1. Climatic implications of changes in O3
How well do we know radiative forcing due to tropospheric O3 change?
What is the temperature response to changing O3?
How useful is radiative forcing as a measure of temperature response?
Loretta J. Mickley, Daniel J. Jacob
Harvard University
David Rind
Goddard Institute for Space Studies

2. How well do we know the forcing due to changing O3?
IPCC 2001

3. Photochemical models tend to overpredict preindustrial O3
Marenco et al., 1994
Preindustrial
ozone models }

4. Attempt to model preindustrial O3, Harvard model
observations
Uncertainties: Lightning NO emissions + soil NO emissions (O3 source) emissions of biogenic hydrocarbons (O3 sink in low-NOx atmosphere)

5. Harvard-GISS General Circulation Model
GISS GCM II’ meteorology
9 sigma layers, 4o by 5o horizontal grid
24 chemical tracers
Detailed O3-NOx-hydrocarbon chemistry
80 chemical species, > 400 chemical reactions
Preindustrial atmosphere:
No fossil fuel combustion, 10% present-day biomass burning
Approach to predindustrial O3 problem: construct a test simulation, adjusting the natural emissions within (or not far from) uncertainties.

6. Uncertainties in natural emissions
Biogenic emissions
Isoprene Tg C y Tg y-1
Monoterpenes 130? Tg C y Tg y-1
Lightning NO Tg N y Tg y-1
Soil NO Tg N y Tg y-1
Adjusted simulation

7. Uncertainty of radiative forcing due to O3 is quite large.
Test simulation with
Decreased lightning NOx and soil NOx emissions and
Increased biogenic emissions:
DF = 0.80 Wm-2 (about 1/2 DF of CO2)
Standard simulation,
DF = 0.44 Wm-2

8. How has the change in O3 affected climate?
3 pairs of simulations with climate model
O3 fields allowed to influence meteorology
75 years each, “qflux ocean” (SST allowed to change)
1. Calculated O3 : Present-day O3 vs Preindustrial O3
2. Does the inhomogeneity of O3 change matter?
Preindustrial with18 ppb increase O3 everywhere vs Preindustrial O3
(18 ppb = globally averaged increase since preindustrial times)
3. Is the climate more sensitive to O3 change than to CO2?:
Control run vs Control run with 25 ppm decrease in CO2
(25 ppm yields same globally averaged forcing as change in O3.)

9. Response of surface temperature to tropospheric O3 change
Temperature diverges for the two simulations
DT = ~0.3o C

10. Response of temperature throughout atmosphere to changing O3
50-year averages
Stronger temperature response in NH
Temperature increases throughout troposphere, but decreases in stratosphere due to decreased upward flux in 9.6m band.

11. Temperature response to uniform O3 change
Changing O3 everywhere by 18 ppb leads to smaller interhemispheric temperature differences.

12. Response of temperature to changing CO2
Temperature increase at surface due to CO2 is almost 0.4o C, compared to 0.3o C for O3.
In lower stratosphere, CO2 change leads to an increase in temperature, while O3 change leads to a decrease.

13. How do forcings of CO2 and uniform O3 compare?
Difference plot, CO2 – uniform O3
CO2 shows weaker forcing over tropics, but stronger forcing over poles, where climate is more responsive due to positive albedo feedback.

14. Water vapor limits CO2 radiative forcing over tropics
Longwave forcing from change in CO2 correlates with specific humidity at 500 mb over tropics.
R2 O3 = 0.07
R2 CO2 = 0.69
Water vapor absorbs strongly at CO2 wavelengths, swamping the effect of CO2.
Same globally avgd forcing leads to stronger stronger forcing at poles for CO2.

15. Clouds also limit longwave CO2 forcing
CO2 LW forcing correlates even better with high cloud cover over tropics.
R2 O3 = 0.13
R2 CO2 = 0.86

16. How do forcings of CO2 and calculated O3 compare?
SW forcing
CO2 forcing is more uniform and stronger in SH.
O3 forcing at high Northern latitudes is mostly shortwave forcing due to Arctic snow and ice, together with high O3 concentrations.

17. Increased O3 leads to large albedo decrease over Arctic
Surface temperature increases over Arctic for both CO2 and calculated O3 change.
Large albedo decrease over Arctic = negative feedback on O3 forcing
Snow+ice melt, albedo decreases, smaller shortwave forcing, smaller than expected temperature change

18. How does albedo change affect O3 forcing?
Calculate forcing using:
1. present-day O3 + albedo of snow and ice decreased by 2%
2. preindustrial O3 + usual albedo
DF = 0.37 Wm-2
Change in albedo counteracts increase in tropospheric O3 at high latitudes.

19. Conclusions
Uncertainty in forcing due to tropospheric ozone added to the atmosphere since preindustrial times is larger than usually thought.
DF may be as much as 0.8 Wm-2 .
Globally averaged radiative forcing of changing tropospheric O3 may have limited value as indicator of climate change.
Reasons for apparent smaller sensitivity in O3 case:
Water vapor and clouds limit CO2 forcing over tropics, so for the same globally averaged DF, CO2 forcing is larger over poles.
Shortwave forcing due to O3 change at high latitudes is reduced due to a negative albedo feedback.

20. Extra slides

21. Relationship of isoprene and OH
In low-NOx preindustrial atmosphere, OH is depleted at surface, and isoprene builds.
Isoprene then becomes a sink of ozone.

22. Contribution of lightning NOx to surface O3
Second test simulation:
Decrease in lightning NOx only
Lightning from tropics influences surface ozone at mid-latitudes in winter.
Only lightning NOx decreased

23. Surface distribution of present-day O3 in July (ppb)
Greater concentrations over industrial areas and regions of biomass burning.

24. What is the effect of shortwave forcing due to O3?
LW + SW – 0.49 Wm-2
LW only – 0.37 Wm-2
SW forcings for calculated O3 strongest over Arctic due to ice, snow, and high O3 concentrations.

25. Response of temperature to 2xCO2
Stronger temperature response in NH due to stronger positive albedo effect.
Warmer temperature, less ice+snow cover, more incoming sunlight absorbed, still warmer tempertures