AR5: main findings & knowledge gaps
on Short-Lived Climate Forcers andtheir Radiative Forcing
Olivier Boucher
Institut Pierre-Simon Laplace, Paris, FranceSLCF
Some gas
Some particulate
Short-lived (<>CH4)
Often co-emitted
Emissions could
decrease in the future
In part driven by air
quality measures or asa by-product of GHG
mitigation

Myhre et al., Chapter 8, AR5, 2013

Myhre et al., Chapter 8, AR5, 2013

Radiative forcing ofclimate changeby emitted species=> Some cool=> Some warm=> Large uncertainties

Myhre et al., Chapter 8,AR5, 2013

=> uncertainties are large, here for ERFaci…

Boucher et al., Chapter 7, AR5, 2013

AR5 expert
judgement

AEROCOM IImodels

Boucher et al.,
Chapter 7,AR5, 2013

AAOD from AERONET- in principle valid for AOD>0.4- but used for all AOD values

Bond et al., JGR, 2013

Yet roughly 15% of AOD is more than 0.4
which corresponds to <10% of BC rad. effects• What should be the role of SLCF in
mitigating climate change?
• Is it possible and desirable to decrease
emissions of warming species while
keeping emissions of cooling species thesame?
• What are the co-benefits of decreasing
emissions of SLCF?
• How well do we know emissions,concentrations and radiative forcings ofSLCF?

Black Carbon

Aviation-induced cloudiness

Kärcher, Nature Comms, 2018

IPSL-CM5A-LR
climate model
No anthropogenic
aerosols
Anthropogenic
aerosols fixed at2005 values

Cooling due toanthropogenic
aerosols inhistorical period

historical

RCP45

Warming due todecreasing
aerosols in
RCP45

Reconstructionof NH and SH Tchange

NHSHwith a large TCRof 2.5 K and alarge (negative)aerosol ERF of -2
Wm-2
with a small TCRof 1 K and small(negative)
aerosol ERF of -1
Wm-2NHSH A factor of 7reduction

 Clear environmental and health benefits (eg, Sofiev et al,
Nature Comms, 2017) but what climate effects?

Malavelle et al., Nature, 2017

Scaling theMalavelle et al.
results to shippingemissions…

2 Tg SO2 mth-1 emissions from Holuhraun => - 0.21 Wm-2 RF in Sept-Oct 2014

100 Tg SO2 yr-1 global emissions => - 0.9 Wm-2 RF from sulphate ACI
Reducing shipping emissions of 10 Tg SO2 yr-1 by a factor 7 =>- 0.15 Wm-2 => 0.15 K cooling within 10 years(with a factor of 2 enhancement because of the spatio-temporal distribution)

Conclusions• SLCF offer in principle some climate
mitigation opportunities but …
– only few SLCFs have warming effects andmay not be so easy to mitigate– other species are often co-emitted– there are trade-offs involving climate warming– uncertainties are large• In any case monitoring of SLCFs isneeded hence knowing emissions andtheir evolution is paramountEND
https://atmosphere.copernicus.eu/services/climate-forcing

Kärcher, Nature Comms, 2018

Boucher et al., Chapter 7, AR5, 2013

Executive Summary – Clouds and Aerosols
MACC AOD• Climaterelevant
aerosol
processes are
betterunderstood,
and climaterelevant
aerosol
properties
betterobserved, thanat the time ofAR4.
CALIPSO

Executive Summary – Clouds and Aerosols• Climate-relevant aerosol processes are better understood,
and climate-relevant aerosol properties better observed,
than at the time of AR4. However, the representation ofrelevant processes varies greatly in global aerosol and climate
models and it remains unclear what level of sophistication isrequired to model their effect on climate. Globally, between 20
and 40% of aerosol optical depth (medium confidence) andbetween one quarter and two thirds of cloud condensation
nucleus concentrations (low confidence) are of anthropogenicorigin.

Executive Summary – Clouds and Aerosols• Climate-relevant aerosol processes are better understood,
and climate-relevant aerosol properties better observed,
than at the time of AR4. However, the representation ofrelevant processes varies greatly in global aerosol and climate
models and it remains unclear what level of sophistication isrequired to model their effect on climate. Globally, between 20
and 40% of aerosol optical depth (medium confidence) andbetween one quarter and two thirds of cloud condensation
nucleus concentrations (low confidence) are of anthropogenic
origin.
No statement on the anthropogenic fraction of AAOD
No statement on the anthropogenic fraction of ice nuclei

Need forhigher
resolution

R. Wang(Beijing University),
Balkanski & Boucher