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
Inhomogeneity of tropospheric ozone change and radiative
forcing: what is the effect on climate?
JJA Ozone column change
Results from GISS GCM 2’ with
embedded photochemistry
Annual mean DF = 0.49 W m-2
JJA Ozone radiative forcing
9.6 mm
UV
Ozone
Forcing in longwave and shortwave:
snow, ice
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.
GCM equilibrium simulation for present-day climate with
present-day vs. preindustrial tropospheric ozone
equilibrium
climate
Present-day ozone
Preindustrial ozone
DF = 0.49 W m-2
DT = 0.3oC
Mickley et al., 2003
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)
Global
NH
Strong cooling in stratosphere
(>1oC in Arctic winter):
Stratospheric
ozone
SH
Tropospheric
ozone
9.6 mm
Surface
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)
Temperature response to ozone increase compared to
25-ppm increase in CO2
25-ppm DCO2
Realistic DO3
Uniform DO3
25-ppm DCO2 corresponds to
DF =0.47 W m-2 ~ DF for ozone
CO2 more effective “global
warmer” than
tropospheric ozone per
unit forcing.
Global average obscures
regional sensitivities!
Temperature cools in lower stratosphere due to
increase in tropospheric ozone
DCO2
DO3 realistic
DO3 uniform
DJF, 100 hPa
Strong cooling at high
northern latitudes in
winter stratosphere:
remote effect.
Controlling tropospheric
ozone could hasten
recovery of stratospheric
ozone.
(an unexpected benefit!)
Vertical variation of radiative forcing: DO3 DO3unif DCO2
O3 longwave forcing
increases with altitude
O3 absorbs incident & reflected uv
SW
LW
O3 diminishes downward uv flux
CO2 forcing
saturates
Water vapor diminishes LW forcing
CO2 exhibits stronger total forcing in mid-troposphere
Total forcings match
DF CO2 > DF Ozone
in mid-troposphere
Surface temperature
responds to forcing
throughout troposphere
Seasonal variation of temperature changes
Surface temperature
CO2
O3
DCO2 temperature
change largest in NH
winter due to albedo
feedback
O3unif
Integrated trop temperature
CO2
O3
O3unif
DO3 integrated
temperature change
largest in NH summer
due to strong forcing and
vertical mixing
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
Realistic DO3
Strong warming downwind of
ozone source regions
DT
White areas =
insignificant values
GCM surface warming patterns from increasing tropospheric
ozone over 20th century – JJA
D Realistic O3
Difference DO3 – DCO2
Equivalent DCO2
DCO2 shows stronger warming
over dry Sahara
DO3 yields stronger warming over
Arctic and midlatitude continents
White areas = insignificant values
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
Extra slides
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.
Variation of forcing with altitude: DO3unif, DCO2
SW
Experimental forcings normalized at tropopause
LW
DCO2 forcing with water
vapor removed from
calculation, no DCO2 in
stratosphere
DO3 uniform forcing with 1000 x O3 concentrations
Cloud cover changes amplify surface temperature
response
Increased cloud cover in
upper troposphere warms,
especially for DCO2
Decreased cloud cover in
lower troposphere warms