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Climate response to the increase in tropospheric ozone since preindustrial times: signature of ozone forcing

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Climate response to the increase in tropospheric ozone since preindustrial times: signature of ozone forcing Loretta J. Mickley, Daniel J. Jacob, Brendan D. Field – PowerPoint PPT presentation

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Title: Climate response to the increase in tropospheric ozone since preindustrial times: signature of ozone forcing


1
Climate response to the increase in tropospheric
ozone since preindustrial times signature of
ozone forcing
  • Loretta J. Mickley, Daniel J. Jacob, Brendan D.
    Field
  • Harvard University
  • David Rind, GISS

2
Inhomogeneity of tropospheric ozone change and
radiative forcing what is the effect on climate?
JJA Ozone column change
JJA Ozone radiative forcing
Results from GISS GCM 2 with embedded
photochemistry Annual mean DF 0.49 W
m-2 Forcing in longwave and shortwave
3
Main Questions
  • Does the inhomogeneity of tropospheric ozone
    forcing matter?
  • Is the climate response wavelength-dependent?
  • How does the response to Dozone differ from
    response to DCO2?
  • Approach
  • Perform GISS GCM II equilibrium climate
    simulations.
  • Feed in monthly mean preindustrial and
    present-day ozone fields.
  • Keep well-mixed GHGs, aerosol, stratospheric
    ozone constant.
  • Also, compare response to DCO2 with similar
    forcing.

4
GCM equilibrium simulation for present-day
climate with present-day vs. preindustrial
tropospheric ozone
equilibrium climate
DF 0.49 W m-2
Present-day ozone
DT 0.3oC
Preindustrial ozone
Mickley et al., 2003
5
Inhomogeneity of climate response to
tropospheric ozone change over 20th century
  • Greater warming in northern hemisphere (due to
    more ozone and albedo feedback in Arctic)
  • Strong cooling in stratosphere (gt1oC in Arctic
    winter)

Global
NH
Stratospheric ozone
Tropospheric ozone
SH
9.6 mm
Surface
6
Comparison with simulation with uniform 18-ppb
increase of tropospheric ozone
18-ppb average increase globally since
preindustrial times in troposphere
Realistic ozone increase
Uniform ozone increase
Increasing ozone uniformly results in small
interhemispheric temperature difference (0.03 oC)
7
Temperature response to ozone increase compared
to 25-ppm increase in CO2
25-ppm DCO2 corresponds to DF 0.47 W m-2 DF
for ozone
25-ppm DCO2
Realistic DO3
CO2 more effective global warmer than
tropospheric ozone per unit forcing. Global
average obscures regional sensitivities!
Uniform DO3
8
Temperature cools in lower stratosphere due to
increase in tropospheric ozone
DCO2
Strong cooling at high northern latitudes in
winter stratosphere remote effect.
Controlling tropospheric ozone could hasten
recovery of stratospheric ozone. (an unexpected
benefit!)
DO3 realistic
DO3 uniform
DJF, 100 hPa
9
Vertical variation of radiative forcing DO3
DO3unif DCO2
O3 longwave forcing increases with altitude
O3 absorbs incident reflected uv
SW
CO2 forcing saturates
LW
Water vapor diminishes LW forcing
O3 diminishes downward uv flux
10
CO2 exhibits stronger total forcing in
mid-troposphere
Total forcings match
DF CO2 gt DF Ozone in mid-troposphere
Surface temperature responds to forcing
throughout troposphere
11
Seasonal variation of temperature changes
Surface temperature
DCO2 temperature change largest in NH winter due
to albedo feedback
CO2
O3
O3unif
Integrated trop temperature
DO3 integrated temperature change largest in NH
summer due to strong forcing and vertical mixing
CO2
O3
O3unif
12
GCM surface warming patterns from increasing
tropospheric ozone over 20th century JJA
Warming over interior NA increased O3 in mid-
to lower trop reduced static stability in lower
trop reduced cloud cover precipitation
Strong warming downwind of ozone source regions
White areas insignificant values
Realistic DO3
DT
13
GCM surface warming patterns from increasing
tropospheric ozone over 20th century JJA
D Realistic O3
Equivalent DCO2
Difference DO3 DCO2
DCO2 shows stronger warming over dry Sahara DO3
yields stronger warming over Arctic and
midlatitude continents White areas
insignificant values
14
Conclusions
  • Limitations of radiative forcing as a yardstick
    to gauge the relative importance of a greenhouse
    gas
  • Need to include monthly mean tropospheric ozone
    fields in climate models
  • Remote impact of increasing tropospheric ozone on
    stratospheric ozone recovery over coming decades

Mickley et al., 2003
15
Extra slides
16
Water vapor diminishes CO2 forcing over low
latitudes
Difference DF O3unif DF CO2 mostly due to
water vapor interference with CO2 DF.
For same global forcing, CO2 forcing shifted
toward poles, where climate is more sensitive.
17
Variation of forcing with altitude DO3unif, DCO2
Experimental forcings normalized at tropopause
SW
DCO2 forcing with water vapor removed from
calculation, no DCO2 in stratosphere
LW
DO3 uniform forcing with 1000 x O3 concentrations
18
Cloud cover changes amplify surface temperature
response
Increased cloud cover in upper troposphere warms,
especially for DCO2
Decreased cloud cover in lower troposphere warms
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