΔTmax via the GWP100• CO2-only studies show ΔTmax is constrained by total emissions over time (ΣECO2) 1• GTP measures ΔT, but still relies on time horizon and doesn’t signal need to limit ΣECO2• So how might ΣECO2 fit into a multi-gas emissions policy?1 Allen et al. (2009) Warming caused by cumulative carbon emissions towards the trillionth tonne, Nature Matthews et al. (2009) The proportionality of global warming to cumulative carbon emissions, Nature
Hypothesis• On timescales relevant to 2°C, GHGs can be divided into two baskets: – ‘long-lived’: ΔTmax ΣE – ‘shorter-lived’: ΔTmax Esustained• Does this give a reliable prediction for ΔTmax?• Where is the dividing line?
ΔT for RCP2.6 implied by PCTs & SETs 2 0.7Contribution to peak T from PCT (°C) Contribution to peak T from SET (°C) 1.8 0.6 1.6 1.4 0.5 1.2 0.4 1 0.8 0.3 0.6 0.2 0.4 0.1 0.2 0 0.0 2000 2020 2040 2060 2080 2100 2000 2020 2040 2060 2080 2100 Year Year Fossil CO2 Other CO2 N2O CH4 HFC32 HFC43_10 CF4 C2F6 C6F14 HFC125 HFC134a HFC143a HFC23 SF6 HFC227ea HFC245fa
Comparison with realised ΔT 2.0 1.8 Total SET ΔT (°C above pre-industrial) 1.6 1.4 1.2 Total PCT 1.0 0.8 RCP2.6 all 0.6 0.4 RCP2.6 0.2 GHGs only 0.0 2000 2020 2040 2060 2080 2100 Year
Conclusions• For the 2°C limit GHGs could be split into two baskets: – ‘long-lived’: committed ΔTmax = PCTx × ΣEx – ‘shorter-lived’: future ΔTmax (if sustained) = SETx × Ex• Sum of PCTs & SETs gives a guide to size & timing of ΔTmax• Reduces reliance on (arbitrary) time horizons• 2°C could be met by limiting cumulative ΣECO2e for long-lived GHGs including N2O• Allowable ΣECO2e depends on emissions of shorter-lived GHGs near time of peaking only (but near-term emissions do influence warming rate as well)