IPCC Expert Meeting on Short-lived Climate Forcers Geneva 28-31 May 2018 Emission metrics for SLCFs Keith P Shine Department of Meteorology, University of Reading, UK k.p.shine@reading.ac.uk With particular thanks to: Myles Allen, Michelle Cain, Jan Fuglestvedt •  Background and how we got to where we are •  Metrics in the context of long-term temperature goals Note: In this talk, I often use CO, as the example long- lived gas and methane (CH 4 ) as the example SLCF. But conclusions apply more generally to other SLCFs
•  Climate emission metrics provide an "exchange rate”. They allow the climate effect of emissions of species to be compared with emissions of CO, ● Emissions of all species can then be placed on a common scale ("CO ,-equivalent (CO ,-e) emissions") ● If a metric is perfect, the same CO – e emissions from a different mix of species would produce the same climate effect; in practice conventional metrics fail to do this •  Many choices have to be made in choosing an appropriate metric •  Ultimately, choices should be guided by the policy that the metric aims to serve

IPCC, the Kyoto Protocol and GWP
•  Kyoto Protocol uses the  100-year GWP (GWP 100), mostly from IPCC’s 2nd Assessment (1995) •  NDC’s use values from a variety of assessments • GWP is generally accepted as an appropriate measure by the user community, and has played an important role in enabling Kyoto •  At the time of Kyoto, GWP was the only metric that IPCC had assessed: Kyoto chose GWP 100 ●  AR5 also assessed the Global Temperature-change Potential (GTP) but recommended  neither the GWP or GTP (AR4 did recommend GWP) ●  The COン-e problem is shared by all these conventional metrics The Global Warming Potential (GWP) - the view from IPCC’s First Assessment Report ... Section 2.2.7: "... there is no universally accepted methodology for combining ... relevant factors into a single (metric) ... A  simple approach [i.e. the GWP] has been adopted here to illustrate the difficulties inherent in the concept ….” It presented three time-horizons (20, 100 and 500 yr)...  'as candidates for discussion [that] should not be considered as having any special significance
The time-integrated radiative forcing in response to a pulse emission of a species (relative to the same quantity for a emission of the same mass of CO,) Note! Persistence of CO, GWP has a strong memory of short-lived emissions even after they have disappeared from the atmosphere IPCC AR5 WG1 Chapter 8
●  It does not represent the temperature impact: CO,’s impact persists; CH₄’s is small after ≈50 years (neglecting carbon-cycle feedbacks) ● Long-term temperature impact of CO, pulse emission can only be matched by sustained  SLCF emissions Shine et al, Cli Change  (2005)
. IPCC GWP 100 has changed with time: reflects changing understanding of CH 4 lifetime and indirect effects, CO  properties, etc. Volatility ≥ for other SLCFs – important in policy usage Stated uncertainty in CH4 GWP100 is ±40%. Greater for SLCFs. (If post-AR5 science developments are assessed to be robust by AR6, GWP 100 could increase to ≈ 35)
Global to global Regional to global Regional to regional Figures from Jan Fuglestvedt
Table 8.A.3 | GWP and GTP for NO, from surface sources for time horizons of 20 and 100 years from the literature. All values are on a per kilogram of nitrogen basis. Uncerta for numbers from Fry et al (2012) and Collins et al (2013) refer to 1-σ. For the reference gas CO), RE and IRF from AR4 are used in the calculations. The GWP 100 and GTP 100 can be scaled by 0.94 and 0.92, respectively, to account for updated values for the reference gas CO 3 . For 20 years the changes are negligible.
IPCC AR5 WG1 Chapter 8 NOx as a example. AR5 included additional SLCFs. How would/could  this regionality be handled? Fuglestvedt et al. Atmos Env 2010 Short-lived ozone (warming) methane- induced ozone
(cooling) Returning to the global perspective ...
nate change PUBLISHED ONLINE: 2 MAY 2016 | DOI: 10.10. New use of global warming potentials to compare cumulative and short-lived climate pollutants Myles R. Allen¹,²*, Jan S. Fuglestvedt³, Keith P. Shine⁴, Andy Reisinger⁵, Raymond T. Pierrehumbert² and Piers M. Forster6 And: Allen et al. (2018) to appear in npj Climate and Atmospheric Science on 5 June 10.1038/s41612-018-0026-8 (not yet active) Environ. Res. Lett. 13 (2018) 054003 https://doi.org/10.1088/1748-9326/aab89 Increased importance of methane reduction for a 1.5 degree target William J Collins 1,6®, Christopher P Webber 1, Peter M Cox 2, Chris Huntingford 3, Jason Lowe 4,5 , Stephen Sitch ², Sarah E Chadburn ²,5 , Edward Comyn-Platt ³, Anna B Harper ², Garry Hayman ³ and Tom Powell ²
(38 GtCO 2 is the 2011 anthropogenic emissions of CO 2 ; total CH₄ emissions are the same in both frames) Allen et al. Nature CC (2016),
•  The conventional usage of GWP 100 says that the CO 2 equivalence of CH 4 emission is given by CO₂-e[tonnes]=GWP₁00 x CH₄ Emission[tonnes] •  The "equivalence" is s such that the integrated radiative forcing over 100 years is the same for the CH4 pulse and the equivalent pulse of CO2 •  Under GWP*, the CO, equivalence comes from the change in CH4 emission  rate CO 5-e*[tonnes]=H x GWP H x change in CH 4 emission rate [tonnes per year] •  The "equivalence" is temperature change rather than integrated forcing: arguably more aligned Paris goals
•  Under GWP, a 1 tonne CH 4 pulse is "equivalent" to a 28 tonne CO₂ pulse (IPCC AR5 GWP₁00 for CH₄=28) •  Under GWP* = H x GWP(H), a 1 tonne per year increase in CH 4 emission rate is equivalent to a 100x28 = 2800 tonne (one-off) CO₂ pulse. (Dependence on H is quite modest) •  And similarly, a 1 tonne per year decrease in CH 4 emission rate is equivalent to a 2800 tonne (one- off)  removal of CO, •  Equivalence only holds if CH 4 decrease is sustained indefinitely. If emissions go back up, equivalence is lost. A policy challenge.
•  Constant CH 4 emissions cause no further ΔT ●  CO,-e using GWP100 would say they continue to warm •  (Constant CH 4 emissions continue to elevate temperature and so retain mitigation potential)
Cain et al. Martin School Brief

Falling emissions ●  Falling CH 4 emissions are equivalent to CO, removal; they cause a cooling ●  Conventional (GWP 100) CO,-e says that they cause additional warming until emissions reach zero * Assuming the SLCF causes a warming!
Cain et al. Martin School Brief

● CO₂-e using GWP might be reasonable when emissions increase;  it fails when they are constant or falling •  Arguably the greatest challenge to the "integrity" of GWPs since IPCC’s First Assessment (1990)
Annual emissions in CO,-e using GWP 100 Cumulative CO − e emissions using GWP 100 ●  The CO 7-e is calculated using GWP 100 •  CO, and CH4 emissions rise and then fall; but using CO₂−e, CH₄ seems to accumulate in the atmosphere † RCP2.6: IPCC’s Representative Concentration Pathway aiming for 2 deg C Allen et al. 2018 to appear in npj Climate and Atmospheric Science
Annual emissions in CO - e using GWP 100 Cumulative CO − e and temperature changes (dashed) •  Cumulative CO ,-e works well for CO , (!) •  For CH4, even though emissions fall and cause temperature to decrease, CO₂-e using GWP 100 cannot capture this Allen et al. 2018 to appear in npj Climate and Atmospheric Science
Annual emissions in CO,-e* using GWP* Cumulative CO¸–e* emissions using GWP* •  Under GWP*, the change in CH4 emissions holds the CO₂  equivalence • Once CH 4 emissions begin to fall, they become equivalent to removal of CO, Allen et al. 2018 to appear in npj Climate and Atmospheric Science
Annual emissions in CO,-e* using GWP* the total) Cumulative CO 2 –e* and temperature changes (dashed) Using GWP100 Allen et al. 2018 to appear in npj Climate and Atmospheric Science
• GWP 100 seems poorly suited for characterising CO 2 - equivalence for constant/falling SLCF emissions in the temperature context •  The problem could be resolved via a new GWP usage that we call GWP*. This equates a sustained step decrease in SLCF emission rate with a one-off removal of CO, from the atmosphere • GWP* seems better than GWP for monitoring progress to a long-term temperature goal, but the comparison of pulse (long-lived) and sustained (SLCF) emissions requires a change of thinking •  Any change in the metric used in international agreements would be disruptive and likely to be resisted by some/many •  Dependence of global impact on time and location of SLCF emissions is also a challenge