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There is growing international interest in mitigating climate change during the early part of this century by reducing emissions of short-lived climate pollutants (SLCPs), in addition to reducing emissions of CO2. The SLCPs include methane (CH4), black carbon aerosols (BC), tropospheric ozone (O3) and hydrofluorocarbons (HFCs). Recent studies have estimated that by mitigating emissions of CH4, BC, and O3 using available technologies, about 0.5 to 0.6 °C warming can be avoided by mid-21st century. Here we show that avoiding production and use of high-GWP (global warming potential) HFCs by using technologically feasible low-GWP substitutes to meet the increasing global demand can avoid as much as another 0.5 °C warming by the end of the century. This combined mitigation of SLCPs would cut the cumulative warming since 2005 by 50% at 2050 and by 60% at 2100 from the CO2-only mitigation scenarios, significantly reducing the rate of warming and lowering the probability of exceeding the 2 °C warming threshold during this century.
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Atmos. Chem. Phys., 13, 6083–6089, 2013
www.atmos-chem-phys.net/13/6083/2013/
doi:10.5194/acp-13-6083-2013
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The role of HFCs in mitigating 21st century climate change
Y. Xu
1
, D. Zaelke
2
, G. J. M. Velders
3
, and V. Ramanathan
1
1
Scripps Institution of Oceanography, UC San Diego, 9500 Gilman Dr., La Jolla, CA 92093, USA
2
Program on Governance for Sustainable Development, Bren School of Environmental Science & Management,
UC Santa Barbara, CA 93106, USA
3
National Institute for Public Health and the Environment (RIVM), P.O. Box 1, 3720 BA Bilthoven, the Netherlands
Correspondence to: Y. Xu (yangyang@ucsd.edu)
Received: 21 November 2012 – Published in Atmos. Chem. Phys. Discuss.: 18 December 2012
Revised: 16 May 2013 – Accepted: 23 May 2013 – Published: 26 June 2013
Abstract. There is growing international interest in mitigat-
ing climate change during the early part of this century by re-
ducing emissions of short-lived climate pollutants (SLCPs),
in additionto reducing emissions of CO
2
. The SLCPs include
methane (CH
4
), black carbon aerosols (BC), tropospheric
ozone (O
3
) and hydrofluorocarbons (HFCs). Recent studies
have estimated that by mitigating emissions of CH
4
, BC, and
O
3
using available technologies, about 0.5 to 0.6
C warm-
ing can be avoided by mid-21st century. Here we show that
avoiding production and use of high-GWP (global warming
potential) HFCs by using technologically feasible low-GWP
substitutes to meet the increasing global demand can avoidas
much as another 0.5
C warming by the end of the century.
This combined mitigation of SLCPs would cut the cumula-
tive warming since 2005 by 50% at 2050 and by 60% at
2100 from the CO
2
-only mitigation scenarios, significantly
reducing the rate of warming and lowering the probability of
exceeding the 2
C warming threshold during this century.
1 Introduction
The ozone depleting substances (ODSs) (e.g., chloroflu-
orocarbons (CFCs), hydrochlorofluorocarbons (HCFCs)),
halons, and HFCs are part of a family of gases known as
halocarbons. Halocarbons are used as refrigerants, propel-
lants, cleaning and foam blowing agents, and fire extinguish-
ers, etc. Molina and Rowland (1974) identified the potent
stratospheric ozone depleting effects of CFCs. This was fol-
lowed, within a year, by the discovery of the potent green-
house effect of the halocarbons CFC-11 and CFC-12 (Ra-
manathan, 1975). Many studies confirmed this finding and
estimated that the global warming potential (GWP) of CFC-
11 and CFC-12 (using a 100yr time horizon) at 4750 and
10900 respectively, as summarized by the Intergovernmental
Panel on Climate Change Fourth Assessment Report (Forster
et al., 2007). Ramanathan (1975) set the stage for identifying
numerous other non-CO
2
greenhouse gases (GHGs) in the at-
mosphere such as CH
4
and O
3
among others (see Wang et al.,
1976 and Ramanathan et al., 1985a). The first international
assessment of the climate effects of non-CO
2
gases was con-
ducted in 1985 (Ramanathan et al., 1985b) and it concluded
that CO
2
was the dominant contributor to greenhouse forcing
until 1950s, and since the 1960s non-CO
2
gases have begun
to contribute as much as CO
2
. A more recent list of the non-
CO
2
GHGs can be found in Pinnock et al. (1995) and Forster
et al. (2007).
Most of HFCs now in use, along with CH
4
, O
3
, and BC
(black carbon aerosols), have relatively short lifetimes in the
atmosphere in comparison with long-lived GHGs, such as
CO
2
and N
2
O (nitrous oxide) (e.g., see Smith et al., 2012),
and are therefore referred to as short-lived climate pollutants
(SLCPs). The lifetime of BC is several days to weeks, tro-
pospheric O
3
is a few months, and CH
4
is about 12yr. The
global average lifetime, weighted by the production of the
various HFCs now in commercial use, is about 15yr, with
a range of 1 to 50yr (Table 1). Because the lifetimes of the
SLCPs are much shorter than that of CO
2
, a significant por-
tion of which remains in the atmosphere for centuries to mil-
lennia, the radiative forcing by SLCPs will decrease signif-
icantly within weeks to a few decades after their emissions
are reduced.
Motivated by modeling studies (e.g., Ramanathan and Xu,
2010; Shindell et al., 2012), policy makers are showing
Published by Copernicus Publications on behalf of the European Geosciences Union.
6084 Y. Xu et al.: The role of HFCs in mitigating 21st century climate change
Table 1. Indicative applications using HFCs
.
HFC Indicative applications Lifetime 100yr GWP
UNEP and WMO (2011) WMO (2011) WMO (2011)
HFC-134a Mobile and stationary air conditioning and refrigeration, foams, 13.4 1370
medical aerosols and cosmetic and convenience aerosol products
HFC-32 In blends for refrigeration and air conditioning 5.2 716
HFC-125 In blends for refrigeration and air conditioning 28.2 3420
HFC-143a In blends for refrigeration and air conditioning 47.1 4180
HFC-152a Foams, aerosol products 1.5 133
HFC-227ea Foams, medical aerosols, fire protection 38.9 3580
HFC-245fa Foams 7.7 1050
HFC-365mfc Foams 8.7 842
HFC-43-10mee Solvents 16.1 1660
HFC-23 is not included in the scenarios discussed here. Although it is currently the second most abundant HFC in the atmosphere, it is assumed that the
majority of this chemical is produced as a byproduct of HCFC-22 production and not because of its use as a replacement for CFCs and HCFCs. Hence, the
emissions of HFC-23 depend on a different set of assumptions than the other HFCs (Velders et al. 2009). Miller and Kuijpers (2011) estimated that HFC-23
emissions increase could contribute 0.014Wm
2
to radiative forcing in 2050. Therefore, the contributed warming due to potential HFC-23 will be only
about 0.01
C by our estimation.
increasing interest in fast-action climate mitigation strategies
that target SLCPs (Wallack and Ramanathan, 2009; Molina
et al., 2009). Ramanathan and Xu (2010) (hereafter RX10)
concluded that as much as 0.6
C warming can be avoided
by mid-21st century using current technologies to reduce
all four SLCPs, with mitigation of HFCs contributing about
20% (0.1
C) to the avoided warming by 2050. Furthermore,
RX10 also showed that exceeding the 2
C warming thresh-
old can be delayed by three to five decades beyond 2050
by these efforts. Based on an international assessment com-
missioned by the United Nations Environment Programme
(UNEP) and the World Meteorological Organization (WMO)
(UNEP and WMO, 2011), Shindell et al. (2012) used a
3-dimensional climate model to account for reductions in
CH
4
, O
3
, and BC emissions (but not HFCs) using mitiga-
tion scenarios similar to those employed in RX10. UNEP
and WMO (2011) as well as Shindell et al. (2012) calcu-
lated the avoided warmingto be0.5(± 0.05)
C by 2070. This
estimate is consistent with RX10, which would also yield
0.5
C avoided warming if only CH
4
, O
3
, and BC were mit-
igated. All three studies calculated that full implementation
of mitigation measures for these three SLCPs can reduce the
rate of global warming during the next several decades by
nearly 50%. Furthermore, Arctic warming can be reduced
by two-thirds over the next 30yr compared to business as
usual (BAU) scenarios (UNEP and WMO, 2011).
However, with the exception of the RX10 study, HFCs
have thus far not been included in analyses of the temper-
ature mitigation benefit from SLCP mitigation. Even RX10
did not recognize the full potential of the radiative forcing
increase, as shown recently by Velders et al. (2012), due to
an unconstrained use of HFCs toward the end of this century.
Therefore, what has been missing in the previous studies is
the potentially large increase in HFC use. The present study
builds upon RX10 to further account for the newly devel-
oped projections of HFC emissions and provides a detailed
analysis of the implication of HFC mitigation on global tem-
perature.
2 Methods
2.1 HFC emission projection
Because of their catalytic destruction of stratospheric ozone,
production and consumption of CFCs, HCFCs and other
ODSs are being phased out under the Montreal Protocol (An-
dersen and Sarma, 2002; Andersen et al., 2007). With the
phase-out of CFCs under the Montreal Protocol completed
in 1996 in developed countries and in 2010 in developing
countries (UNEP, 2010), and with the scheduled phase-out
of HCFCs by 2030 in developed countries, and 2040 in de-
veloping countries (UNEP, 2007), HFCs are increasingly be-
ing used as alternatives in applications that traditionally used
CFCs, HCFCs and other ODSs to meet much of the de-
mand for refrigeration, air conditioning, heating and thermal-
insulating foam production (Velders et al., 2012). HFCs do
not destroy the ozone layer (Ravishankara et al., 1994) but
are potent GHGs (Velders et al., 2009).
The demand for HFCs is expected to increase in both de-
veloped and developing countries, especially in Asia, in the
absence of regulations, as is the demand for HCFCs for feed-
stock (Velders et al., 2009). HFCs are the fastest growing
GHGs in the US, where emissions grew nearly 9 % between
2009 and 2010 compared to 3.6% for CO
2
(EPA, 2012).
Globally, HFC emissions are growing 10 to 15 % per year
and are expected to double by 2020 (WMO, 2011; Velders et
al., 2012). The presence of HFCs in the atmosphere results
almost completely from their use as substitutes for ODSs
(Table 1).
Atmos. Chem. Phys., 13, 6083–6089, 2013 www.atmos-chem-phys.net/13/6083/2013/
Y. Xu et al.: The role of HFCs in mitigating 21st century climate change 6085
The future HFC projection in this study is estimated us-
ing (1) the growth rates of gross domestic product (GDP)
and populations from the Special Report on Emissions Sce-
narios (SRES) (IPCC, 2000), and (2) the replacement pat-
terns of ODSs with HFCs and not-in-kind technologies as
observed in the past years in Western countries. We assumed
that these replacement patterns will stay constant and will
hold for developing countries. The European Union regu-
lation (842/2006) aimed at moving away from high-GWP
HFCs is also included in the HFC projections used here.
Readers are referred to Velders et al. (2009) for more de-
tails in HFC scenario development (e.g., emissions by sub-
stance, region and years). Because the projected forcing from
HFC-23 is much smaller than that from intentionally pro-
duced HFCs, it is not included in this study. In spite of poten-
tial large increases in HFC-23 from the continued production
of HCFC-22 for feedstock, the HFC-23 forcing in 2050 is
0.014Wm
2
(Miller and Kuijpers, 2011) and the associated
warming is only about 0.01
C.
2.2 Other emission projection
The future emission scenarios of CO
2
are adopted from the
Representative Concentration Pathway (RCP, van Vuuren et
al., 2011) database. We take RCP 2.6 (van Vuuren et al.,
2007) as mitigation case and RCP 6.0 (Hijioka et al., 2008)
as BAU case for CO
2
. CO
2
emissions in the mitigation case
will decline by half in mid-21st century, while the BAU CO
2
emissions are projected to continue to increase until 2080.
The peak CO
2
atmospheric concentration is 660 and 440
ppm under BAU and mitigation cases, respectively. We note
that CO
2
scenarios under RCP 6.5 and 2.6 may have differ-
ent assumptions with regard to emission sectors and there-
fore the difference between those two pathways may not di-
rectly represent the effect of mitigation efforts. The SLCP
projections, except for HFCs, are retained from RX10. Under
a BAU scenario, CH
4
emissions are predicted to rise by 40%
in 2030, and BC emissions are projected to increase by 15%
by 2015 and then level off. The mitigation scenarios follow
recommendations from studies by the International Institute
for Applied Systems Analysis (IIASA) (Cofala et al., 2007)
and the Royal Society (2008) that maximum feasible reduc-
tions of air pollution regulations can result in reductions of
50% in CO emissions and 30% in CH
4
emissions by 2030,
as well as reductions of 50% in BC emissions by 2050.
2.3 Models
The model used in RX10 is an integrated carbon and radi-
ant energy balance model. It adopts the Bern CO
2
geochem-
istry model (Joos et al., 1996) to estimate the atmospheric
CO
2
concentration from emissions. The model links emis-
sions of pollutants with their atmospheric concentrations and
the change in the radiative forcing. The carbon-geochemistry
model is then integrated with an energy balance climate
24
422
Fig 1. HFC radiative forcing change (W/m2) since the year of 2005. Note we include both upper 423
(red solid line) and lower limit (red dash line) of HFC growth under BAU scenarios. The 424
scenarios previously adopted in RX10 and from various other sources are shown for reference. 425
426
Fig. 1. HFC radiative forcing change (W m
2
) since the year of
2005. Note we include both upper (red solid line) and lower limits
(red dash line) of HFC growth under BAU scenarios. The scenar-
ios previously adopted in RX10 and from various other sources are
shown for reference.
model with a 300 m ocean mixed layer and a climate sen-
sitivity of 0.8 (0.5 to 1.2)
C/(Wm
2
) to simulate the tempo-
ral evolution of global mean surface temperature. The model
also accounts for historical variations in the global mean ra-
diative forcing to the system attributable to natural factors,
GHGs and air pollutants including sulfates, nitrates, carbon
monoxide, ozone, BC, and organic carbons. The model is ca-
pable of simulating the observed historical temperature vari-
ations (Fig. 2), as well as the historical CO
2
concentration
and ocean heat content (Box 1 in RX10).
3 Results and discussions
3.1 Large increase of HFC forcing
The radiative forcing of HFCs in 2008 was small at less
than 1% of the total forcing from long-lived GHGs (WMO,
2011). However, without fast action to limit their growth,
the radiative forcing of HFCs could increase from nearly
0.012Wm
2
in 2010 to up to 0.4Wm
2
in 2050 (BAU high
in Fig. 1). The 0.4 Wm
2
is equal to nearly 30 to 45% of
CO
2
forcing increase by 2050 (if CO
2
follows BAU and mit-
igation scenarios, respectively; see Sect. 2.2 for scenario de-
scriptions), or about the same forcing contributed by current
CO
2
emissions from the transportation sector (IEA, 2011).
In the scenarios discussed here, the demand for HFCs for the
period 2050 to 2100 is assumed to maintain at the 2050 lev-
els (assuming complete market saturation), which results in
increasing HFC abundances and radiative forcing past 2050,
with HFC forcing possibly reaching as high as 0.8Wm
2
in
2100 (BAU high in Fig. 1).
www.atmos-chem-phys.net/13/6083/2013/ Atmos. Chem. Phys., 13, 6083–6089, 2013
6086 Y. Xu et al.: The role of HFCs in mitigating 21st century climate change
25
427
Fig 2. Model simulated temperature change under various mitigation scenarios that include CO
2
428
and SLCPs (BC, CH
4
, HFCs). BAU case (red solid line with spread) considers both high and low 429
estimates of future HFC growths (as shown in red solid and dash lines in Fig. 1). Note this 430
uncertainty of temperature projection related to HFC scenarios is around 0.15°C at 2100. The 431
vertical bars next to the curve show the uncertainty of temperature projection at 2100 due to 432
climate sensitivity uncertainty. For simplicity, we only show the cases of CO
2
mitigation (red 433
dash line) and full mitigation (black line). 434
Fig. 2. Model simulated temperature change under various miti-
gation scenarios that include CO
2
and SLCPs (BC, CH
4
, HFCs).
BAU case (red solid line with spread) considers both high and low
estimates of future HFC growths (as shown in red solid and dash
lines in Fig. 1). Note this uncertainty of temperature projection re-
lated to HFC scenarios is around 0.15
C at 2100. The vertical bars
next to the curve show the uncertainty of temperature projection at
2100 due to climate sensitivity uncertainty. For simplicity, we only
show the cases of CO
2
mitigation (red dash line) and full mitigation
(black line).
We also calculate HFC forcing from emission data pro-
vided by the RCP database and compare them with our forc-
ing projections. Similar comparisons of future emission and
forcing are also shownin Fig. 5.5 of the WMO (2011) assess-
ment. However, a direct comparison is difficult, because with
the exception of RCP 8.0 (Riahi et al., 2007), the scenarios
include various mitigation policies as assumptions and there-
fore cannot be considered as “BAU” scenarios. The Global
Change Assessment Model (GCAM) group that produces
RCP 4.5 (Wise et al., 2009; Thomson et al., 2011) does,
however, make available a “BAU” scenario (GCAM base-
line in Fig. 1), which does not include explicit mitigation ac-
tions (Smith et al., 2011). As a result, HFC forcing is two
times larger in the GCAM baseline scenario than the asso-
ciated RCP 4.5 scenario. The HFC BAU projections used in
this study are substantially higher over the long-term than the
projections of other studies, including RCP 8.0 and GCAM
baseline. In the GCAM baseline, for example, HFC forcing
increase from 2005 is less than 0.2Wm
2
in 2100, as com-
pared to 0.5 to 0.8Wm
2
in our BAU scenarios.
There are several reasons for the discrepancies in HFC
projections. (1) HFC scenarios have not received much at-
tention in the development of the RCP database. Individual
integrated assessment modeling groups have adopted vari-
ous assumptions and techniques in developing those RCP
projections and associated reference scenarios. However, de-
tailed descriptions of HFC projection in RCPs and associ-
ated reference scenarios are not available in relevant papers,
which are more focused on long-lived GHGs. (2) Most, if
not all, of the RCP scenarios for HFCs were developed be-
fore 2007. Therefore, they did not take into account the ac-
celerated HCFC phase-out in both developing and developed
countries agreed by the parties at the 19th Meeting of the Par-
ties to the Montreal Protocol in September 2007, which will
lead to lower future HCFC emissions and higher HFC emis-
sions (Meinshausen et al., 2011). (3) At least some RCP sce-
narios did not take into account the large observed growth in
HFC use and atmospheric concentrations since 2000 (WMO,
2011). The linear growth in RCP scenarios (Fig. 1 and also
see Fig. 3.22 of Clarke et al., 2007) is distinctly different
from the assumptions of a growing market in developing
countries, which was the basis of our HFC scenario. (4) Fi-
nally, another recent HFC scenario up to 2050 (Gschreyet al.,
2011), which includes detailed market analysis, also shows
emissions much higher than RCPs, but smaller than ours.
The smaller emissions in Gschrey et al. (2011) compared to
ours are the result of two assumptions: first, a larger frac-
tion of non-fluorocarbon alternatives in several sectors; and
second, and second, the market saturation in several sectors
after about 2030, 10yr earlier than in our scenarios, which
considered consumption saturation for a few sectors at 2040
(Fig. 1b of Velders et al., 2009) and a complete saturation
after 2050. Note that the emission of HFCs will continue in-
creasing for a short time period after the market saturation at
2050, and the mixing ratio and forcing of HFC will further
grow toward end of 21st century (Fig. 1). We acknowledge
that some RCP models project HFC in a detailed method
(e.g., by gas and sector based on the evolution of multiple
drivers over time including vehicle demand, building air-
conditioning use, etc.). Some RCP models and Gschrey et
al. (2011) have also assumed some emission drivers do not
scale with GDP over the long-term due to saturation effects
(e.g., floor space of buildings), which could be even more
important on a century timescale. The differences in consid-
ering saturation effects may be a reason that our projections
yield larger emissions.
In conclusion, differences in HFC scenarios arise from
large differences in their underlying assumptions and to the
level they take into account recent information. HFC projec-
tions insome RCP scenarios do not use themore recent infor-
mation as in Velders et al. (2009) and Gschrey et al. (2011).
The scenarios in Velders et al. (2009) are based on assump-
tions similar to those of IPCC-SRES with respect to growth
rates in GDP and population, but have incorporated new cur-
rent informationon (1) reported recent increases in consump-
tion of HCFCs in Article 5 (developing) countries of about
20% per year (through 2007), (2) replacement patterns of
HCFCs by HFCs as reported in non-Article 5 (developed)
countries, and (3) accelerated phase-out schedules of HCFCs
in Article 5 and non-Article 5 countries (2007 Adjustment of
the Montreal Protocol). We note that this HFC scenario is not
necessarily a more accurate forecast of future HFC emissions
than other scenarios, but a projection of what can happen if
Atmos. Chem. Phys., 13, 6083–6089, 2013 www.atmos-chem-phys.net/13/6083/2013/
Y. Xu et al.: The role of HFCs in mitigating 21st century climate change 6087
developed countries continue current practices in replacing
ODSs with HFCs and if developingcountries follow this path
as well.
In contrast to the large increase under BAU scenarios, re-
placing those HFCs currently in use with low-GWP HFC
alternatives that have lifetimes of less than one month can
eliminate future HFC forcing increase (Velders et al., 2012).
Under the mitigation scenario, the total HFC radiative forc-
ing in 2050 would be less than its current value (Mitigation
in Fig. 1). Alternatives with no direct impact on climate, in-
cluding ammonia, carbon dioxide, and hydrocarbons, as well
as low-GWP HFCs and not-in-kind alternatives, are already
in commercial use in a number of sectors. For other sectors,
alternatives are being evaluated or further developed (UNEP,
2011). The calculation of climate mitigation assumes that the
selected alternatives do not compromise energy efficiency,
an assumption that appears reasonable in light of the his-
toric trend of increased energy efficiency when chemicals
are phased out under the Montreal Protocol (Andersen and
Morehouse, 1997; Andersen and Sarma, 2002; Andersen et
al., 2007).
3.2 Implication for global temperature
The simulated temperature trends (Fig. 2) agree with the ear-
lier studies (Shindell et al., 2012; UNEP and WMO, 2011)
that combined mitigation of CH
4
, BC and O
3
can miti-
gate 0.5
C of warming by the mid-century. It also agrees
with RX10 that HFCs contribute about 0.1
C to the avoided
warming of 0.6
C by 2050 and that SLCPs are critical for
limiting the warming below 2
C. CO
2
mitigation, although
begun in 2015, has very little effect for the near-term (see
difference between red solid line and red dash line in Fig. 2).
Focusing on the longer timescale of the end of the century
(Fig. 2), CO
2
mitigation plays a major role in reducing addi-
tional warming by as much as 1.1
C by 2100. Next, the com-
bined measures (CO
2
, CH
4
, BC and O
3
) considered in UNEP
and WMO (2011) and Shindell et al. (2012) are not suffi-
cient to limit the warming below 2
C (blue line in Fig. 2),
had these studies included the updated projected HFC growth
patterns of Velders et al. (2009) in their BAU scenarios. Miti-
gation of the potentialgrowth of HFCsis shown to play a sig-
nificant role in limiting the warming to below 2
C and could
contribute additional avoided warming of as much as 0.5
C
by 2100 (blue and black line in Fig. 2). Using the lower lim-
its of BAU increase of HFC (red dash line in Fig. 1), 0.35
C
warming will be avoided.
The results are consistent with RX10 for the near-term,
but the avoided warming from HFCs towards the end of the
century is 100% higher in this study, due to the updated
forcing scenarios accounting for the high HFC growth rate
(green lines in Fig. 1 for a comparison with RX10 forcing
scenarios). Replacing HFCs with available low-GWP sub-
stitutes that have a lifetime of one month or less, or with
other materials or technologies, can provide up to 0.35 to
0.5
C of warming mitigation by 2100 in the scenarios used
here. The important point to note is that assessing the role
of HFCs in climate change depends on what BAU (i.e., ref-
erence/baseline) scenarios the climate models assume for
HFCs in their simulations. Many climate models assume
much smaller growth of HFC emission, because of the im-
plicit assumption that replacements with low impact on cli-
mate for high-GWP HFCs will be adopted extensively dur-
ing this century, an assumption that largely depends on the
extent of policy interventions, as well as technological and
economic developments. Our study, however, shows that if
current growth rates of high-GWP HFCs continue, the addi-
tional warming from HFCs alone will be as much as 0.5
C
during this century. The potential temperature mitigation by
the end of this century, from HFC replacement, is in addi-
tion to the 1
C potential mitigation from other SLCP re-
ductions (Fig. 2; also see RX10). When mitigation effort to
reduce high-GWP HFCs is combined with that on BC and
CH
4
, 0.6
C warming can be avoided by 2050 and 1.5
C
by 2100 (black solid line vs. red dash line in Fig. 2). This
would cut the cumulative warming since 2005 by 50% at
2050 and by 60% at 2100 from the corresponding CO
2
-only
mitigation scenarios (red dash line in Fig. 2). Based on our
high HFC growth scenarios, the contribution to the avoided
warming at 2100 due to HFC emission control is about 40%
of that due to CO
2
emission control. Considering the near-
term (2050) timescale, HFC emission is even more effective
(140% of CO
2
mitigation) in curbing the warming. Given
the limited knowledge regarding climate sensitivity (0.5 to
1.2
C/(Wm
2
)), the absolute valueof projected temperature
at the end of 21st century is also uncertain (vertical bars in
Fig. 2), but the relative contribution of HFC to reducing the
warming is still significant and less subject to such uncer-
tainty.
4 Conclusions
The concept of “short-lived climate pollutants” highlights
the shorter lifetime of those pollutants (including HFCs) as
compared to long-lived GHGs (including CO
2
and CFCs).
Our paper demonstrates the benefits of replacing high-GWP
HFCs with low-GWP alternatives, so the overall forcing
and associated warming due to HFC growth can be signifi-
cantly reduced. The results presented here could strengthen
the interest of policymakers in promoting fast-action strate-
gies to reduce SLCPs, including HFCs, as a complement
to immediate action to reduce CO
2
emissions. There are
several policy options for limiting HFC growth, separate
from those for BC and CH
4
, including using the Mon-
treal Protocol to phase down the production and consump-
tion of HFCs (Molina et al., 2009; UNEP, 2012a, b), which
would preserve the climate benefits the treaty has already
achieved through its success in phasing out nearly 100 sim-
ilar chemicals (Velders et al., 2007, 2012). Without the
www.atmos-chem-phys.net/13/6083/2013/ Atmos. Chem. Phys., 13, 6083–6089, 2013
6088 Y. Xu et al.: The role of HFCs in mitigating 21st century climate change
Montreal Protocol, the projected radiative forcing by ODSs
would have been roughly 0.65Wm
2
in 2010 (Velders et al.,
2007), and the global temperature would have been higher
(green line in Fig. 2). It is also important to emphasize that
HFC mitigation should not be viewed as an “alternative”
strategy for avoiding the 2
C warming, but rather as a critical
component of a strategy that also requires mitigation of CO
2
and the other SLCPs. The focus of this study is on near-term
warming over the next several decades to end of the century.
For the longer-term (century and beyond), mitigation of CO
2
would be essential for a significant reduction in the warming.
Acknowledgements. The study was supported by the National
Science Foundation (ATM07-21142) and University of California,
San Diego Open Access Fund (Pilot). We thank Nathan Borgford-
Parnell, Stephen O. Andersen, and Dennis Clare for reading the
manuscript; and Allison Thomson and Keywan Riahi for sharing
data. We acknowledge two anonymous reviewers and Steven J.
Smith for their constructive comments that greatly improved the
paper.
Edited by: D. Shindell
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The emissions of hydrofluorocarbons (HFCs) have increased significantly in the past 2 decades, primarily as a result of the phaseout of ozone-depleting substances under the Montreal Protocol and the use of HFCs as their replacements. In 2015, large increases were projected in HFC use and emissions in this century in the absence of regulations, contributing up to 0.5 ∘C to global surface warming by 2100. In 2019, the Kigali Amendment to the Montreal Protocol came into force with the goal of limiting the use of HFCs globally, and currently, regulations to limit the use of HFCs are in effect in several countries. Here, we analyze trends in HFC emissions inferred from observations of atmospheric abundances and compare them with previous projections. Total CO2 eq. inferred HFC emissions continue to increase through 2019 (to about 0.8 GtCO2eq.yr-1) but are about 20 % lower than previously projected for 2017–2019, mainly because of the lower global emissions of HFC-143a. This indicates that HFCs are used much less in industrial and commercial refrigeration (ICR) applications than previously projected. This is supported by data reported by the developed countries and the lower reported consumption of HFC-143a in China. Because this time period preceded the beginning of the Kigali provisions, this reduction cannot be linked directly to the provisions of the Kigali Amendment. However, it could indicate that companies transitioned away from the HFC-143a with its high global warming potential (GWP) for ICR applications in anticipation of national or global mandates. There are two new HFC scenarios developed based (1) on current trends in HFC use and Kigali-independent (K-I) control policies currently existing in several countries and (2) current HFC trends and compliance with the Kigali Amendment (KA-2022). These current policies reduce projected emissions in 2050 from the previously calculated 4.0–5.3 GtCO2eq.yr-1 to 1.9–3.6 GtCO2eq.yr-1. The added provisions of the Kigali Amendment are projected to reduce the emissions further to 0.9–1.0 GtCO2eq.yr-1 in 2050. Without any controls, projections suggest a HFC contribution of 0.28–0.44 ∘C to global surface warming by 2100, compared to a temperature contribution of 0.14–0.31 ∘C that is projected considering the national K-I policies current in place. Warming from HFCs is additionally limited by the Kigali Amendment controls to a contribution of about 0.04 ∘C by 2100.
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