1. Climate feedbacks on tropospheric ozone
David Stevenson
Institute of Atmospheric and Environmental Science
University 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

2. 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

3. 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’

4. Year 2000 Annual Zonal Mean Ozone (24 models)

5. Year 2000
Ensemble mean of 25 models
Annual Zonal Mean
Annual Tropospheric Column

6. Year 2000
Inter-model
standard deviation (%)
Annual Zonal Mean
Annual Tropospheric Column

7. Comparison of ensemble mean model with O3 sonde measurements
UT 250 hPa
Model ±1SD
Observed ±1SD
J F M A M J J A S O N D MT
500
hPa LT
750
hPa
90-30°S
30°S-Eq
30°N-Eq
90-30°N

8. 2030 CLE - 2000 2030 MRF - 2000 2030 A2 - 2000 +5 ppbv +10 ppbv

9. Change in tropospheric O3 burden (2000-2030)
CLE + ΔClimate
ΔO3 / Tg(O3)

10. Loss increases by more than production
Stratospheric
influx increases

11. Impact of Climate Change on Ozone by 2030 (ensemble of 9 models)
Positive stratospheric influx feedback
Negative water vapour feedback
Mean - 1SD
Mean
Mean + 1SD
Positive and negative feedbacks – no clear consensus

12. Tropospheric water vapour in 6 GCMs
Differences of
± 10% in tropics
Tropospheric H2O column / g(H2O) m-2
90S Eq N

13. (also leads to a negative feedback on CH4)
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…

15. Radiative forcing implications
Forcings (mW m-2) for the 3 scenarios:
-23%
+37%
CO2
CH4 O3