1. Global Change Welcome Meeting, Edinburgh, October 15th 2010
Short-lived Climate Forcers: Let’s not forget Earth’s other heating control knobs
David Stevenson
Global Change Welcome Meeting, Edinburgh, October 15th 2010

2. Quick outline Radiative forcing of climate change
Short-lived climate forcers: SLCFs
Gases: CH4, O3
Aerosols: SO4, BC, etc.
Contribution towards climate change since 1750
Implications for climate sensitivity
Rapid influence on future radiative forcing
Recent changes in SLCFs – can we control them?
Problems
Cleaning up some air pollutants exacerbates warming
Poorly known processes – high uncertainties
Mustn’t take our eyes off CO2

3. Forster et al. (2007) IPCC-AR4 WG1 Chapter 2

4. Radiative Forcing Earth’s energy budget (in balance):
If there is a ‘radiative forcing’ (RF), the budget is out of balance until the climate system responds so as to restore balance.
Equilibrium global mean surface temperature response (ΔTs) is:
ΔTs = λ RF where λ is the climate sensitivity, units °C/(W m-2)
NB Climate sensitivity sometimes defined as the temperature change for a
2xCO2 radiative forcing; current best estimate is ~3 °C

5. SLCFs Specifying the time period is crucial.
NB the diagram says nothing about the time evolution of each RF, which may be complex
SLCFs
Ice cores + direct observations
Models + limited observations
Large RF uncertainties → climate sensitivity uncertain
Forster et al. (2007) IPCC-AR4 WG1 Chapter 2

6. ~1°C less warming by 2050
Red = 5°C climate sensitivity
Blue = 2°C climate sensitivity
Top curves follow A2 scenario
Lower curves linearly reduce
CH4, tropospheric O3 and BC
from 2010 to PI levels in 2050
If we know RFs and T accurately enough, we can constrain climate sensitivity
Penner et al., 2010, Nature Geoscience

7. Recent trends and efforts to control SLCFs
Emissions control efforts so far have been mainly motivated by ‘clean air act’ type legislation – i.e. driven by concerns for air quality and human health, not climate
Sulphate aerosol
Black Carbon aerosol
Ozone (tropospheric/stratospheric)
Methane

8. UK SO2 emissions have fallen dramatically…
(RoTAP, under review)

9. Global SO2 emissions also now falling:
Measured/modelled SO4 deposition to Greenland (but this only reflects regional emissions)
Global SO2 emissions also now falling:
Suggests we can simulate SO4 deposition to Greenland reasonably well…
So the negative RF from sulphate aerosols is reducing, i.e. a warming influence on global climate since ~1980.
(Lamarque et al., 2010)

10. Effect of removing the entire burden of sulphate aerosols in the year 2000 on temporal evolution of global and annual mean surface air temperature anomalies (°C).
Figure 7.24 IPCC WG1 AR-4 (from Brasseur & Roeckner, 2005)

11. Global BC emissions rising:
Measured/modelled BC deposition to Greenland (but this only reflects regional emissions)
Global BC emissions rising:
Global BC emissions are rising.
But Greenland BC is declining – due to reductions in European/N. American emissions.
Suggests BC emissions controls have an effect.
(Lamarque et al., 2010)

12. Contributors to Arctic Black Carbon
Europe dominates in the Arctic – so European BC emissions controls could be effective
Multi-model results:
% contributions of regional emissions to surface BC concentrations
Shindell et al. (2008)

13. Methane
Methane had stabilized, but is rising again. Is this humans or nature?
NB decadal lifetime – significant inertia in response
Anthropogenic Emissions
About ½ sources of CH4 are anthropogenic. Wetlands largest natural source.
Its sink (OH) also influenced by anthropogenic emissions (esp. NOx and CO).
Lack of a clear explanation of past variations make it difficult to know how easily controllable CH4 is.

14. Tropospheric ozone
Europe has reduced O3 precursor emissions by ~30% in last 20 years
Royal Society, 2008

15. Surface ozone measurements: peak O3 falling – but background O3 rising…
SE England
Remote NH sites
West coast Ireland
West coast USA
Royal Society, 2008
Parrish et al 2009

16. O3 trends partly explained by models
Observations
But quite large discrepancies between models and observations
i.e. we don’t fully understand what controls ozone
Lamarque et al., 2010

17. Can we control SLCFs?
Strongest emissions controls so far have been SO2, reducing SO4 aerosol – but this warms!
Black carbon aerosol has also responded to regional air quality emissions controls
Regional O3 precursor emissions controls have not stopped background O3 rising
Methane (a Kyoto gas) appears to be rising despite apparent decreases in anthropogenic emissions
SO2 and BC dominated by anthropogenic sources
CH4 and O3 have significant natural sources, and complex chemistry

18. Conclusions
SLCFs are large contributors to radiative forcing since 1750
Large uncertainties in their RFs feed into estimates of climate sensitivity, and hence magnitude/speed of future climate change
Reductions in CH4, O3 and BC could significantly reduce near-term warming (but CH4 & O3 currently rising)
Reductions in SO4 have already significantly increased warming – further reductions will exacerbate GHG warming
Many of these emissions changes driven by air quality (not climate) policies
Feasibility and effectiveness of emissions reductions requires a deeper understanding of processes that control SLCFs
CO2 controls still urgently required