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

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

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

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

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

~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

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

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

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)

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)

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)

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)

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.

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

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

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

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

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