1. Chemistry-Climate Modelling: Impacts of climate change on tropospheric chemical composition
David Stevenson
Institute of Atmospheric and Environmental Science School of GeoSciences The University of Edinburgh
Thanks to:
Ruth Doherty (Univ. Edinburgh)
Dick Derwent (rdscientific)
Mike Sanderson, Colin Johnson, Bill Collins (Met Office)
Frank Dentener (JRC Ispra), Markus Amann (IIASA)

2. STOCHEM Global Lagrangian 3-D chemistry-climate model
Meteorology: HadAM3 + prescribed SSTs
GCM grid: 3.75° x 2.5° x 19 levels
CTM: 50,000 air parcels, 1 hour timestep
CTM output: 5° x 5° x 9 levels
Detailed tropospheric chemistry
CH4-CO-NOx-hydrocarbons
detailed oxidant photochemistry
Interactive lightning NOx, C5H8 from veg.
~1 year/day on 36 processors (Cray T3E)

3. Model experiments Two transient runs: 1990 → 2030 Driving meteorology
1. Fixed SSTs (mean of )
2. SSTs from a climate change scenario (is92a)
shows ~1K surface warming 1990s-2020s
New IIASA* global emissions scenario:
Business as usual (BAU) [aka CLE Current Legislation]
Stratospheric O3 is a fixed climatology
Vegetation (land-use) also a fixed climatology
*IIASA: International Institute for Applied Systems Analysis (Austria)

4. IIASA Emissions scenarios
Global totals – there are significant regional variations
Courtesy of Markus Amann (IIASA) & Frank Dentener (JRC)

5. Compare changes between the 1990s and 2020s
Model experiments
BAU, fixed SSTs
Compare changes between the 1990s and 2020s
BAU, is92a SSTs
1990
2030

6. Decadal mean values
1990s

7. BAU 2020s

8. BAU Change in surface O3, BAU 2020s-1990s +2 to 4 ppbv over
>+10 ppbv India
+2 to 4 ppbv over
N. Atlantic/Pacific
A large fraction is
due to ship NOx
Change in surface O3, BAU 2020s-1990s
BAU

9. BAU+climate change 2020s

10. MRF BAU BAU+cc Change in surface O3, BAUcc 2020s-1990s
Look at the difference between these
two to see influence of climate change
Change in surface O3, BAUcc 2020s-1990s
MRF
BAU
BAU+cc

11. destruction over the oceans
ΔO3 from climate change
Warmer temperatures &
higher humidities
increase O3
destruction over the oceans
But also a role
from increases
in isoprene
emissions from
vegetation?

12. Isoprene emissions

13. Lightning NOx

14. Zonal mean H2O increase 2020s (climate change – fixed climate)

15. Zonal mean C5H8 change 2020s (climate change – fixed climate)
Increased
emissions
from veg
as T
increases

16. Zonal mean lightning NOx change 2020s (climate change – fixed climate)
More convection
in N mid-lats?
Less, but higher,
tropical convection?
No overall trend
in Lightning NOx
emissions

17. Zonal mean PAN decrease 2020s (climate change – fixed climate)
Colder LS
Increased
PAN
thermal
decomposition,
due to
increased T

18. Zonal mean NOx change 2020s (climate change – fixed climate)
Less
tropical
convection
and
lightning?
Increased
N mid-lat
convection
and lightning?
Increased
PAN
decomposition

19. Zonal mean O3 NCP change 2020s (climate change – fixed climate)
Looks quite like
NOx +
H2O increase
C5H8 increase
Longer O3 lifetime in MT/UT means
reduced NCP
there dominates
globally.

20. Zonal mean O3 decrease 2020s (climate change – fixed climate)

21. Zonal mean OH change 2020s (climate change – fixed climate)
Complex
function:
F(H2O,
NOx,
O3,
T,…)

22. CH4, CH4 & OH trajectories 1990-2030
Current CH4 trend
looks like MRF –
coincidence?
All scenarios show increasing OH

23. Conclusions
Anthropogenic emissions will be the main determinant of future tropospheric O3
Ship NOx looks important
Climate change will introduce feedbacks that modify air quality