JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, D01107, doi:10.1029/2003JD004320, 2004
Updated radiative forcing estimates of four halocarbons
L. K. Gohar
Department of Meteorology, University of Reading, Reading, UK
G. Myhre
Department of Geosciences, University of Oslo, Oslo, Norway
K. P. Shine
Department of Meteorology, University of Reading, Reading, UK
Received 3 November 2003; accepted 14 November 2003; published 13 January 2004.
[1] There is a large discrepancy (greater than 20%) between two recently published
estimates of radiative forcing of four hydrofluorocarbons (HFCs), two of which, HFC-23
and HFC-134a, are the most abundant HFCs in the atmosphere. We report an
intercomparison, using two different radiative transfer methods, aimed at clarifying the
forcing values for HFC-23 and HFC-134a, as well as HFC-227ea and HFC-32. The
calculated global, annual mean radiative forcings differed by 12% or less and are within
INDEX TERMS: 1699 Global Change:
the expected errors in the radiative forcing estimates.
General or miscellaneous; 3359 Meteorology and Atmospheric Dynamics: Radiative processes; 3399
Meteorology and Atmospheric Dynamics: General or miscellaneous; KEYWORDS: radiative forcing,
halocarbons
Citation: Gohar, L. K., G. Myhre, and K. P. Shine (2004), Updated radiative forcing estimates of four halocarbons, J. Geophys. Res.,
109, D01107, doi:10.1029/2003JD004320.
1. Introduction
[2] Assessing the relative importance of the potential
impacts of hydrofluorocarbons (HFCs) and similar gases,
some of which are chlorofluorocarbon replacement gases,
can be done by a clear, comprehensive, and consistent set of
forcing calculations. Two such forcing calculations have
been reported recently by Sihra et al. [2001], who document
65 gases, and by Jain et al. [2000], who report forcing
estimates of 39 gases. The majority of forcings for gases
common to both studies agreed to within 10%. However,
four HFCs, HFC-134a, HFC-227ea, HFC-23, and HFC-32,
disagreed by greater than 20%. This was noted by Sihra et
al. [2001], who could not fully explain the reasons for the
differences and stated that the differences in absorption
cross section could only partly explain the discrepancies
for two of the four gases. Both studies used similar radiative
transfer models, and so this appeared unlikely to be a large
contributor to the discrepancies. To resolve the large differences in forcing estimates, we performed an intercomparison of the radiative forcing estimates for the four HFCs.
[3] The particular need to resolve the discrepancies arises
from the fact that radiative forcing values are used to
generate the global warming potentials (GWPs) [e.g.,
Intergovernmental Panel on Climate Change (IPCC),
2002], which are used within, for example, the Kyoto
Protocol to the United Nations Framework Convention on
Climate Change. The Kyoto Protocol adopts values from
IPCC [1996], but any future protocols are likely to require
Copyright 2004 by the American Geophysical Union.
0148-0227/04/2003JD004320$09.00
updated values, particularly as IPCC has adopted a modified
per molecule radiative forcing for CO2 in its calculations of
GWPs [IPCC, 2002]. Note in particular that two of the four
gases studied here, HFC-23 and HFC-134a, are the two
most abundant HFCs in the atmosphere, and so it is
especially important that there is confidence in their radiative forcing values.
2. Method
[4] Two different methods of calculating the forcing were
used here. In one method (‘‘Reading’’) we used a narrowband model (NBM) [Shine, 1991] and a line-by-line radiative transfer model (RFM) [Dudhia, 1997] to calculate the
cloudy sky adjusted forcing as described by Sihra et al.
[2001]. The second method (‘‘Oslo’’) used a discreteordinate radiative transfer code (DISORT) [Stamnes et al.,
1988] with a line-by-line model, GENLN2 [Edwards,
1992], as in the work of Myhre and Stordal [1997] with
the incorporation of cloudy sky adjusted radiative forcing
(G. Myhre et al., manuscript in preparation, 2003).
[5] The atmospheric profiles of temperature, cloud cover,
water vapor, carbon dioxide, ozone, methane, and nitrous
oxide concentrations used in the two methods were the
standard profiles used by the two groups. We deliberately
chose not to adopt identical profiles to see if this was a
possible source of discrepancies. The input to the Reading
method used three vertical profiles to represent the global
mean, and the Oslo method used two vertical profiles, all
derived from recent global analyses.
[6] The absorption cross-section spectra for the four
HFCs were derived at the Ford Motor Company; details
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GOHAR ET AL.: BRIEF REPORT
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Table 1. Integrated Absorption Cross Sections of the Spectra
Used in the Present Work (200 – 2000 cm 1) and Those Employed
in the Work of Sihra et al. [2001] (450 – 2000 cm 1)a
Integrated Cross Section,
10 17 cm2 molecules 1 cm
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Table 2. Global, Annual Mean Clear-Sky and Cloudy Sky
Instantaneous Radiative Forcing and Cloudy Sky Adjusted
Radiative Forcing for a 1.0 ppbv Uniform Change
Radiative Forcing, W m
2
ppbv
1
1
Gas
Ford 2003
Ford Data Given
by Sihra et al. [2001]
HFC-134a
HFC-227ea
HFC-23
HFC-32
13.07
23.27
11.82
5.77
12.40
23.04
11.61
5.65
Instantaneous
Clear
Instantaneous
Cloudy
Adjusted
Cloudy
Gas
Reading
Oslo
Reading
Oslo
Reading
Oslo
HFC-134a
HFC-227ea
HFC-23
HFC-32
0.199
0.313
0.231
0.143
0.199
0.300
0.228
0.146
0.156
0.249
0.178
0.110
0.149
0.229
0.171
0.110
0.175
0.279
0.200
0.121
0.164
0.251
0.187
0.119
a
Spectra were recorded at 296 K in 700 torr of air diluent (Ford Motor
Company).
of the observational techniques are given in the work of
Christidis et al. [1997]. The integrated absorption crosssection values are given in Table 1. These absorption cross
sections are updated spectra to those described by Sihra et
al. [2001] and include spectral structure in the region down
to 200 cm 1. The integrated absorption cross section of
HFC-134a has increased by 5%, bringing it into better
agreement with studies including the lower wave number
bands [Highwood and Shine, 2000]; the other three HFCs
cross sections’ increased by 2% or less. Note that Jain et al.
[2000] also used the Ford cross-section data in their
calculations, and their values should have been similar to
the values in the final column of Table 1, although they do
not report the integrated cross sections.
[7] The radiative forcings are initially calculated for a
0.1-ppbv uniform vertical profile perturbation to remain in
the weak limit; the values were then scaled up to obtain per
ppbv radiative forcing, which is normally reported. The
instantaneous clear-sky and cloudy sky radiative forcings
were calculated first, and then the cloudy sky adjusted
radiative forcings were calculated. Finally, the lifetime
adjusted values, which account for the falloff in concentrations above the tropopause, were calculated following the
method detailed by Sihra et al. [2001] to compare with Jain
et al. [2000].
3. Results
[8] Table 2 shows the global, annual mean, clear-sky,
cloudy sky, and cloudy sky adjusted forcings. Consider the
clear-sky instantaneous radiative forcing in Table 2. The
differences between values from the two methods are small
(0 – 4%) and presumably reflect small differences in the
radiation codes and vertical profiles used at Reading and
Oslo. To provide insight into the relative magnitude of these
two factors, the Oslo profiles were used in the Reading
model to calculate the forcing of HFC-134a; this changed
the result by 2%. We conclude that the differences in the
radiation codes and vertical profiles used at Reading and at
Oslo contribute approximately equally to the small differences in the results evident in Table 2. In the case of the
clear-sky calculation for HFC-134a these two factors compensated for each other.
[9] The instantaneous cloudy sky forcings are also given
in Table 2. The addition of clouds reduces the radiative
forcing estimates for all gases, with a greater reduction for
Oslo than for Reading, even though both models yield
realistic values of the global mean, outgoing longwave
radiation at the top of the atmosphere (235 W m 2).
The difference between the two methods increases for all
gases, except HFC-32, because of the different cloud
descriptions used. The largest difference between the two
methods is for HFC-227ea and is approximately 8%.
[10] The cloudy sky adjusted radiative forcings are shown
in the final two columns of Table 2. The average difference
in values between the two methods for the cloudy sky
adjusted radiative forcing is approximately the same as for
the cloudy sky instantaneous forcing, and the stratospheric
temperature adjustment does not generally increase the
average difference in forcing values in the two methods.
The largest difference (11%) is still for HFC-227ea, and
again this is mostly due to the different cloud specifications.
[11] The lifetime adjusted global, annual mean, cloudy
sky radiative forcings for the two methods and Jain et al.
[2000] are given in Table 3. The lifetime adjustment forcing
is calculated following the method given by Sihra et al.
[2001]. Note that the values for the Reading results differ
slightly from the Sihra et al. [2001] values not only because
of the revised Ford cross sections (Table 1) but also because
cross sections from a variety of sources, where available,
were used in that work. Our results are in much closer
agreement (12% or better) with each other than with the
forcings of Jain et al. [2000]. The average of our forcings
is lower than the Jain et al. [2000] values by 25% for
HFC-134a and HFC-227ea, 33% for HFC-23, and 40% for
HFC-32.
4. Conclusions
[12] The new calculations presented in this note show a
satisfactory (12% or better) agreement of the radiative
forcing of the four halocarbons, HFC-134a, HFC-227ea,
Table 3. Global, Annual Mean, and Cloudy Sky Adjusted
Radiative Forcing for a 1.0 ppbv Change After Applying the
Adjustment for Lifetime Described by Sihra et al. [2001], With the
Jain et al. [2000] Values Given in the Third Column
Radiative Forcing, W m
2
ppbv
1
Lifetime Adjusted
Gas
Reading
Oslo
Jain et al. [2000]
HFC-134a
HFC-227ea
HFC-23
HFC-32
0.166
0.271
0.193
0.111
0.155
0.243
0.181
0.110
0.200
0.322
0.248
0.155
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GOHAR ET AL.: BRIEF REPORT
HFC-23, and HFC-32, and are between about 25 and 40%
lower than those given by Jain et al. [2000]. We still do not
have a satisfactory explanation for this discrepancy, but we
believe that the level of agreement between the two independent calculations presented here favors the use of the
lower values. Despite the higher level of agreement between
the Reading and Oslo results, we note that the agreement
between the two models was not perfect, and clearly
methodological differences in the way clouds are included
in the models are enough to generate differences of up to
12% in the forcing values of some gases.
[13] Acknowledgments. L. K. Gohar was funded by NERC grant
NER/L/S/2001/00661 and the EC project CRYOSTAT (EV2K-CT-2001000116). We thank K. Sihra for his help, Tim Wallington for his useful
comments, and T. Wallington, M. D. Hurley, C. Basher, and O. J. Neilsen
for providing the updated spectrum.
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L. K. Gohar and K. P. Shine, Department of Meteorology, University of
Reading, Earley Gate, Reading, RG6 6BB, UK. (l.k.gohar@reading.ac.uk;
k.p.shine@reading.ac.uk)
G. Myhre, Department of Geosciences, University of Oslo, P. O. Box
1022 Blindern, N-0315 Oslo, Norway. (gunnar.myhre@geofysikk.uio.no)
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