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GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L02813, doi:10.1029/2006GL028252, 2007
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Indirect radiative forcing of the ozone layer during the 21st century
Robert W. Portmann1 and Susan Solomon1
Received 21 September 2006; revised 5 November 2006; accepted 8 December 2006; published 19 January 2007.
[1] The response of a coupled two-dimensional radiativechemical-dynamical model to possible 21st century changes
of the greenhouse gasses (GHGs) carbon dioxide, nitrous
oxide and methane are explored using a range of IPCC
marker scenarios of GHG emissions. The changes to the
ozone layer caused by these GHGs are found to be
relatively large (e.g., up to 5% global mean column ozone
changes and 30% local changes for CO2 using the IPCC A2
scenario between 2000 and 2100) and the mechanisms for
these changes are discussed. The ozone changes are
compared to the recovery of ozone due to expected
decreases in chlorine containing compounds. Since carbon
dioxide, nitrous oxide, and methane affect ozone they
induce an indirect radiative forcing in addition to their direct
radiative forcing. These indirect radiative forcings are
computed using a combination of accurate line-by-line
and band radiative transfer models and are compared to the
radiative forcing of ozone during the 1979– 2000 time
period. Although the changes in ozone are large at some
altitudes over the 2000 – 2100 time horizon, the range of
associated future indirect radiative forcings from ozone over
the range of IPCC scenarios are found to be 0.1 to 0.1 W
m2, which is small compared with the corresponding range
of total direct radiative forcing of 2.2 to 6.2 W m2 for these
GHGs over this time horizon. Citation: Portmann, R. W., and
S. Solomon (2007), Indirect radiative forcing of the ozone layer
during the 21st century, Geophys. Res. Lett., 34, L02813,
doi:10.1029/2006GL028252.
1. Introduction
[2] The greenhouse gases (GHGs) CO2, CH4, and N2O
are thought to be the most important gases that will affect
changes in climate in the 21st century, through their direct
radiative forcing [Intergovernmental Panel on Climate
Change (IPCC), 2001]. During the years 1980 – 2000 the
indirect radiative forcing caused by ozone depletion due
largely to CFC increases has been important as well, and
reduced the total radiative forcing during this period. This
indirect radiative forcing should reverse itself during the
21st century, mostly by 2050 as CFCs decrease [World
Meteorological Organization (WMO), 2003]. However, it
has long been known that the GHGs CO2, CH4, and N2O
will also affect O3 through their thermal and chemical roles
in the stratosphere (see discussion below). In this paper, we
consider whether the indirect radiative forcing from these
O3 changes, caused by GHG changes, could be significant
1
Chemical Science Division, Earth System Research Laboratory,
NOAA, Boulder, Colorado, USA.
This paper is not subject to U.S. copyright.
Published in 2007 by the American Geophysical Union.
in the 21st century. An estimate of this radiative forcing will
be presented in this paper.
2. Modeled Future Ozone
[3] There is a long history of model estimates of the
effects of GHGs and CFCs on ozone, including Haigh and
Pyle [1982], Brasseur et al. [1985], and Wuebbles et al.
[1983]. These papers show that local ozone changes of the
order 10– 50% locally are possible due to feasible perturbations, and that the effect of increasing CO2 could cancel
the effects of increasing CFCs and N2O under some circumstances. However, the changes expected were in the middle
and upper stratosphere and thus the impact on the ozone
column was not large. Note that these studies were all
carried out prior to the discovery of the ozone hole and
thus do not contain heterogeneous chemistry, which greatly
increases the potency of the CFCs at high latitudes and
causes many other chemical changes in the lower stratosphere. More recently, studies by Randeniya et al. [2002],
Chipperfield and Feng [2003] and Rosenfield et al. [2002]
have modeled the future behavior of ozone due to several of
these GHGs. Their results are qualitatively consistent with
the earlier studies, with the exception of the Randeniya
study that did not consider coupling to the future temperature changes.
[4] We use the NOCAR two-dimensional chemicalradiative-dynamical model in this study [Garcia et al.,
1992; Solomon et al., 1998; Portmann et al., 1999] to
compute ozone distributions. This model has been used
extensively to study past and future changes in ozone and
includes comprehensive chemistry, detailed radiative transfer, and innovative dynamics including planetary and gravity
wave breaking schemes (see references above for details).
We force the model with the WMO scenarios for halogens
[WMO, 2003] and use the IPCC A2 scenario for future
concentrations of CO2, N2O, and CH4. The IPCC A2
scenario is generally considered to be a relatively high
estimate for these gases compared to other scenarios. We
choose it to maximize the effects of the GHGs on the ozone
layer. This study focuses on the stratospheric ozone response and does not include emission scenarios for tropospheric ozone precursors such as NOx or CO.
[5] Figure 1 shows the time evolution of ozone global
mean anomalies for the 1980– 2100 period using the WMO
[2003] halogen gases and the IPCC A2 GHG scenarios.
This calculation shows the percent changes relative to 1980
for the model and the combined satellite record [Fioletov et
al., 2002]. The global mean changes in the 1980 – 2000 time
period are about 5% and are caused primarily by CFC
increases during this time period. There is then a recovery
from these decreases due to the reduction of CFCs in the
stratosphere, which is nearly complete by 2050. However,
there is a ‘‘super’’ recovery due to the temperature changes
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sphere and are primarily caused by temperature decreases at
these altitudes, which reduce key gas-phase chemical rate
coefficients. The largest change occurs in the temperature
dependence of the reaction
O þ O3 ! 2 O2
ð1Þ
which is a primary sink for odd-oxygen (i.e., O + O3) in the
upper stratosphere. In the middle and lower stratosphere at
low and midlatitudes (and the global mean) there are ozone
decreases that are caused by a reversed ‘‘self healing’’ (i.e.,
the reduced penetration of UV wavelengths due to increased
ozone column overhead causes reduced ozone production
from O2 photolysis). The opposition of the upper and
middle stratospheric changes reduces the effect of these
changes on the ozone column, although the overall effect is
still positive as shown in Figure 1.
[7] Figure 3 shows the effect of increasing N2O on the
ozone profile in the 21st century. The primary response is
for increased N2O to cause increased NOx-induced ozone
losses in the middle stratosphere where the NOx effect on
ozone is the largest. These changes are reduced in the lower
stratosphere due to the buffering effect of increased NOx on
HOx (e.g., HOx is reduced by increases in HNO3). In
addition, NOx increases in the lower stratosphere can cause
increased ozone production from chemical processes of the
same character as tropospheric smog chemistry. The net
effect of competing chemical processes in the lower statosphere leads to ozone increases at low latitudes and ozone
decreases at high latitudes [see Nevison et al., 1999].
Figure 1. (a) The IPCC A2 scenario used in the model
calculations presented in Figures 1b, 2, 3 and 4. (b) The
variation of global mean total ozone anomalies during the
21st century due to the combined effects of Halogens and
GHGs computed using the NOCAR 2-D model. The
anomalies are computed with respect to years 1980 – 1985
and have been smoothed with a 25 month smoothing
function. The effect of varying GHGs has been separated
out by individually keeping the gas at year 2000 levels
compared with the full variation. The smoothed anomalies
computed from the NIWA assimilated satellite record are
also shown for the past [Fioletov et al., 2002].
from the GHGs in the stratosphere [Brasseur and Hitchman,
1998; Pitari et al., 1992; Rosenfield et al., 2002], which is
discussed below. The individual effects of the GHGs are
separated out in Figure 1 by re-doing the calculation with
each GHG held fixed at its year 2000 value. This shows that
increasing CO2 and CH4 cause increases in future ozone,
while increasing N2O causes ozone decreases, and that these
changes are all on the order 2 – 4% by 2100. Thus the future
changes due to the GHGs have the potential to be nearly as
large as the CFCs on this long time-scale. It is important to
note that this is only true in the global mean and at low
latitudes. At high latitudes, the ozone changes by CFC on
the 1980– 2000 time-scale are much larger than the changes
expected for GHGs in 2100.
[6] Figure 2 shows the profile changes in ozone due to
increases in CO2 between 1980 and 2100 for several latitude
ranges. The ozone changes are largest in the upper strato-
Figure 2. Profile change in ozone due to increases in CO2
during the 21st century (the percent change from 2000 to
2100) for six 30° wide latitude bands computed with the
NOCAR 2-D model. Ozone in the upper stratosphere
increases due to changes in gas-phase rate coefficients
caused by temperature decreases. The temperature decreases
due to increased long-wave radiative cooling from CO2. The
ozone decreases in the lower stratosphere at low and
midlatitudes from reversed ‘‘self healing.’’
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Figure 3. Same as Figure 2, except for N2O. The primary
response is for increased N2O to cause increased NOxinduced ozone losses in the middle stratosphere where the
NOx effect on ozone is the largest. These changes are
reduced in the lower stratosphere due to the buffering effect
of HOx on NOx.
Figure 4. Same as Figure 2, except for CH4. CH4 causes
increased H2O and HOx in the stratosphere and thus
increased HOx induced ozone losses, especially in the upper
stratosphere/lower mesosphere. In the troposphere increased
CH4 causes increased ozone due to NOx induced ozone
production (smog chemistry).
[8] The profile changes in ozone due to increases in CH4
during the 21st century are shown in Figure 4. CH4
increases cause increased H2O in the stratosphere and thus
increases in HOx (especially in the middle and upper
stratosphere). The increased HOx causes enhanced HOxinduced ozone losses in the upper stratosphere/lower mesosphere. These effects are quite large near and above the
stratopause (10%). In the troposphere increased CH4
causes increased ozone due to NOx induced ozone production (smog chemistry [see, e.g., Finlayson-Pitts and Pitts,
2000]). The magnitude of this effect is strongly dependent
on the amount of tropospheric NOx, which the present
model, like other two-dimensional models, simulates only
crudely. For this reason, the stratospheric component of this
effect is separated out below when calculating the radiative
forcing.
compute the instantaneous radiative forcing. Then the
stratospheric temperatures are relaxed using a straightforward time-marching relaxation technique using the longwave heating rate changes output from the LBL calculation.
The radiative forcing model used in the FDH adjustment
calculation is the CCM-IR2 code [Briegleb, 1992]. The
calculations were done using six 30° wide latitude bins and
temperature and cloud parameters obtained from the ISCCP
data set [Rossow and Schiffer, 1999]. The line parameters
were obtained from the HITRAN 2000 compilation
[Rothman et al., 2003], including later corrections.
[10] The radiative forcing caused by changes in O3 is
strongly influenced by the altitude at which the change
occurs [Lacis et al., 1990; de F. Forster and Shine, 1997].
Also, unlike most other gases that induce radiative forcing,
the radiative forcing from O3 changes is strongly influenced
by the stratospheric adjustment. The change due to adjustment is on the same order as the instantaneous change for
O3 (for changes in the lower stratosphere), while for most
gases it is less than a 20% change. In general, the radiative
forcing is largest for changes near the tropopause and in the
lower statosphere and much smaller for high and low
altitudes. For stratospheric O3 changes, the shortwave
3. Indirect Radiative Forcing From GHG Change
[9] The ozone changes shown above were input into a
radiative transfer model in order to compute the radiative
forcing due to the changes. These are an indirect radiative
forcing because the changes in ozone can be attributed to
the change in the GHGs (of course these add to the direct
radiative forcings of the GHGs themselves, which are large
as discussed below). Radiative forcing is a measure of the
extra heat (in W m2) input into the troposphere due to the
change and is found by computing the negative of
the change in flux at the tropopause after stratospheric
temperatures have relaxed to the perturbation. The stratospheric adjustment is usually computed using the fixed
dynamical heating approximation. If stratospheric temperatures are not adjusted then the forcing is called the
instantaneous radiative forcing. A line-by-line (LBL) radiative transfer (RT) model is used [Portmann et al., 1997] to
Table 1. Global Mean Indirect Radiative Forcing for Ozone
2000 – 2100
Instantaneous, W m2
CO2
N2O
CH4
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Adjusted, W m2
Stratosphere
Only
Full Profile
Stratosphere
Only
Full Profile
0.031
0.029
0.014
0.036
0.046
0.124
0.077
0.026
0.031
0.081
0.038
0.131
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PORTMANN AND SOLOMON: INDIRECT RADIATIVE FORCING OF O3 LAYER
Table 2. Global Mean Adjusted Radiative Forcing, CO2 + N2O +
CH4, 2000 – 2100
IPCC
Scenarioa
Direct RFb,
W m2
Indirect RFb via O3,
W m2
A2
B1
Rangec
5.6a
2.3a
2.2 to 6.2a
0.09
0.03
0.1 to 0.1
a
IPCC [2001].
RF, radiative forcing.
c
Range across all SRES marker scenarios (A1B, A1T, A1FI, A2, B1,
B2).
b
radiative forcing is generally larger than the longwave
radiative forcing for the instantaneous forcing, while the
stratospheric adjustment causes a comparable offsetting
change in the longwave forcing.
[11] The instantaneous and adjusted radiative forcing for
the ozone changes induced by the GHG changes over the
2000 – 2100 time period are shown in Table 1. All of these
indirect radiative forcings are less than 0.15 W m2, which
can be compared with the total direct radiative forcing of
5.6 W m2 from these GHGs over this time horizon [IPCC,
2001]. The reason for the small indirect radiative forcing
is due to the profiles of the ozone changes shown in
Figures 2 – 4. While the absolute values of the local and
total column O3 changes are reasonably large, nearly none
of the changes occur near the tropopause. In addition, there
are both negative and positive changes in the stratosphere,
especially in the case of CO2, which compete with each
other in the overall effect.
4. Conclusion
[12] The radiative forcing of stratospheric ozone during
the 1980 –2000 period was estimated by IPCC [2001] to be
0.15 ± 0.1 W m2. This is important in attribution studies
for recent decades since it is competing against a much
smaller direct radiative forcing from the GHGs compared to
that expected based upon scenarios for the 21st century. The
changes in the profile and column ozone due to increases in
CO2, N2O, and CH4 are likely to be substantial during the
21st century. This is especially true of the CO2 induced
changes near the stratopause, which can lead to as much as a
30% increase in local ozone abundances. These numbers are
in general agreement with other calculations [Rosenfield et
al., 2002; Chipperfield and Feng, 2003]. However, despite
these large local changes, the radiative forcing of ozone in
the 21st century due to changes of CO2, N2O, and CH4 is
likely negligibly small compared with the direct radiative
forcing of these gases. Table 2 compares the total direct
forcing of these gases against their indirect forcing over the
21st century. It shows that the indirect forcing from ozone
changes is less than a tenth of a W m2 compared with
about 5.6 W m2 for the A2 scenario, or 2.3 W m2 for the
B1 scenario. These scenarios are at the high and low end of
the range of the marker scenarios considered by IPCC
[2001], which is also shown on Table 2. The small size of
the ozone indirect radiative forcing estimates is due to
competing changes in the ozone profile and because the
largest changes are in the middle and upper stratosphere
where they do not strongly influence the radiative changes
at the tropopause.
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[13] The GHGs may cause larger changes to the stratosphere than can be modeled using a two-dimensional model.
For example, there could be large changes in wave driving
which could alter the stratospheric circulation, the polar
vortices could change dramatically, or tropopause heights
could change. In addition, other constituent changes not
considered in this study such as water vapor increases or
changes in stratospheric aerosol could alter ozone distributions. Note that all of these changes are highly uncertain and
no reliable model estimates of their future magnitude exist.
If these changes caused large ozone changes in the lower
stratosphere then they could cause an indirect radiative
forcing larger than that estimated in this paper. However,
given the small radiative forcings found in this study for
relatively large ozone changes it is unlikely that any of these
effects will cause the indirect radiative forcing of ozone to
be significant during the 21st century.
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R. W. Portmann and S. Solomon, Chemical Sciences Division, Earth
System Research Laboratory, NOAA, R/CSD08, 325 Broadway, Boulder,
CO 80305-3328, USA. ([email protected])
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