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GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L01807, doi:10.1029/2006GL027472, 2007
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Article
Radiative forcing due to stratospheric water vapour from CH 4
oxidation
Gunnar Myhre,1,2 Jørgen S. Nilsen,1,3 Line Gulstad,1 Keith P. Shine,4 Bjørg Rognerud,1
and Ivar S. A. Isaksen1
Received 6 July 2006; revised 24 October 2006; accepted 20 November 2006; published 9 January 2007.
[1] Here we report on estimates of the changes in
stratospheric water vapour (SWV) due to methane
oxidation based on observational data. Above the
tropopause oxidation of methane results in a decrease in
its mixing ratio with altitude and this is a major source for
SWV. The vertical profile of SWV changes from methane
oxidation is presented here using satellite observations of
the vertical profile of methane. Trends in the SWV are
shown to be small in the lower stratosphere, but can reach
0.7 ppbv at 30 km at high latitudes over the period 1950 –
2000. The radiative forcing for this indirect effect of
methane increase over the industrial era is estimated to be
slightly weaker than 0.1 Wm 2 which implies a larger
contribution of water vapour to the methane global warming
potential than used in recent Intergovernmental Panel on
Climate Change assessments. Our estimate considers only
chemical changes and not SWV of dynamical causes.
Importantly, we find substantial differences in the
temperature change in the stratosphere for a homogeneous
change in SWV and SWV change from methane oxidation.
This has implications for trend analysis of SWV
and understanding and attribution of the stratospheric
temperature trend. Citation: Myhre, G., J. S. Nilsen,
L. Gulstad, K. P. Shine, B. Rognerud, and I. S. A. Isaksen
(2007), Radiative forcing due to stratospheric water vapour from
CH4 oxidation, Geophys. Res. Lett., 34, L01807, doi:10.1029/
2006GL027472.
1. Introduction
[2] Measurements from a variety of sources show an
increase in stratospheric water vapour (SWV) during the
period from mid-1950s to 2000 [Oltmans et al., 2000;
Randel et al., 2004; Rosenlof et al., 2001]. The change
may have been as large as a doubling over this time period.
However, the uncertainties are large since no long term
global network of SWV observations exists. A decline in
SWV has been observed since 2000 based on several types
of measurements techniques, although the magnitude of the
change differs [Randel et al., 2004]. Randel et al. [2006]
links the recent decrease in SWV to a cold tropical tropopause and the Brewer-Dobson circulation. The apparent
reversal in trend, differences in the trend between various
1
Department of Geosciences, University of Oslo, Oslo, Norway.
Center for International Climate and Environmental Research, Oslo,
Norway.
3
Now at Ullevål University Hospital, Oslo, Norway.
4
Department of Meteorology, University of Reading, Reading, UK.
2
Copyright 2007 by the American Geophysical Union.
0094-8276/07/2006GL027472$05.00
instruments, and lack of a physical explanation raises
questions of the significance of the reported long term trend.
[3] Several studies have investigated the radiative effect
and temperature change based on the observed trends in
SWV [Forster and Shine, 1999, 2002; Oinas et al., 2001;
Smith et al., 2001]. The potential for enhanced SWV to be a
strong radiative forcing mechanism has been shown by
Forster and Shine [2002]. They obtained a radiative forcing
as large as 0.6 Wm 2 over the period 1960 to 2000 using a
SWV trend of 0.05 ppmv/year. Most of the earlier studies
used a vertically independent change in the SWV profile
with cooling in the lower stratosphere of 0.2 – 0.5 K/decade
at high latitudes [Forster and Shine, 1999, 2002; Oinas et
al., 2001; Shine et al., 2003] but more realistic changes in
the SWV vertical profile have also been used [Shine et al.,
2003; Smith et al., 2001]. A cooling in the lower stratosphere of more than 1 K/decade at mid to high latitudes is
observed over recent decades. Observed temperature trends
can be explained partially by stratospheric ozone depletion
but it appears inadequate to explain all of the observed
cooling at 50 hPa [Shine et al., 2003].
[4] In addition to a significant direct radiative forcing,
CH4 has a significant important indirect effects on O3, OH
(therefore affecting other radiative actively gases), and
SWV [Fuglestvedt et al., 1996; Lelieveld and Crutzen,
1992; Lelieveld et al., 1998; Shindell et al., 2005]. CH4 is
oxidized in the atmosphere by OH, which in the stratosphere leads to SWV and is the most important source for
SWV. Since the mid-18th century CH4 has increased from
700 ppmv to 1750 ppmv. A large trend in the SWV cannot
be entirely explained by the increase in CH4. Hansen et al.
[2005] calculated the SWV from CH4 oxidation in a
two-dimensional model. Their radiative forcing due to
SWV for a change in CH4 between the years 1750 to
2000 is 0.07 Wm 2.
[5] Several explanations of the increase in SWV have
been proposed [Joshi and Shine, 2003; Rockmann et al.,
2004; Rosenlof, 2003; Sherwood, 2002]. It is likely that
increased transport of water vapour from the troposphere to
the stratosphere has contributed to the SWV trend [Hansen
et al., 2005; Rosenlof, 2003]. Rockmann et al. [2004]
suggested that increased level of chlorine, reduced ozone,
and more OH as a feedback of increased CH4 in the
stratosphere were three mechanisms that had increased the
oxidation of CH4 in the stratosphere.
[6] In this study we use vertical CH4 profiles from
satellite data to calculate the vertical profile of the increase
in SWV since pre-industrial times from the oxidation of
CH4, and the resulting radiative forcing and temperature
changes in the stratosphere. In addition to changes since
pre-industrial time, we focus on changes since 1950 and
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Figure 1. HALOE zonal mean vertical profile of CH4 over
the period October 1991 throughout 1999.
1979. These time scales are chosen since SWV trends are
observed, at least crudely, since 1950 and satellite data of
stratospheric temperature trends are available after 1979.
2. Methods
2.1. SWV Change Based on Satellite Data
[7] We use data from the Halogen Occultation Experiment (HALOE) onboard the Upper Atmosphere Research
Satellite (UARS), described by Russell et al. [1993]. The
HALOE instrument provides high-quality vertical profiles
of methane and water vapor, derived from solar occultation
experiments. The vertical resolution for the version used
here is approximately 1.3 km. The HALOE measurements
extend almost down to the local tropopause level, below
which cloud effects contaminate a significant fraction of the
retrievals. The data are obtained from the SPARC Reference
Climatology Project (http://www.sparc.sunysb.edu/html/
ref_clim.html). HALOE CH4 measurements from October
1991 throughout 1999 are averaged following the method of
Randel et al. [1998], making use of the Microwave Limb
Sounder (MLS) data, also onboard UARS, to fill in where
HALOE data are lacking. The HALOE CH4 has been
validated by Park et al. [1996].
[8] The HALOE vertical profiles of CH4 are used to
calculate the SWV from the oxidation of CH4 (see Figure 1).
The CH4 concentration is nearly constant up to the tropopause. In the stratosphere the sum of water vapour and twice
the CH4 concentration is nearly constant [Nassar et al.,
2005; Rosenlof, 2002]. This is important for our method to
extract SWV changes from CH4 oxidation. Above the
tropopause oxidation of one CH4 molecule generates two
H2O molecules. Based on the reduction CH4 with altitude
from the HALOE measurements we calculate the increase in
SWV. We consider only the fractional part of the CH4
oxidation to SWV that is anthropogenic. This is obtained
by using the present and pre-industrial (in some cases since
1950 and 1979 concentrations) CH4 concentration. The
time-lag of the methane has not been taken into account
and with the rather modest increase in the tropospheric
methane during the 1990s, this neglect is not expected to
cause a significant error.
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[9] An assumption in our method is that shape of the
vertical profile of CH4 in the stratosphere has not changed
over the industrialized era. This assumption is likely to be
applicable to a large extent, but some caveats should be
mentioned. In reality, a small part of the decrease in water
vapour with height will result from the several years’
delay for the increases in tropospheric methane to propagate to the upper stratosphere. In addition, both Evans et
al. [1998] and Nedoluha et al. [1998] find decreases in
stratospheric methane from HALOE data during the mid1990s which could be driven by an increased rate of
oxidation of methane to water vapour. For example
Nedoluha et al. [1998] estimate that the increase in
tropospheric methane gives a SWV trend at 40– 60 km
of 20 ppbv/year between 1992 and 1997 from the trend in
tropospheric methane, but this is actually exceeded by an
increase in the conversion of methane to SWV (38 ppbv/
year). However, Nedoluha et al. [2003] discuss the lack of
a trend in SWV between 1996– 2002, using observations
from a variety of sources, and so an increased methane
conversion does not appear to be occurring during this
later period.
2.2. Radiative Transfer Calculations
[10] In the calculations of radiative forcing and temperature changes in the stratosphere we use separate longwave and shortwave radiative schemes. For the longwave
calculations a broad band model is adopted [Myhre and
Stordal, 1997], whereas for shortwave a model using the
discrete ordinate method [Stamnes et al., 1988] is used.
The two schemes that are used for global calculations are
compared to line-by-line codes [Myhre and Stordal, 2001]
for a mid-latitude-summer profile. A change of SWV from
6.0 ppmv to 6.7 ppmv gave a shortwave radiative forcing
of 0.11 Wm 2 for the global model and the LBL (with a
6% difference). For similar change in SWV the longwave
radiative forcing was 0.32 Wm 2 in the broad band model
and the LBL model (2 % difference). Stratospheric temperature adjustment is calculated using the fixed dynamical heating approximation as in Myhre and Stordal
[1997]. The meteorological data (including water vapour)
used for the reference calculation is from ECMWF on a
T21 (5.6 5.6 degree) resolution and with 60 vertical
layers.
3. Results
3.1. SWV Change
[11] Figure 2a shows the SWV change from CH4 oxidation based on the HALOE data over the industrialized era,
with small changes in the lower stratosphere but almost
2 ppmv at altitudes at 50 –60 km. The latitudinal gradient in
the SWV is particularly strong in the lower stratosphere. In
the upper stratosphere the contribution from CH4 is significant and can amount to nearly one third of the present
abundance of SWV [Rosenlof, 2002]. More than half of the
CH4 change since pre-industrial time has occurred after
1950. The SWV from CH4 oxidation since 1950 is shown in
Figure 2b; it has a similar pattern to Figure 2a, but with
maximum changes of somewhat more than 1 ppmv (or
20 ppbv/year) at high latitudes in the upper stratosphere. A
significant increase of 0.7 ppmv in SWV is obtained in the
middle stratosphere (30 km) at high latitudes, but only half
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shown in Figure 2c, with also maximum changes up to
20 ppbv/year.
3.2. Radiative Forcing
[12] Table 1 shows results of the radiative forcing calculations due to the increase in SWV from CH4 oxidation.
Results are given for changes since 1750 (the beginning of
the industrial era), 1950, and 1979 until present. The net
radiative forcing over the industrial era of 0.083 Wm 2
consists of a longwave forcing of 0.10 Wm 2 and a
shortwave forcing of 0.02 Wm 2. This net forcing is
slightly larger than that presented by Hansen et al. [2005]
based on a two-dimensional model with a radiative forcing
of 0.07 Wm 2 over the period 1750 to 2000, but much
larger than the value of 0.02 Wm 2 over the period 1850–
1992 estimated by Lelieveld et al. [1998]. The radiative
forcing of the indirect effect of CH4 oxidation to SWV is
15– 20% of the direct radiative forcing due to methane.
Note that this result is of significance for the inclusion of the
indirect effect of stratospheric water vapour changes on the
Global Warming Potential for methane. In recent IPCC
reports [e.g., Intergovernmental Panel on Climate Change
(IPCC), 2001] it is assumed that water vapour contributes
up to 5% of the forcing due to methane; our results indicate
that this figure should be much higher, and indeed closer to
the values given in earlier IPCC reports [see IPCC, 1995,
section 4.2.2]. Further, the radiative forcing from 1950 to
present is 60% of the methane-induced SWV forcing over
the industrial era. Our estimate of SWV from CH4 oxidation
accounts for less than 15% of the upper estimate of radiative
forcing of total SWV change since 1950 by Forster and
Shine [2002]. Additional simulations we have performed
show that SWV change from CH4 oxidation above 30 km is
of relatively minor importance in terms of the radiative
forcing of SWV changes.
Figure 2. Stratospheric water vapour (ppmv) increase
from CH4 oxidation (a) from 1750 until 2000, (b) from 1950
until 2000, and (c) from 1979 until 2000.
of this at the same height over the equator. At lower levels
in the stratosphere the SWV from CH4 oxidation gives only
a small percentage change. Therefore, CH4 oxidation can
only partly explain the proposed trend in SWV since 1950.
The change in SWV from CH4 oxidation since 1979 is
3.3. Temperature Change
[13] To illustrate the importance of the vertical profile of
the SWV change Figure 3a shows the calculated temperature change in the stratosphere based on a constant SWV
change from 6.0 to 6.7 ppmv throughout the stratosphere.
The cooling is around 0.5K in the lower part of the
stratosphere at high latitudes and weaker at higher altitudes
as well as in the whole tropical stratosphere. The pattern is
very similar to the simulations of Forster and Shine [1999]
for the same SWV change. The more realistic change in the
SWV profile of Shine et al. [2003]; Smith et al. [2001] show
weaker cooling for SWV than for a constant profile. Figure
3b shows the temperature change for SWV increase due to
CH4 oxidation based on the HALOE data over the industrial
era. Importantly the temperature change in this case differs
significantly from the change with constant SWV change in
Figure 3a. The cooling is shifted upward and we find a
warming in the lower stratosphere. This warming is a result
of amplified absorption of downwelling longwave radiation
due to increased SWV at higher altitudes. The cooling in the
lower stratosphere in Figure 3a is mainly a result of local
cooling from the SWV increase. This local cooling is almost
absent in the results in Figure 3b since the lower SWV
change due to CH4 oxidation is so small. Cooling of more
than 1 K at high latitudes upward from the middle stratosphere and globally at 50 km is simulated. Observations
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Table 1. Radiative Forcing Due to SWV Change From CH4
Oxidation and the Direct Radiative Forcing Due to CH4
Radiative Forcing, Wm
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responding values since 1979 are approximately a factor
of 3 lower.
2
Period
SWV Change From CH4
Direct Due to CH4
1750 – 2000
1950 – 2000
1979 – 2000
0.083
0.050
0.016
0.48
0.24
0.087
show that the upper stratosphere is cooling [Shine et al.,
2003] with models indicating that ozone depletion and the
increase in CO2 are the dominant contributors. Figures 3c
and 3d show the temperature change from SWV from CH4
oxidation over the period since 1950 and 1979, respectively.
For 1950 to present the simulated temperature change is less
than 0.2 K warming in the lower stratosphere and a
maximum cooling around 50 km of 0.7 K. The cor-
4. Summary
[14] In this study we use satellite data from HALOE of
CH4 to estimate the SWV change from CH4 oxidation and
the corresponding radiative forcing. The simulations presented here indicate a radiative forcing slightly weaker than
0.1 Wm 2 and that a significant fraction of the forcing has
occurred since 1950. The forcing from water vapour is
approximately 15 – 20% of the direct methane forcing,
which is much higher than the value of 5% used in recent
IPCC reports for the water vapour contribution to the global
warming potential for methane.
[15] An important result from this study is that the
temperature change due to increases in SWV from CH4
oxidation is quite different from a homogeneous SWV
change. Shine et al. [2003]; Smith et al. [2001] showed
Figure 3. Temperature change (K) from stratospheric water vapour: (a) change in SWV from 6.0 to 6.7 ppmv, (b) change
in SWV from CH4 oxidation in the period from 1750 until 2000, (c) change in SWV from CH4 oxidation in the period from
1950 until 2000, and (d) change in SWV from CH4 oxidation in the period from 1979 until 2000.
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that HALOE deduced SWV changes result in different
temperature change compared to homogeneous SWV
change. Actually, the results presented here show a small
increase in temperature just above the tropopause for the
SWV increase from CH4 oxidation. Therefore, emphasis
should be made to investigate further the change in the
vertical profile of SWV. The contribution to SWV from CH4
increases since pre-industrial time seems to account for only
a fraction of the reported increase, particularly in the lower
stratosphere, although these observations are uncertain.
Furthermore, the impact on the stratospheric temperature
trend from SWV increase is strongly dependent on the
height profile of the increase. Therefore, both the magnitude
of the SWV increase and impact on stratospheric temperatures are uncertain. Further modeling of the SWV from
CH4 oxidation should be performed to investigate the
importance of increased chlorine level in the atmosphere
and its impact on CH4 destruction in the stratosphere. If this
reaction is of large importance our SWV change may be
slightly overestimated. However, global transport models
that should be used for such purposes must have a top
boundary above the stratosphere, otherwise the boundary
conditions decide to a large extent the SWV change, since a
large portion of the CH4 destruction occurs in the upper
stratosphere.
[16] The largest uncertainty in our estimate of SWV
change and the resulting radiative forcing is probably
related to the observed vertical profile of CH4. It is shown
that the vertical profile in the CH4 from HALOE has some
interannual variability [Ruth et al., 1997]. This interannual
variability is also found in high precision balloon-borne
measurements [Patra et al., 2003] and importantly the
gradient in the vertical profile of CH4 from these measurements compare well with HALOE data.
[17] Acknowledgments. GM acknowledges The Research Council of
Norway project 165533/S30 and KPS acknowledges NERC grant NERC/L/
S/2001/00661.
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L. Gulstad, I. S. A. Isaksen, G. Myhre, and B. Rognerud, Department of
Geosciences, University of Oslo, PO Box 1022, Blindern, N-0315 Oslo,
Norway. (gunnar.myhre@geo.uio.no)
J. S. Nilsen, Ullevål University Hospital, N-0407 Oslo, Norway.
K. P. Shine, Department of Meteorology, University of Reading, Earley
Gate, P.O. Box 243, Reading RG6 6BB, UK.
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