Radiative Forcing of Ozone

Combination of measurements from satellites (SBUV) and ground (Dobson Spectrometer)

1964-2003 trend is -9 Dobson

Units

Sum of tropospheric enhancement and stratospheric depletion

(WMO 2006)

Stratospheric ozone depletion

•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

Antarctic ozone

(WMO 2006)

Tropospheric Ozone Changes

•Observations are too scarce to get a globally comprehensive value

•Instead, chemical transport models, forced with ozone precursor emissions, are used

•Results suggest a 7.9 -13.8 Dobson Unit increase in tropospheric ozone since 1850

Percent Change since pre-industrial

(Gauss 2006)

Ozone Change and Radiative Forcing

• Radiative forcing doesn’t scale linearly with concentration changes

• Net ozone change and radiative forcing have different signs!

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

IPCC definition of radiative forcing

• IPCC definition“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.”

Why this definition?Tropospheric Radiative Budget

HEATING

COOLING

•Troposphere is not in Radiative Equilibruim

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

Ozone Radiative Heating

• Relatively little UV is absorbed by ozone in the troposphere• But it has a huge climate impact

Tropopause

Shortwave Radiative Forcing (W/m2) 5 10 15 20 25

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)

• 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

Conclusions