Short-Lived Climate Forcers
and Climate Metrics
Bill Collins
Met Office Hadley Centre
© Crown copyright Met Office
Outline
• Motivation from Air Quality policies
• Radiative forcing of short-lived climate forcers
• Direct & indirect
• Climate metrics
• Timescales
• Regional metrics
• Summary
© Crown copyright Met Office
Short-lived climate forcers
UNEP O3 & BC report
• CO2 measures are
needed now to get
post 2050 benefits
• CO2 measures alone
don’t keep us < 2°
• Co-emitted SO2
• => SLCF measures
• 0.4° cooling
• CH4, CO, VOCs
• BC (offset by OC)
IEA 450 (not CH4)
Trade-offs:
Radiativve forcing over the EMEP domain
Health impacts vs. radiative forcing vs. mitigation costs
Pareto-optimal solutions
for health vs costs
Paretooptimal solutions
for health vs.
vs RF
Health impacts (years of life lost)
Baseline 2020
Ai Q
Air
Quality
lit P
Policy
li
Radiative forcing from aerosols over the EMEP region
(I iti l results
(Initial
lt ffrom IIASA/MSC
IIASA/MSC-W/U.Oslo
W/U O l cooperation)
ti )
0.4
02
0.2
0.0
W/m2
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
2000
2020 CLE
BC
2020 MFR
OC
SO4
Minimum achievable
forcing in 2020
Sum
•Can try to optimise control measures for both air quality and climate
•Need some metric of the climate impacts of air quality pollutants
Di t and
Direct
d indirect
i di t effects
ff t
Impact of short-lived
short lived climate forcers
(SLCFs)
Emissions
CH4
Atmospheric Processes
Climate Forcing Species
Additional Feedbacks
Additional Climate Forcing
NOX
CO
VOC
NH3
SO2
OCaer
BCaer
Atmospheric Chemistry & Microphysics
ΔCH4
ΔO3
S + N + SOA + OC + BC aerosols
Biosphere Interactions
ΔCH4
ΔCO2
ΔN2O
Cloud Interactions
Δclouds
From Michael Prather
Indirect climate effects
• Aerosols have indirect effects via cloud microphysics
• Precursor pollutants have indirect radiative forcings through chemical reactions
• NOX, VOCs,
VOCs CO and CH4 all produce ozone
• NOX increases oxidising capacity, CO, VOCs and CH4 reduce it
• Removal of methane
• Formation of sulphate aerosol
• NOX, SO2 and NH3 produce nitrate and sulphate aerosols
• Some VOCs can p
produce OC aerosols
© Crown copyright Met Office
Indirect effects via biosphere
p
• O3 poisons plants
• Pl
Plants
t ttake
k less
l
CO2 outt off the
th
atmosphere
• Extra climate effect
ff
comparable to direct O3
forcing
Sitch et al. 2009; Collins et al. 2010
Cli t M
Climate
Metrics
ti
Climate metrics
• Metrics quantify a climate impact of an emitted species
• Usually
y based on pulse
p
emission of 1kg
g and normalised by
y CO2
GWP: RF integrated out to time horizon H
• ~∝
∝ 1/H for short
short-lived
lived species
GTP: Change in Tsurf at time H
• Depends on timescale of climate response
• Short timescale (<10 years), long timescale (100s of years)
• SLCFs and CO2 act on very different timescales
• No metric can give a general CO2 equivalence – needs to be for a
desired policy-relevant quantity
Timescales
•
1mWm-2yr
pulses
(1.6×1019J)
V. short-lived
GWP
12
12-yr
lif ti
lifetime
• Different lengths,
g , but equal
q
areas
• v.short-lived (O3, aerosol)
ΔT
• 12-year
12 year lifetime (methane)
• GTP is a stronger function
of time than GWP
• 2 tracers have very different
GTP20, but
b t similar
i il GTP100
• Scale these patterns by the
CH4, O3 & aerosol forcings
GTP
Reactive gases
g
Used mean forcing from 11 HTAP models (Fry et al. in prep)
GTPs
• Climate metrics for SLCFs can depend on emission regions
• 4 continents (HTAP), E. Asia, Europe, N. America, S. Asia.
• South Asian emissions have largest effect (NOX, VOCs)
GT
TP20
• For NOX, European emissions have least effect
GTP100
C 4
CH
NOX
VOC
CO
Aerosols
• Direct effects only
• Show less regional dependence than ozone precursors
Using
g metrics
• Pulse emissions aren’t meant to be a realistic
scenario
• Response to emission pulse gives Greens
function G(t)
• Response to any emission profile E(t) given by
• ΔT=∫
ΔT ∫0tt’E(t)G(t’-t)dt
E(t)G(t’ t)dt
• Some possible forms for E(t) for AQ control
• Step function – instantaneous change that is
maintained
• Ramp – emission change increases linearly with time.
BC
• Different time constants
8.4 yr
6.8 yr
4 0 yr
4.0
Pulse
• ->Different GTPs for “BC”
• Factor of 3 in GTP20
• So can we say anything useful to
policy makers?
• However responses to step or
ramp in emissions are robust
GTP
CH4
Step
• ->Can say something useful
Ramp
Regional
g
climate metrics
NOX reduction in E. Asia
Radiative forcing
• Forcings are regional
• Zero net forcing doesn’t
mean no regional
temperature change
• NOX emissions cool
globally, but warm northern
mid-high latitudes
• Even more model
dependent & uncertain!
• Shindell & Faluvegi 2010
ARGTP100 (uK
A
K/Tg)
• Absolute Regional
Temperature-change
Potentials
NOx
0.8
0.6
0.4
0.2
0
2
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
S. Hem
Tropics
N. Mid
Region
-lat
Arctic
Global
Summaryy
• Short-lived species have important climate effect
• Chemistry and aerosol models → climate metrics
• Multi-model analysis shows significant spread
• Climate metrics can depend on the emission region
• Regional temperature impacts differ from the global mean
• Starting to have tools to assess (& optimise?) the climate
impacts of air quality policies