1. Radiative Forcing of Ozone

2. 1964-2003 trend is -9 Dobson Units
Combination of measurements from satellites (SBUV) and ground (Dobson Spectrometer)
trend is -9 Dobson Units
Sum of tropospheric enhancement and stratospheric depletion
(WMO 2006)

3. Stratospheric ozone depletion
Antarctic ozone
(WMO 2006)
Anthropogenic CFC emissions cause Antarctica stratopheric ozone depletion
40%-50% of the Antarctic ozone layer destroyed
corresponds to a modeled global mean depletion between 14.1 and 28.6 Dobson Units

4. Tropospheric Ozone Changes
Percent Change since pre-industrial
Observations are too scarce to get a globally comprehensive value
Instead, chemical transport models, forced with ozone precursor emissions, are used
Results suggest a Dobson Unit increase in tropospheric ozone since 1850
(Gauss 2006)

5. Ozone Change and Radiative Forcing
Radiative forcing doesn’t scale linearly with concentration changes
Net ozone change and radiative forcing have different signs!

6. Radiative properties Ozone absorbs UV
This absorption is responsible for the thermal structure of atmosphere
Ozone is also an effective greenhouse gas in the atmospheric window

7. IPCC definition of radiative forcing
“The radiative forcing of the surface-troposphere system due to the perturbation in or the introduction of an agent (say, a change in greenhouse gas concentrations) is the change in net (down minus up) irradiance (solar plus long-wave; in Wm-2) at the tropopause AFTER allowing for stratospheric temperatures to readjust to radiative equilibrium, but with surface and tropospheric temperatures and state held fixed at the unperturbed values.”

8. Why this definition? Tropospheric Radiative Budget
COOLING
HEATING
Troposphere is not in Radiative Equilibruim

9. Why this definition?
Tropospheric Radiative Budget/Radiative-Convective Equilibrium
Dynamics (not Radiation) determines temperature profile
Net radiation change at the tropopause determines the temperature change of the troposphere as a whole

10. Ozone Radiative Heating
Tropopause
Shortwave Radiative Forcing (W/m2)
Relatively little UV is absorbed by ozone in the troposphere
But it has a huge climate impact

11. Stratospheric Ozone Radiative Forcing
Shortwave
Longwave
Total
Shortwave tropospheric heating that results from stratospheric ozone depletion partially offsets it’s longwave cooling
(Forster and Shine 1997)

12. Tropospheric ozone has increased a little
Conclusions
Tropospheric ozone has increased a little
Stratospheric ozone has decreased a lot
Ozone Radiative forcing is positive, reflecting a greater contribution from tropospheric changes
Stratospheric ozone depletion leads to shortwave tropospheric cooling that offsets its effect as a greenhouse gas
Thus stratospheric ozone radiative forcing is the sum of shortwave warming and longwave cooling and is thus fairly neutral