Climate feedbacks on tropospheric ozone

David Stevenson

Institute of Atmospheric and Environmental ScienceUniversity of Edinburgh

F.J. Dentener, M.G. Schultz, K. Ellingsen, T.P.C. van Noije, O. Wild, G. Zeng, M. Amann, C.S. Atherton, N. Bell, D.J. Bergmann, I. Bey, T. Butler, J. Cofala, W.J. Collins, R.G. Derwent, R.M. Doherty, J. Drevet,

H.J. Eskes, A.M. Fiore, M. Gauss, D.A. Hauglustaine, L.W. Horowitz, I.S.A. Isaksen, M.C. Krol, J.-F. Lamarque, M.G. Lawrence, V. Montanaro, J.-F. Müller, G. Pitari, M.J. Prather, J.A. Pyle, S. Rast,

J.M. Rodriguez, M.G. Sanderson, N.H. Savage, D.T. Shindell, S.E. Strahan, K. Sudo, and S. Szopa

Introduction

• Tropospheric O3 is the no.3 GHG

• Closely coupled to OH and CH4 lifetime

• Ground-level O3 is a major air pollutant

• Most studies of future O3 focus on emissions trends (NOx, CO, VOCs etc.)

• BUT climate feedbacks may also be important

ACCENT Intercomparison

• Target IPCC-AR4• 25 models participated• Simulations:

– Year 2000 (reference or base year)– Three year 2030 scenarios:

• IIASA CLE (medium)• IIASA MFR (low)• IPCC SRES A2 (high)

– Plus one climate change case:• 2030 CLE + prescribed 2030 climate• (Performed by nine models)

ACCENT: ‘Atmospheric Composition Change: the European Network of Excellence’

Year 2000 Annual Zonal Mean Ozone (24 models)

Year 2000Ensemble meanof 25 models

AnnualZonalMean

Annual TroposphericColumn

Year 2000Inter-modelstandard deviation (%)

AnnualZonalMean

Annual TroposphericColumn

Comparison of ensemble mean model with O3 sonde measurements

J F M A M J J A S O N D

Observed ±1SD

Model ±1SD

90-30°S 30°S-Eq 30°N-Eq 90-30°N

UT250 hPa

MT500hPa

LT750hPa

2030 CLE - 2000 2030 MRF - 2000 2030 A2 - 2000

+5 ppbv -5 ppbv +10 ppbv

-30

-20

-10

0

10

20

30

40

50

60

70

CLE MFR A2

CLE +ΔClimate

Change in tropospheric O3 burden (2000-2030)Δ

O3 /

Tg

(O3)

Climate impact on tropospheric O3 budget

-150

-100

-50

0

50

100

150

200

ΔP ΔL Δ(P-L) ΔD ΔSinf

CHASER_GCM

LMDzINCAc

NCAR

STOCHEM_HadAM3

UM_CAM

MOZECH

STOCHEM_HadGEMLoss increasesby more than

productionStratospheric

influx increases

Impact of Climate Change on Ozone by 2030(ensemble of 9 models)

MeanMean - 1SD Mean + 1SD

Negative watervapour feedback

Positive stratospheric

influx feedback

Positive and negative feedbacks – no clear consensus

90S Eq 90N

Tro

po

sp

her

ic H

2O

co

lum

n /

g(H

2O

) m

-2

Tropospheric water vapour in 6 GCMs

Differences of± 10% in tropics

Conclusions• Two important feedbacks of climate on tropospheric ozone:

– Negative feedback due to water vapour, via the ozone loss process:O3 + hν → O(1D) + O2

O(1D) + H2O → 2OH(also leads to a negative feedback on CH4)

– Positive feedback due to an increase in the stratospheric influx of O3, mainly due to enhanced Brewer-Dobson circulation, but also possibly because LS O3 increases.

• Models show no consensus on which process dominates• Need to reduce uncertainties in modelling water vapour and STE of

O3 to further constrain these feedbacks• Feedbacks on lightning and isoprene emissions appear less

important globally• There are other potential feedbacks not yet analysed, e.g. wetland

CH4, biomass burning emissions…

Radiative forcing implications

-500

0

500

1000

1500

mW

/ m

2

CO2 795 795 1035

CH4 116 0 141

O3 63 -43 155

CLE MRF A2

Forcings (mW m-2) 2000-2030 for the 3 scenarios:

-23% +37%

CO2

CH4

O3