Radiative forcing due to stratospheric ozone
Gunnar Myhre1, Frode Stordal2 1
Bjrg Rognerud1, and Ivar S.A. Isaksen1
;
1
Department of Geophysics, University of Oslo, Norway
Norwegian Institute for Air Research (NILU), Norway
2
Abstract. Radiative forcing and stratospheric
temperature change due to changes in stratospheric
ozone from a 2D chemistry-transport model (CTM)
in the period 1960-2004 is calculated. A thermal infrared scheme and a short wave scheme are used in
the calculations of the latitudinal and seasonal variation in the radiative forcing. The sensitivity of the
radiative forcing to the altitude of the ozone changes
is investigated. Changes of ozone in the upper stratosphere give positive radiative forcing as the solar
component dominates over the longwave component,
while the opposite is the case for ozone changes in
the lower stratosphere. In the calculations presented
here a weak positive radiative forcing is calculated as
the ozone changes from the CTM are largest in the
upper stratosphere.
perature change [WMO, 1994]. The temperature decreases are largest at high latitudes, but also marked
at lower latitudes.
In this study the sensitivity of the radiative forcing to the altitude of ozone changes is rst investigated. Thereafter, radiative forcing and temperature
changes are presented based on ozone changes from a
2D stratospheric chemistry-transport model (CTM)
in the period 1960-2004.
Input data and models
A stratospheric 2D CTM is used to calculate the
ozone changes from 1960 to 2004. The model has a
vertical resolution of 2 km up to 50 kilometer and a
horizontal resolution of 10 degrees in latitude [Isaksen
et al., 1990]. The model and the results are discussed
in Zerefos et al. [1997].
In the radiative transfer calculations a model of the
thermal infrared radiation and a model of the shortwave radiation are used. A broad band model is used
in the calculation of the thermal infrared radiation
[Myhre and Stordal, 1997]. The main band of ozone
at 9.6 m and the spectroscopically weaker band at
14 m are included.
The shortwave radiative transfer model uses the
discrete ordinate method [Stamnes, 1988] for calculating eects of ozone changes in the ultraviolet and
visible region. The spectral resolution is 1 nm and
8 streams are used in the calculations. Clouds are
neglected in the shortwave code, as opposed to in the
longwave code, where they are included.
Results from the shortwave and longwave models
used here have been shown to compare well to the
results of the comparative study of Shine et al. [1995]
see [Stordal et al., 1996].
In the radiative calculations, the same input data
as in the CTM are used for water vapour and temperature, except at the surface where ECMWF temperatures are used. The spatial resolution is also the
same as in the 2D CTM.
In all the radiative calculations the stratospheric
temperature is adjusted to the changes in heating
rates due to the change in ozone in the stratosphere,
where the adjustment is performed using the xed
dynamical heating formulation (see e.g. Shine et al.
[1995]).
Introduction
Observations from ground based instruments and
satellites indicate a decrease in stratospheric ozone
concentrations during the two last decades [WMO,
1994]. The largest decrease is reported at high latitudes during the winter and spring. However, substantial ozone depletion is reported also for other latitudes and times of the year.
As ozone absorbs solar radiation in the ultraviolet and visible spectrum as well as thermal infrared
radiation, stratospheric ozone decrease can have signicant climatic eects on the surface-troposphere
system [WMO, 1994; IPCC, 1994]. A decrease in
stratospheric ozone allow more solar radiation penetrate through the stratosphere, hence warming the
surface-troposphere system. On the other hand less
absorption of ozone in the thermal infrared region
gives a cooling. Previous estimates of the global
and annual mean radiative forcing due to changes in
stratospheric ozone since pre-industrial time is negative and around -0.1 W=m2 [Ramaswamy et al., 1992;
IPCC, 1994]. Observations of changes in the vertical
prole of ozone are limited. It is well documented
that there is a signicant sensitivity to the altitude of
the ozone change [Lacis et al., 1990; Ramaswamy et
al., 1992; Schwarzkopf and Ramaswamy, 1993; Wang
et al., 1993].
Temperature decreases in the lower stratosphere is
observed [WMO, 1994], and there is evidence that
ozone depletion is the main contributor to this tem1
Results
Altitude (km)
Figure 1 shows the net (longwave and shortwave)
50
radiative forcing at the tropopause when the ozone
80N
amount in each 2 kilometer layer is decreased by 10
EQ
40
%, one by one. Calculations are performed at 80N,
80S, and at the equator for January month. Similar
30
to previous studies [Lacis et al., 1990; Ramaswamy et
al., 1992; Schwarzkopf and Ramaswamy, 1993; Wang
20
et al., 1993] we nd a signicant sensitivity to the altitude where the ozone changes take place. In lower
10
latitudes the net radiative forcing changes even sign
with altitude. The solar component that gives a
0
positive radiative forcing when ozone decreases, is
-0.15
-0.10
-0.05
-0.00
Radiative forcing (W/m )
only slightly dependent upon the height at which
the ozone change take place. Therefore, the solar
component of the radiative forcing is mostly depen- Figure 2: Longwave radiative forcing at the tropopause
dent upon the total ozone change in the stratosphere. due to a temperature decrease of 1 K in each 2 km layer.
On the other hand the longwave radiation depends
strongly upon the altitude of the ozone change. This
is mainly due to the fact that an ozone decrease results in locally less absorption and therefore a local
cooling. Allowing the temperature to adjust to the
50
change in the heating rate has the largest eect on
the longwave radiation in the lower stratosphere, as
40
can be seen in Figure 2. The solar radiation changes
negligibly due to a temperature change [Shine et al.,
30
1995].
2
-10-5
-15
-20
-10-5
-15
-20
-25
-20
-20
-15
-15
-10
-10
-5
0
-10
-5
Altitude (km)
5
-2
20
50
Altitude (km)
10
-5
0
80
NH
30
20
0
60
40
20
0
-20
Latitude(Deg)
-40
-60
-80
SH
Figure 3: Ozone change from 1969 to 1996 as calculated
with the 2D CTM (values in percent).
10
0
-0.10
0
40
5
-10
80N
80S
EQ
-0.08
-0.06
-0.04
-0.02
Radiative forcing (W/m2)
0.00
0.02
Figure 1: Radiative forcing at the tropopause due to an
Radiative forcing (W/m2)
ozone change of 10 % in each 2 km layer.
0.30
0.20
LW
SW
Net
Figure 3 shows the yearly average ozone change
0.10
from 1969 to 1996 calculated with the CTM. Large
changes are calculated near 40 km at all latitudes,
0.00
in fact somewhat larger than the ones observed by
SAGE I/II [WMO, 1994]. The discrepancy is large in
the lower stratosphere, where the SAGE reductions -0.10
are much higher than modelled, except at high latitudes. The modelled reductions at mid latitudes are -0.201960 1970 1980 1990 2000 2010
Year
closer to those observed by ozone sondes.
Figure 4 shows the longwave, shortwave, and net
radiative forcing from 1960 to 2004. In the period Figure 4: Longwave, shortwave, and net globally aver1985 to 1996 the calculations are performed for each aged
radiative forcing in the period 1960 to 2004.
year, otherwise it is 3-5 years between the calculations. The net radiative forcing varies only slowly, ex-
tive forcing due to change in stratospheric ozone of
about -0.1 W=m2 . These were based on the large
ozone reductions in the lower stratosphere deduced
from the SAGE observations and changes in the upper and middle stratosphere were in some cases neglected. It is also worth noticing that changes in the
lower stratospheric ozone observed by ozone sondes
show far less ozone reductions than those found in
SAGE data which have their largest uncertainty in
the lower stratosphere. E.g. in the 30-50 N region
in the 16-18 km region, SAGE data show a 15-20 %
ozone reduction in the 1980s whereas only 5-7 % is
found based on sonde data. The 2D CTM used in
this study estimated a 2-3 % reduction in the same
period. We have therefore performed some additional
sensitivity experiments to test the sensitivity of the
radiative forcing to the ozone changes in the lower
stratosphere, allowing a better comparison with earlier radiative forcing calculations. First, the positive
ozone changes in the lower stratosphere (due to the
self healing eect) are removed, reducing the radiative forcing from 0.046 to 0.039 W=m2 . Second, in
0.40
the two 2 km layers above the tropopause the ozone
LW
SW
0.30
amount
is decreased by 20 and 10 % respectively, in
Net
0.20
better agreement with the SAGE data (change from
0.10
1979 to 1990). The ozone reductions elsewhere are
-0.00
unchanged. A radiative forcing of -0.003 W=m2 is
-0.10
now calculated. We therefore argue that with ozone
-0.20
reductions in the lower stratosphere in line with those
observed by ozone sondes combined with ozone re-0.30
ductions also in the upper stratosphere, the radiative
-0.40
forcing due to stratospheric ozone is probably very
-0.50
-0.60
small on a global scale. The radiative forcing cal80
60
40
20
0
-20 -40 -60 -80
culated here is more in line with the calculation in
NH
Latitude
SH
Hauglustaine et al. [1994] that also used modelled
ozone change. It is important to notice that local
Figure 5: Radiative forcing as a function of latitude due forcing may be signicant with positive forcing in the
to changes in ozone from 1969 to 1996.
tropics and negative forcing at high latitudes, leading
possibly to a modied circulation and climate change.
Radiative forcing (W/m2)
cept for the years 1992 and 1993. The radiative forcing is lower in these two years due to higher levels of
sulfate aerosols in the period after the Mt. Pinatubo
eruption in 1991, resulting in signicant ozone losses
in the lower stratosphere.
The yearly average longwave, shortwave and net radiative forcing due to change in stratospheric ozone
in the period from 1969 to 1996 is shown in Figure
5 as a function of latitude. Both the longwave and
shortwave radiative forcing have their largest absolute values at high latitudes. The longwave component dominates over the shortwave component at latitudes above 60 degrees in both hemispheres. The
seasonal variation in the net radiative forcing is relatively small, except that during the winter and summer the net radiative forcing is negative at lower latitudes and higher latitudes respectively. Table 1 summarizes the results for the four seasons and the yearly
average. The net radiative forcing is positive for all
seasons and in both hemispheres.
Table 1: Longwave, shortwave, and net radiative forcing; global (Gl) northern hemispheric (NH), and southern
hemispheric (SH) averages, for the four seasons and the
yearly average (YA).
-0.151 -0.194
0.192 0.232
0.041 0.038
-0.184 -0.139
0.232 0.194
0.048 0.055
30
-1.0
-3.0
-1.5
.4
-0.6
-0
-0.167 -0.167
0.212 0.213
0.044 0.046
-
-7.0
.5
-0.2
-0.4.6
-0
-0.2
20
10
0
80
NH
0.2
-1.0
4-0.6
-0.
-0.2
60
-1.0
LW -0.170 -0.103
SW 0.240 0.163
Net 0.071 0.060
YA
-7.0
5.0
-0.4
LW -0.127 -0.252
SW 0.135 0.272
Net 0.008 0.020
SON
-5.0
0
-1
-3.0
-0.2
LW -0.148 -0.177
SW 0.188 0.217
Net 0.039 0.040
JJA
Gl
-0.173
0.235
0.061
NH
-0.246
0.329
0.082
SH
-0.100
0.141
0.040
Altitude (km)
MAM
-3.
40
0 0.6
-1. --0.4 0.0-0.2
DJF
-3.0
50
0.3
0.0
0.0
40
20
0
-20
Latitude(Deg)
-40
-60
-80
SH
Figure 6: Calculated temperature change between the
years 1969 and 1996.
As noted earlier, most of previous global and an- The temperature adjustments are shown in Fignual mean studies have calculated a negative radia- ure 6. The temperature decrease is large in the up-
per stratosphere, over 7 K. At high latitudes there
chemistry in the calculation of radiative forcing
is also a large cooling in the lower stratosphere. In
on the climate system, J. Geophys. Res., 99, 1173the lower stratosphere at low latitudes there is a
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the Research Council of Norway (Programme for
Organization, Geneva
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