Equivalence of GHG emissionsunder the 2°C limitSteve Smith1, 2Jason Lowe2Laila Gohar21UKCommittee on Climate Change2Met Of...
Motivation• Internationally agreed policy goal: ΔTmax < 2°C• Climate policies are multi-gas, but only weakly linked to  ΔT...
Hypothesis• On timescales relevant to  2°C, GHGs can be divided  into two baskets:   – ‘long-lived’: ΔTmax  ΣE   – ‘short...
Experimental setup                            MAGICC v5.3                     RCP2.6 background forcings
Defining long- and shorter-lived GHGs                          1                                      HFC-32 (4.9yr)      ...
ΔTmax vs. ΣE for long-lived GHGs                                        5                                                 ...
Peak Commitment Temperatures        Species   Lifetime (yr)     PCT (°C/kg)         CO2e    CO2                         - ...
ΔTmax vs. Esustained for shorter-lived GHGs   dΔT (t ) τ          = λΔRF(t ) - ΔT (t )     dtFor sustained emissions: ΔTma...
Sustained Emission Temperatures     Species           Lifetime (yr)     Radiative efficiency (Wm-2ppb-1)      SET (°Ckg-1y...
ΔT for RCP2.6 implied by PCTs & SETs                                        2                                             ...
Comparison with realised ΔT                                 2.0                                 1.8                       ...
Conclusions• For the 2°C limit GHGs could be split into two baskets:   – ‘long-lived’: committed ΔTmax = PCTx × ΣEx   – ‘s...
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Equivalence of GHG emissions under the 2oC limit - Steve smith et al

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Equivalence of GHG emissions under the 2oC limit - Steve smith et al

  1. 1. Equivalence of GHG emissionsunder the 2°C limitSteve Smith1, 2Jason Lowe2Laila Gohar21UKCommittee on Climate Change2Met Office Hadley Centre
  2. 2. 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
  3. 3. 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?
  4. 4. Experimental setup MAGICC v5.3 RCP2.6 background forcings
  5. 5. Defining long- and shorter-lived GHGs 1 HFC-32 (4.9yr) HFC-227ea (34.2yr) CH4 (12yr) HFC-125 (29yr) HFC-143a (52yr) 0.95 r2 (ΔTmax, Epeak) HFE-125 (136yr) N2O (114yr) 0.9 SF6 (3,200yr) CO2 0.85 0.85 0.9 0.95 1 r2 (ΔTmax, ƩE)
  6. 6. ΔTmax vs. ΣE for long-lived GHGs 5 1 CO2 N2O (114yr) Peak warming (°C) Peak warming (°C) 4 0.8 3 0.6 2 0.4 1 0.2 0 0 0 1000 2000 3000 0 2000 4000 6000 Total emissions 2000-2400 (GtC) Total emissions 2000-2400 (MtN) 0.05 SF6 (3,200yr)Peak warming (°C) 0.04 0.03 0.02 ΔTmax = (PCTx )ΣE x 0.01 0 0 500 1000 1500 2000 2500 Total emissions 2000-2400 (kt)
  7. 7. Peak Commitment Temperatures Species Lifetime (yr) PCT (°C/kg) CO2e CO2 - 4.32E-16 1 N2O 114 1.33E-13 309 HFC-23 270 7.43E-12 17200 HFC-236fa 240 4.83E-12 11200 SF6 3200 1.73E-11 40100 NF3 740 1.11E-11 25600 CF4 50000 6.01E-12 13900 C2 F 6 10000 9.73E-12 22500 C3 F 8 2600 6.31E-12 14600 c-C4F8 3200 7.53E-12 17400 C4F10 2600 6.37E-12 14700 C5F12 4100 6.86E-12 15900 C6F14 3200 6.68E-12 15500 C10F18 >1000 ≥4.83E-12 ≥12100 SF5CF3 800 1.11E-11 25700 HFE-125 136 6.68E-12 15400 PFPMIE 800 6.42E-12 14900
  8. 8. ΔTmax vs. Esustained for shorter-lived GHGs dΔT (t ) τ = λΔRF(t ) - ΔT (t ) dtFor sustained emissions: ΔTmax = ΔT (∞ ) sustained = λαx Ax E x sustained ΔTmax = (SETx )E xwhere: λ = climate sensitivity parameter αx = atmospheric lifetime of gas x Ax = radiative efficiency of gas x
  9. 9. Sustained Emission Temperatures Species Lifetime (yr) Radiative efficiency (Wm-2ppb-1) SET (°Ckg-1yr) * CH4eCH4† 12 3.70E-04 1.74E-12 1HFC-32 4.9 0.11 4.66E-11 27HFC-125 29 0.23 2.50E-10 144HFC-134a 14 0.16 9.88E-11 57HFC-143a 52 0.13 3.62E-10 208HFC-152a 1.4 0.09 8.58E-12 5HFC-227ea 34.2 0.26 2.35E-10 135HFC-245fa 7.6 0.28 7.14E-11 41HFC-365mfc 8.6 0.21 5.49E-11 32HFC-43-10mee 15.9 0.4 1.14E-10 65HFE-134 26 0.45 4.46E-10 256HFE-143a 4.3 0.27 5.22E-11 30HCFE-235da2 2.6 0.38 2.41E-11 14HFE-245cb2 5.1 0.32 5.56E-11 32HFE-245fa2 4.9 0.31 4.54E-11 26* Values calculated for λ=0.8K(Wm-2)-1† CH4 SET includes OH lifetime enhancement and indirect O3 & stratospheric H2O effects
  10. 10. 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
  11. 11. 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
  12. 12. 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)

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