1. Climatic implications of changes in O 3 Loretta J. Mickley, Daniel J. Jacob Harvard University David Rind Goddard Institute for Space Studies How well do we know radiative forcing due to tropospheric O 3 change? What is the temperature response to changing O 3 ? How useful is radiative forcing as a measure of temperature response?

2. How well do we know the forcing due to changing O 3 ? IPCC 2001

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

4. Attempt to model preindustrial O 3, Harvard model model observations Uncertainties: Lightning NO emissions + soil NO emissions (O 3 source) emissions of biogenic hydrocarbons (O 3 sink in low-NOx atmosphere)

5. Harvard-GISS General Circulation Model GISS GCM II’ meteorology 9 sigma layers, 4 o by 5 o horizontal grid 24 chemical tracers Detailed O 3 -NO x -hydrocarbon chemistry 80 chemical species, > 400 chemical reactions Preindustrial atmosphere: No fossil fuel combustion, 10% present-day biomass burning Approach to predindustrial O 3 problem: construct a test simulation, adjusting the natural emissions within (or not far from) uncertainties.

6. Uncertainties in natural emissions Biogenic emissions Isoprene 200-600 Tg C y -1 825 Tg y -1 Monoterpenes 130? Tg C y -1 200 Tg y -1 Lightning NO 1-20 Tg N y -1 1.0 Tg y -1 Soil NO 4-13 Tg N y -1 2.0 Tg y -1 Adjusted simulation

7. Uncertainty of radiative forcing due to O 3 is quite large. Test simulation with Decreased lightning NOx and soil NOx emissions and Increased biogenic emissions:  F = 0.80 Wm -2 (about 1/2  F of CO 2 ) Standard simulation,  F = 0.44 Wm -2

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

9. Response of surface temperature to tropospheric O 3 change Temperature diverges for the two simulations  T = ~0.3 o C

10. Response of temperature throughout atmosphere to changing O 3 50-year averages Stronger temperature response in NH Temperature increases throughout troposphere, but decreases in stratosphere due to decreased upward flux in 9.6  band.

11. Temperature response to uniform O 3 change Changing O 3 everywhere by 18 ppb leads to smaller interhemispheric temperature differences.

12. Response of temperature to changing CO 2 Temperature increase at surface due to CO 2 is almost 0.4 o C, compared to 0.3 o C for O 3. In lower stratosphere, CO 2 change leads to an increase in temperature, while O 3 change leads to a decrease.

13. How do forcings of CO 2 and uniform O 3 compare? Difference plot, CO 2 – uniform O 3 CO 2 shows weaker forcing over tropics, but stronger forcing over poles, where climate is more responsive due to positive albedo feedback.

14. Water vapor limits CO 2 radiative forcing over tropics Longwave forcing from change in CO 2 correlates with specific humidity at 500 mb over tropics. R 2 O 3 = 0.07 R 2 CO 2 = 0.69 Water vapor absorbs strongly at CO 2 wavelengths, swamping the effect of CO 2. Same globally avgd forcing leads to stronger forcing at poles for CO 2.

15. Clouds also limit longwave CO 2 forcing CO 2 LW forcing correlates even better with high cloud cover over tropics. R 2 O 3 = 0.13 R 2 CO 2 = 0.86

16. How do forcings of CO 2 and calculated O 3 compare? CO 2 forcing is more uniform and stronger in SH. O 3 forcing at high Northern latitudes is mostly shortwave forcing due to Arctic snow and ice, together with high O 3 concentrations. SW forcing

17. Increased O 3 leads to large albedo decrease over Arctic Surface temperature increases over Arctic for both CO 2 and calculated O 3 change. Large albedo decrease over Arctic = negative feedback on O 3 forcing Snow+ice melt, albedo decreases, smaller shortwave forcing, smaller than expected temperature change

18. Conclusions Uncertainty in forcing due to tropospheric ozone added to the atmosphere since preindustrial times is larger than usually thought.  F may be as much as 0.8 Wm -2. Globally averaged radiative forcing of changing tropospheric O 3 may have limited value as indicator of climate change. Reasons for apparent smaller sensitivity in O 3 case: Water vapor and clouds limit CO 2 forcing over tropics, so for the same globally averaged  F, CO 2 forcing is larger over poles. Shortwave forcing due to O 3 change at high latitudes is reduced due to a negative albedo feedback.

19. Extra slides

20. 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.

21. Contribution of lightning NOx to surface O 3 Second test simulation: Decrease in lightning NOx only Lightning from tropics influences surface ozone at mid-latitudes in winter. Only lightning NOx decreased

22. Surface distribution of present-day O 3 in July (ppb) Greater concentrations over industrial areas and regions of biomass burning.

23. What is the effect of shortwave forcing due to O 3 ? LW + SW – 0.49 Wm -2 LW only – 0.37 Wm -2 SW forcings for calculated O 3 strongest over Arctic due to ice, snow, and high O 3 concentrations.

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