Near-term climate forcers and climate policy: methane and black carbon Daniel J. Jacob

Atmospheric black carbon: absorber of solar radiation diesel engines residential fuel open fires freshly emitted BC particle Global BC emission [Wang et al., 2014] Loss of BC is by wet deposition (lifetime ~ 1 week)

Gorillas and chimpanzees of climate change CO 2 : the 800-lbs gorilla Methane and BC: the chimps Do we care about the chimps?

Radiative forcing of climate change Solar flux F in Terrestrial flux F out =σ T 4 Global radiative equilibrium: F in = F out Perturb greenhouse gases or aerosols radiative forcing  F = F in - F out Global equilibrium surface temperature responds as  T o ~  F

Radiative forcing referenced to emissions, Radiative forcing from methane emissions is 0.97 W m -2, compared to 1.68 W m -2 for CO 2 Radiative forcing from black carbon aerosol (BC) is 0.65 W m -2, highly uncertain Together methane and BC have radiative forcing comparable to CO 2 But atmospheric lifetimes of methane (10 years) and BC (~1 week) are shorter than CO 2 (> 100 years) What does that mean for priorities in controlling future emissions? [IPCC, 2014]

Climate policy metrics consider the integrated future impact of a pulse unit emission of a radiative forcing agent Inject 1 kg of agent X at time t = 0 time Concentration C(t) from pulse time Impact from pulse = f(C(t)) time Discount rate Climate metric = (impact)  (discount rate)  dt …usually normalized to CO 2

Standard IPCC metric: Global Warming Potential (GWP) Integrated radiative forcing over time horizon [0, H] CO 2 methane BC Radiative forcing  F vs. time for pulse unit emission of X at t = 0 GWP for methane vs. chosen time horizon: 28 for H = 100 years  1 Tg CH 4 = 28 Tg CO 2 (eq) IPCC [2014] GWP is easy to compute, but it does not correspond to any physical impact H Discount rate: step function

New IPCC metric: global temperature potential (GTP) Global mean surface temperature change at t = H CO 2 methane BC Temperature change vs. time for pulse unit emission at t = 0 Temperature response to actual 2008 emissions taken as a 1-year pulse IPCC [2014] Methane as important as CO 2 for 10-year horizon, unimportant for 100-year horizon Discount rate: Dirac function H

Why does methane cause only a short-term temperature response? ToTo ToTo T o +  T o ToTo F in t < 0 t = 0 t = 20 years t = 100 years climate equilibrium emission pulse climate response back to original equilibrium F out  F = 0  F < 0  F > 0

Simple calculation of Global Temperature Potential (GTP) Use impulse response function of surface T o to pulse  F of 1 W m -2 at time t = 0: t in years obtained by fitting results of HadCM3 climate model GTP is then given by Boucher and Reddy [2008]

Implication of GTP-based policy for near-term climate forcers Start controlling methane 40 years before target, BC 10 years before target IPCC [2014] Consider a policy aiming to restrict warming to 2 o C in 100 years

Controlling methane and BC should be part of climate policy … but for reasons totally different than CO 2 It addresses climate change on time scales of decades – which we care about It offers decadal-scale results for accountability of climate policy It is less sensitive to arguments over what discount rates should be used It is an alternative to geoengineering by aerosols It has important air quality co-benefits BC has additional regional, hydrological impacts Trend in Arctic sea ice volume Geoengineering: cloud seeding

Black carbon in the atmosphere diesel engines residential fuel open fires freshly emitted BC particle Global BC emission [Wang et al., 2014] Loss of BC is by wet deposition (lifetime ~ 1 week)

BC exported to upper troposphere is major component of forcing frontal lifting deep convection scavenging BC source region (combustion) Ocean Export to upper troposphere Global mean BC profile (chemical transport model) BC forcing efficiency Integral contribution To BC forcing Samset and Myhre [2011] 50% from BC > 5 km …because it’s above white clouds instead of dark surface

Multimodel intercomparisons and comparisons to observations Koch et al. [2009], Schwarz et al. [2010] BC, ng kg -1 TC4 aircraft campaign (Costa Rica) Observed Models Such large overestimate must be due to model errors in scavenging AeroCom chemical transport models (CTMs) used by IPCC overestimate BC by order of magnitude in upper troposphere Pressure, hPa obs models 60-80N obs models 20S-20N Pressure, hPa HIPPO aircraft campaign over Pacific BC, ng kg -1

Previous application to Arctic spring (ARCTAS) CCN Cloud updraft scavenging Large scale precipitation Anvil precipitation IN+CCN entrainment detrainment BC/aerosol scavenging in GEOS-Chem CTM CCN+IN, impaction Meteorological data including convective mass fluxes from NASA GEOS assimilation system Aerosols are scavenged in cloud by similarity with condensed water Additional scavenging below cloud by rain/snow In-cloud scavenging efficiency from freezing/frozen clouds is highly uncertain Additional uncertainty for BC is its efficiency as cloud condensation nucleus (CCN) and ice nucleus (IN) BC lifetime in GEOS-Chem is 4 days (vs. 7±2 days in AeroCom models)

GEOS-Chem BC simulation: source regions and outflow NMB= -27% NMB= -12% NMB= 6.6% Observations (circles) and model (background) surface networks AERONET BC optical depth NMB= -32% Aircraft profiles in continental/outflow regions HIPPO (US) Arctic (ARCTAS) Asian outflow (A-FORCE) US (HIPPO) observed model Wang et al., 2014 Normalized mean bias (NMB) in range of -30% to +10% Tests sources, export

Comparison to HIPPO BC observations across the Pacific Model doesn’t capture low tail, is too high at N mid-latitudes Mean column bias is +48% Still much better than the AeroCom models Wang et al., 2014 Observed Model PDF PDF, (mg m -3 STP) -1

BC top-of-atmosphere direct radiative forcing (DRF) Emission Tg C a -1 Global load (mg m -2 ) [% above 5 km] BC AAOD x100 Forcing efficiency (W m -2 /AAOD) Direct radiative forcing (W m -2 ) fuel+fires This work [8.7%] ( ) AeroCom [2006] 7.8 ± ± 0.08 [21±11%] 0.22± ± ± 0.07 Bond et al. [2013] Our best estimate of 0.19 W m -2 is much lower than IPCC recommendation of 0.65 ( ) W m -2 or the Bond et al. review IPCC value is from models that greatly overestimate BC in upper troposphere Wang et al., 2014 DRF = Emissions X Lifetime X Mass absorption coefficient X Forcing efficiency Global atmospheric load Absorbing aerosol optical depth (AAOD) Better understanding of BC scavenging is critical for radiative forcing estimates