HFCs in the Atmosphere: 

Concentrations, Emissions and Impacts 

Stephen A. Montzka, PhD 

ABSTRACT 

Concentrations of hydrofluorocarbons (HFCs) have increased in the global background atmosphere in recent years. By 2011 global

mean HFC concentrations ranged from ≤ 1 parts per trillion for HFC-227ea to 65 ppt for HFC-134a. Although HFCs in the 

atmosphere do not deplete ozone, they are greenhouse gases (GHGs) that exert a warming influence on Earth’s climate. Total direct 

heating supplied by all HFCs in 2011 was 0.019 W m-2, which is ≤ 1% of the heating supplied by fossil-fuel-derived carbon dioxide 

(1.8 W m-2) or anthropogenic methane (0.5 W m-2) in the atmosphere. This warming influence, known also as radiative forcing, is 

currently small for HFCs because their concentrations are more than 6 orders of magnitude smaller than CO2. A comprehensive 

assessment of climate impacts of different GHGs, however, also accounts for their influence on climate integrated over time following 

their release to the atmosphere. This is important particularly for GHGs because some remain in the atmosphere for only days, while 

others have lifetimes longer than a century. The Global Warming Potential (GWP) provides a measure of the time-integrated radiative 

forcing arising from an emission of a GHG relative to that from an equivalent emission (by mass) of carbon dioxide. Most HFCs 

emitted to the atmosphere today have 100-year GWPs ranging from 100 to 14000, which are many times larger than CO2 (GWP = 

1). Large increases in use and emission of high-GWP HFCs projected in the future imply a similarly large increase in radiative forcing 

from HFCs. Such an increase would offset much of the climate gains supplied by the Montreal Protocol in phasing out 

chlorofluorocarbons. The climate impact of future HFC use, however, would be minimized if the mix of HFCs (or other chemicals)

adopted as substitutes had substantially lower GWPs than those in use today. 

WHY HFCS ARE IN THE ATMOSPHERE 

Hydrofluorocarbons (HFCs) are present in today’s atmosphere primarily as a result of society’s efforts to 

reduce consumption and use of ozone depleting substances (ODSs). Since HFCs do not deplete stratospheric 

ozone and are effective refrigerants, air conditioning and foam-blowing agents, aerosol propellants, and solvents, 

they have received wide use as ODS substitutes in these applications. 

The fully revised and amended Montreal Protocol on Substances that Deplete the Ozone Layer set schedules 

for phasing out industrial production for dispersive uses of ODSs such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), halons, methyl bromide, and some long-lived chlorinated solvents. The success of 

this Protocol has been substantial. Global-scale production and emission of the most potent ODSs (CFCs, halons, 

methyl bromide and long-lived chlorinated solvents) are now a small fraction of what they were in the mid-1980s; 

atmospheric concentrations of nearly all potent ODSs have peaked and are now declining (Figure 1) (Montzka and 

Reimann et al., 2011). In contrast, atmospheric concentrations of HCFCs, which are less potent ODSs, continue 

to increase in the background atmosphere. Production and emission of HCFCs continues at substantial levels 

because these chemicals are used as temporary substitutes for the more potent ODSs (Figure 1). Production and 

Stephen A. Montzka is a research chemist with the National Oceanic and Atmospheric Administration in Boulder, CO, USA. 
Figure 1. Annual global mean atmospheric concentrations (expressed as mixing ratios or mole fractions; units are parts per 

trillion or ppt) of ozone depleting gases and their long-lived substitutes derived from measurements in the remote 

atmosphere. Two panels are presented to show with detail atmospheric changes for trace gases at higher (left panel) 

and lower (right panel) concentrations. These observational records are from the Global Monitoring Division of 

NOAA’s Earth System Research Laboratory and the Advanced Global Atmospheric Gases Experiment or 

AGAGE (Prinn et al., 2000) (compiled in the most recent WMO Scientific Assessment of Ozone Depletion; 

Montzka and Reimann et al., 2011; updated data are from NOAA, see: ftp://ftp.cmdl.noaa.gov/hats/). 

consumption of HCFCs in developed countries is now decreasing, while that in developing countries has increased 

rapidly in recent years and is capped only after 2012 (UNEP, 2012). 

The threat to the ozone layer from ODSs has been declining for over a decade as a result of the Montreal 

Protocol. The summed concentration of ozone-depleting chlorine and bromine has decreased by up to 10% from 

its peak in different regions of the stratosphere (Montzka and Reimann et al., 2011). Despite this success, it will 

still be 2050 to 2080 before the summed concentration of all ozone-depleting halogen declines back to 1980 levels 

(approximately pre-ozone-hole levels). This is primarily because only a few percent of CFCs and halons are 

photochemically destroyed each year by natural processes; these chemicals have “lifetimes” of 20-100 years in the 

global atmosphere and their concentrations decrease only slowly even when emissions are negligible. 

Because CFCs are strong GHGs, the success of the Montreal Protocol has also provided a substantial benefit 

to climate (Velders et al., 2007). In the absence of this Protocol, emissions of CFCs would have been much larger 

and concentrations of these gases would have been significantly higher for decades and centuries thereafter 

(Newman et al., 2009). As of 2010, the emission reduction brought about by the Montreal Protocol is equivalent to 

reducing CO2 emissions by about 10 Gt yr-1, which is about five times the annual emission reduction target of the 

Kyoto Protocol for 2008-2012 (Velders et al., 2007). 

HFCs were produced in large quantities as non-ozone-depleting substitutes for CFCs, HCFCs, and other 

ODSs beginning in the early 1990s. Atmospheric concentrations have increased relatively rapidly since that time, 

although as of 2011, the global HFC concentration of any individual HFCs was still < 100 ppt (Figure 1). 

Although HFCs do not deplete ozone, they do have some environmental impacts that could be of concern if 

concentrations were to increase without control in the future. HFCs oxidize in the atmosphere to produce TFA, a 

toxic chemical that accumulates in biological tissues. Given current projections of increased use of HFCs, 

however, HFCs are not likely to contribute substantially to the background amount of TFA relative to other 

existing TFA sources (Montzka and Reimann et al., 2011). 

Of greater concern is that some HFCs, particularly those receiving much use today, are strong GHGs. 

Projections of uncontrolled HFC use in “business as usual” scenarios show large increases in production and 

emissions of HFCs by 2050, primarily as a result of developing countries becoming fully industrialized (Velders et 

al., 2009; UBA, 2009). This future production is not controlled by the Kyoto Protocol, as target dates for this 

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Global surface mean (ppt)

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HCFC-141b

HCFC-142b

HFC-143a

HFC-125

HFC365mfc

HFC-32

HFC-152a

CFC-12

CFC-113

CCl4

HCFC-22

CFC-11

HFC-23

HFC-134a

CH3CCl3
Protocol are now expiring and it did not cover all countries. If emissions of the strong GHG HFCs in use today 

increase as projected, the climate benefit that had been achieved by the Montreal Protocol (as annual emissions) 

would be largely erased. As a result, alternatives with smaller climate impacts are being considered, such as HFCs 

with shorter lifetimes, or not-in-kind solutions. 

MEASURING HFCS IN THE ATMOSPHERE 

Assessing the global environmental impact of a trace gas starts with characterizing the concentration of that 

trace gas in the global atmosphere. For chemicals with lifetimes longer than about 0.5 yr, a measurement made at a 

site far removed from sources can be representative of a broad region of the global atmosphere. As a result, the 

global background mean concentration of a trace gas can be fairly accurately derived from measurements at only a 

few sites. 

When measurements are made at more than one site, they more accurately characterize the global mean 

concentration and also supply information about the location of sources (Montzka et al., 2009). For example, 

when CFCs, HCFCs, and HFC are emitted in large quantities relative to losses, higher concentrations are measured 

in the remote northern hemisphere (NH) than in the southern hemisphere (SH), and concentrations in the SH lag 

those in the NH by at least 1 year (Figure 2). These observations are consistent with human activities being the 

primary source of these chemicals to the atmosphere, given that most humans live in the NH. As emissions 

become small (already observed for CFCs), hemispheric concentration differences diminish. Furthermore, 

concentrations in the SH lag those observed in the NH because the largest substantial barrier to meridional mixing 

in the lower atmosphere is near the equator (the inter-tropical convergence zone), so it typically takes much longer 

for an HFC molecule to be transported from North America to South America than from North America to 

central Asia, for example. As a result of these considerations, current sampling networks for characterizing global 

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HFC-134a (mixing ratio, ppt)

alt (83 N)

brw (71 N)

hfm** (42 N)

nwr (40 N)

kum (20 N)

mlo (20 N)

cgo (40 S)

smo (14 S)

spo (90 S)

global mean

Figure 2. Past atmospheric changes for HFC-134a. Points represent monthly mean concentrations determined from 

NOAA flasks sampled approximately weekly at 8 remote stations in both hemispheres and one non-remote site 

in the north-eastern U.S.A. (hfm**). Error bars are shown for remote sites only and represent 1 standard 

deviation of all measurements during that month (error bars are often smaller than points). The black line 

represents the global surface mean concentration. Station latitudes are indicated in the legend. Data are an update 

of Montzka et al. (1996) and are available at: ftp://ftp.cmdl.noaa.gov/hats/hfcs/hfc134a_GCMS_flask.txt. 
atmospheric concentrations of long-lived gases include measurements at remote sites in both the NH and SH, and 

fewer sites at similar latitudes. 

Trends in global atmospheric concentrations of long-lived trace gases are routinely derived from ongoing 

measurements at Earth’s surface. Measurements from periodic aircraft campaigns have demonstrated that longlived trace gases such as CFC, HCFCs, and HFCs are fairly rapidly mixed vertically within the lower atmosphere so 

that measurements made at the surface at a few remote locations provide an accurate representation of 

concentrations and their time-dependent changes throughout the entire lower atmosphere (Wofsy et al., 2011). 

Surface measurements also allow an accurate picture of compositional changes in the stratosphere, once time lags 

for mixing are considered (Schauffler et al., 2003). As a result, surface-based measurements allow one to evaluate 

adverse environmental impacts arising from the presence of trace gases in other regions of the atmosphere such as 

stratospheric ozone depletion and climate change. 

Atmospheric measurements of CFCs, HCFCs, and HFC are typically made with well-established 

chromatographic techniques with instruments deployed at remote sites that regularly and automatically collect and 

analyze samples multiple times per day (Elkins et al., 1993; Prinn et al., 2000), or with flasks collected weekly that 

are subsequently shipped to and analyzed in a central laboratory (Montzka et al., 1996). Measurements made with 

chromatographic techniques can be highly precise and accurate. Repeatability for replicate measurements is 

typically <1% for trace gases at concentrations above a few parts per trillion. Differences in results reported by 

the two main independent laboratories making global measurements of these gases are typically < 5%, and most 

differences can be traced to the preparation of calibration standards as opposed to instrumental artifacts. 

OBSERVED CHANGES IN ATMOSPHERIC HFC CONCENTRATIONS 

Measurements of HFCs currently in use by society and produced by industry show increasing atmospheric 

concentrations throughout the global atmosphere (Figure 1). The observations show higher atmospheric 

concentrations and larger spatial and temporal concentration variations at NH surface sites, consistent with most 

emissions emanating from the NH as a result of use by humans in a variety of applications (Figure 2). Even higher 

variability is observed at sites near large populations (i.e., non-remote locations such as hfm in Figure 2). NOAA 

measurements at a number of continental sites in the U.S. during the past few years, for example, show 

concentrations of HFC-134a in individual samples that are up to 6 times higher than observed at remote sites. 

Measurements of HFCs from independent sampling networks provide a consistent picture of global mean 

concentrations and yearly increases for HFCs measured by both networks (Montzka and Reimann et al., 2011). 

The most abundant HFC in the atmosphere currently is HFC-134a, followed by HFC-23. Other HFCs are present 

at lower concentrations, and most are increasing relatively rapidly at the present time. HCFC-22 is the only HCFC 

present at higher concentrations than HFC-134a in the global background atmosphere currently, and the 

concentration of the three major CFCs are still higher than any HFC (Figure 1). 

Natural emissions of HFCs are thought to be extremely small or non-existent. Some trace-gas measurement 

networks existed in the late 1980s when HFCs were first produced. As a result, atmospheric records for some 

HFCs start concurrently with the first release of these industrially produced chemicals into the atmosphere. 

Concentrations of these particular HFCs increase from near zero (< 1 parts-per-trillion) in the measurement 

record, suggesting that any natural sources of these gases are insignificant (Figure 2). For HFCs for which ongoing 

measurements began years after initial production, societal use and emission, atmospheric concentrations during 

these initial years are derived from the collection and analysis of archived canister air samples (Fraser et al., 1999 

and Miller et al., 2010) and the extraction and analysis of air trapped in the snowpack above glaciers in Antarctica 

and Greenland (Butler et al., 1999; Montzka et al., 2010). Both methods provide a reliable way to derive 

atmospheric composition of many stable gases. The negligible concentrations measured in the oldest air samples 
demonstrate that HFCs, CFCs and HCFCs are present in the atmosphere entirely as a result of industrial 

production and use by humans. 

GREENHOUSE GASES THAT INFLUENCE CLIMATE 

Many different trace gases, water vapor, clouds, and aerosols interact with light energy across the 

electromagnetic spectrum (from high energy ultraviolet to lower-energy infrared wavelengths) and affect the 

balance of heat in Earth’s climate system. The main contributors to Earth’s greenhouse are currently water vapor, 

carbon dioxide (CO2), clouds, aerosols, methane (CH4), ozone, nitrous oxide (N2O), and halocarbons (Schmidt et 

al., 2010). The GHGs, for example, absorb infrared radiation emitted from Earth’s surface and allow less of that 

emitted heat to escape to space. This additional absorption adds heat to Earth’s climate system. Water vapor and 

clouds provide much of the atmospheric greenhouse heating in today’s atmosphere but are “highly active 

components of the climate system that respond rapidly to changes in temperature and air pressure by evaporating, 

condensing, and precipitating” (Lacis et al., 2010). Water vapor and clouds are different from other GHGs in that 

they respond to the physical atmospheric conditions set by other chemicals. As a result, water vapor and clouds 

supply important feedbacks to climate conditions set by GHGs and other processes (e.g., orbital variations), rather 

than being the primary drivers of climate change. 

The primary drivers of recent climate change are the non-condensing GHGs including CO2, CH4, ozone, 

N2O, and halocarbons, and aerosols. Of these chemicals, many are emitted to the atmosphere from both natural 

processes and human activities. Emissions from both sources affect the balance of heat in the climate system. In 

quantifying the human contribution to present-day climate change, only trace-gas concentration changes since the 

beginning of the industrial revolution (1750) are considered. Atmospheric concentrations of trace gases in 1750 

are fairly well known based on measurements of air extracted from glacial ice bubbles (Etheridge et al., 1996; 

Etheridge et al., 1998). 

Radiative efficiencies and direct radiative forcing 

The instantaneous contribution of GHGs to climate change is typically evaluated with the metric of radiative 

forcing (Forster and Ramaswamy et al., 2007). Radiative forcing (units of W m-2) refers explicitly to the climate 

forcing or heating influence arising from the change in the atmospheric concentration of a trace gas between 1750 

and the present day. The direct radiative forcing of a trace gas is determined by the trace gas concentration and 

how efficiently it absorbs infrared (longwave) radiation emanating from Earth’s surface (i.e., its radiative efficiency 

expressed in W m-2 ppb-1). For trace gases at very low concentrations (e.g., HFCs, CFCs, and HCFCs), direct 

radiative forcing is linearly proportional to atmospheric concentration. For trace-gases at higher concentrations 

(i.e., CO2, CH4, N2O), however, radiative forcing is non-linearly related to atmospheric concentration. Radiative 

efficiencies for HFCs in use today range between 0.02 and 0.4 W m-2 ppb-1, which is orders of magnitude larger 

than the current radiative efficiencies of CO2 (0.000014 W m-2 ppb-1), CH4 (0.00037 W m-2 ppb-1), and N2O 

(0.00303 W m-2 ppb-1) (Forster and Ramaswamy et al., 2007). In other words, HFCs are potent GHGs; the 

additional heat trapped from an added HFC molecule is orders of magnitude larger that the additional heat trapped 

by an added molecule of CO2. Interestingly, the radiative efficiencies of HFCs in use today are comparable to 

those of previously used CFCs and currently used HCFCs (0.140.32 W m-2 ppb-1) . 

Although most HFCs in use today are potent GHGs on a per-molecule basis, their summed contribution to 

climate forcing currently is less than 1% of that from all long-lived GHGs. The radiative forcing from HFCs in 

2011 totaled about 0.019 W m-2 (Figure 3). HFC-134a accounts for most radiative forcing from HFCs (0.0096 W 

m-2), followed by HFC-23 (0.0045 W m-2), and other HFCs. In comparison, the radiative forcing contributed by 

all long-lived GHGs was 2.8 W m-2 in 2011 (http://www.esrl.noaa.gov/gmd/aggi/) (Figure 3). In other words, 

the total direct heating influence from HFCs was 0.6% of the direct heating supplied from all long-lived, non- 
Figure 3. Direct radiative forcing from long-lived greenhouse gases. These forcings are calculated from global annual surface 

mean mixing ratios and published radiative efficiencies (Daniel and Velders et al., 2011; Forster and 

Ramaswamy et al., 2007). Left-hand panels show the contributions from all the substantial, long-lived GHGs or 

groups of gases; right-hand panels show subsets of lesser contributors (note the 1000-fold change in y-scale). The 

“ODSs” grouping includes CFCs, HCFCs, and chlorinated solvents, the “Sum, Kyoto synthetics” includes all 

synthetic gases in the Kyoto Protocol (HFCs, PFCs, and SF6); and “other HFCs” includes HFC-143a, HFC125, and HFC-152a. Data are from NOAA and AGAGE compiled in Montzka and Reimann et al. 

(2011); updated data are from NOAA; PFC data are from Mühle et al. (2010). 

condensing GHGs in 2011 (when only anthropogenic contributions are included). 

It is useful to put the direct heating influence supplied by long-lived GHGs in perspective. Approximately 

240 W m-2 of incoming solar energy is absorbed by Earth’s surface and atmosphere (Hansen et al., 2007). The 2.8 

W m-2 supplied by human-caused increases in long-lived GHGs in 2011 is about 1.2% of this absorption and is 

quite large compared to the peak-to-peak solar variability across solar cycles (~0.1%) or the long-term increase in 

radiative forcing (~0.1 W m-2) from changes in total solar irradiance since the 17th-century (Forster and 

Ramaswamy et al., 2007). Put in another context, the heat added from long-lived GHGs is equivalent to the heat 

from about 14 trillion 100-W light bulbs distributed evenly across Earth’s surface, operating continuously. This 

corresponds to a 100-W light bulb positioned on every 6×6 m (or 20×20 ft) grid covering Earth’s entire surface. 

Total direct radiative forcing from all long-lived gases has increased over the past 2 decades at a fairly 

constant annual rate (Figure 3). The atmospheric increase in CO2 accounts for most of this radiative forcing 

increase. HFCs account for a small portion of the overall increase. Over the past 5 years, for example, increases in 

HFCs have accounted for about 4% of the increase in total radiative forcing from all long-lived GHGs. 

At NOAA, global mean concentrations of long-lived GHGs are measured with a global sampling network on 

a continuous basis. The results provide a rich source of ongoing information about atmospheric composition and 

how it is changing. One metric derived from these data is the NOAA Annual Greenhouse Gas Index (AGGI), 

which is a measure of the radiative forcing supplied by these gases relative to the forcing supplied in 1990, the 

baseline year for the Kyoto Protocol (Hofmann et al., 2006; http://www.esrl.noaa.gov/gmd/aggi/). The AGGI in 

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Sum, 

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Other HFCs

SF6

Sum, 

HCFCs

HCFC-22

Sum, Kyoto 

synthetics HCFC141b

HCFC-142b

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HFCs

Radiative Forcing (W m-2)

Radiative Forcing (milliW m-2)
2011 was 1.30, indicating that in 2011 the radiative forcing arising from anthropogenic activities has increased by 

30% since 1990. 

Global Warming Potentials: Estimating the time-integrated climate impact of a trace-gas emission 

Direct radiative forcing is an estimate of the instantaneous heating influence supplied by a trace gas at a given 

atmospheric concentration. This metric does not capture the full climate impact of a trace gas emission, however, 

because different GHGs persist in the atmosphere from weeks to centuries. To compare time-integrated climate 

impacts of GHG emissions, the global warming potential (GWP) is often used. The GWP is a measure of the 

radiative forcing from a trace gas emission, integrated over a specific time period and expressed relative to the 

climate influence of an equivalent mass emission of CO2 (Forster and Ramaswamy et al., 2007; Daniel and Velders 

et al., 2011; Montzka et al., 20011a). The GWP derived for a 100 year period is often used to compare the 

integrated climate impacts of different trace gas emissions. Multiplying a trace-gas emission amount by its 

respective 100-yr GWP allows a ‘CO2-equivalent’ (CO2-eq) emission to be calculated. 

Emissions derived from inventory accountings or global atmospheric observations (see Box at end of paper) 

can be converted to CO2-eq emissions. The resulting values allow an assessment of the relative contributions of 

the current emissions of different GHGs to future climate change (Figure 4). At 33.5 Gt yr-1 in 2010, CO2 

Figure 4. CO2-equivalent emissions of greenhouse gases. Carbon dioxide emissions are derived from inventories of fuel use 

and cement production (additional amounts from land-use change of about 3 GtCO2 in recent years are not 

shown) (source: http://cdiac.ornl.gov/ftp/ndp030/global.1751_2008.ems and http://cdiac.ornl.gov/ 

trends/emis/prelim_2009_2010_estimates.html). Emissions of non-CO2 gases were derived from measured 

global concentrations (Figure 3) and estimates of annual losses (see Box). CO2-equivalent emissions are derived by 

multiplying emissions by 100-year Global Warming Potentials (100-year GWPs from Daniel and Velders et al., 

2011). Different colors represent different chemicals or chemical classes, and the multiple lines given for a single 

chemical or chemical class represent emissions derived from independent global sampling networks (NOAA; and 

AGAGE—see Prinn et al., 2000; data compiled in Montzka and Reimann et al., 2011). Right-hand panel 

shows CO2-equivalent emissions of individual HFCs and their sum derived similarly (note 1000-fold expanded 

scale). Contributions from individual HFCs that are less than about 10 Mt CO2-eq yr-1 in 2011 are not shown. 

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emissions are larger than the sum of all non-CO2 GHG emissions by a factor of about 2. HFCs emissions in 2010 

amounted to 0.6 Gt CO2-eq yr-1, or slightly more than 1 percent of total greenhouse gas emissions during that year. 

CO2-eq emissions of ODSs have decreased by about 10 Gt CO2-eq yr-1 owing to the Montreal Protocol but were 

still 1.8 Gt CO2-eq yr-1 in 2010 (Figure 4). At the present time the increases in annual HFC emissions have only 

slightly offset the reduction in annual ODS emissions brought about by the Montreal Protocol. 

Although halocarbons such as CFCs, HCFCs, and HFCs have fairly similar radiative efficiencies, their GWPs 

span more than two orders of magnitude because they have a range of lifetimes (Figure 5). Differences in lifetime 

stem from differences in chemical reactivity: Chemicals with longer lifetimes have correspondingly larger GWPs 

because they contribute to radiative forcing for longer periods after emission and because the accumulated 

atmospheric concentration following emission is higher than for a short-lived gas. For example, although the 

radiative efficiencies for HFC-23 and HFC-152a are different only by a factor of about 2 (0.19 for HFC-23 and 

0.09 for HFC-152a), their 100-yr GWPs are different by a factor of 100 (14,200 for HFC-23 vs. 133 for HFC-152a) 

(Daniel and Velders, 2011). In other words, an emission of HFC-23 has a much larger integrated impact on 

climate than does an equal emission of HFC-152a. 

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Figure 5. The radiative efficiency for halocarbons with a range of lifetimes and their respective 100-yr GWPs. Each point 

represents a different halocarbon (for a list of halocarbons included here, see Table 5-A2 in Daniel and Velders et 

al., 2011). Halocarbons with shorter lifetimes generally have smaller time-integrated impacts on climate and, 

hence, smaller 100-yr GWPs. 

Different values are derived for the GWP of any individual trace gas depending on the time period over 

which climate impacts of a pulse emission are integrated. This is because the time-evolution in concentration and 

radiative forcing after an emission of a trace gas is typically much different than it is for CO2 (see Box). Carbon 

dioxide is unique among trace gases because it is not removed from the atmosphere by processes that can be 

characterized by a single timescale (or lifetime). Despite this, it takes about 100 years for 63% (or 1-1/e, see Box) 

of a pulse emission of CO2 to be removed from the atmosphere (and much longer for remaining amounts to be 

removed). Hence, the integrated radiative influence of a chemical with a lifetime < 100 yr over a short time 

horizon (e.g., 20 years) relative to CO2 is much larger than when that time-integration period is 100 or 500 years. 
Over longer time horizons, the GWP is smaller for a short-lived gas because it becomes completely removed from 

the atmosphere and, hence, its radiative forcing contribution approaches zero while CO2, as the reference, persists 

in the atmosphere for centuries. Accordingly, short-lived chemicals have 20-yr GWPs that are substantially larger 

than their 100-yr or 500-yr GWPs. The opposite is generally true for chemicals with lifetimes longer than about 

200 years. For these chemicals, their 20-yr GWPs are smaller than their 500-yr GWP because 500 years after 

emission a larger fraction of the non-CO2 gas remains in the atmosphere compared to CO2. 

Finally GWPs reported in the scientific literature have changed over time as our scientific understanding 

improves. GWPs in the late 1990s and early 2000s (e.g., those that appeared in Ramaswamy et al., 2001) were 

adopted by the United Nations Framework Convention on Climate Change as the official values to use for 

calculating CO2-equivalent emissions going forward. With each scientific assessment report of ozone depletion or 

climate, updates to these initial values are derived from improved scientific understanding (Forster and 

Ramaswamy et al., 2007; Daniel and Velders et al., 2011). As a result, a range of GWP values for individual gases 

can be found in the literature. 

POTENTIAL FUTURES 

By the different measures discussed above it is clear that the climate influence of HFCs emitted to date is 

relatively small in comparison to other GHGs, particularly CO2, and will remain so in the immediate future. Yet 

there is concern about this influence increasing substantially on longer terms (Figure 6). This concern arises 

Figure 6. Projected future radiative forcings for a single HFC emissions scenario in which the mix of HFCs used in the 

future have range of lifetimes (and, therefore, GWPs): 15 years, 3, years, 1 year, and 1 month (adapted from 

Velders et al., 2012). For reference, the current mix of HFCs has an average lifetime of ~15 years and a 100-

yr GWP of 1630. The average 100-yr GWP of HFCs associated with the scenario with an average HFC 

lifetime of 1-month is 11. The future radiative forcing arising from increased HFC emissions depends 

substantially on the average lifetime (and GWP) of the HFCs chosen for use by society. Also shown are the 

anticipated increases in radiative forcing from CO2 in the future (upper panel, note different scale; green shaded 

area), and the radiative forcing that might have resulted if the Montreal Protocol had not been adopted (blue 

hatched range). 

CO2 range

CFCs, halons, others

HCFCs

No Montreal Protocol

HFCs

Upper range

(~ 15 yr lifetime)

3-yr lifetime

1-yr

1-month

Low-GWP substitutes

Radiative Forcing (W m-2)

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1950 1970 1990 2010 2030 2050
primarily from the potentially large increase in production and emissions of high-GWP HFCs that could occur by 

mid-century in the absence of global controls (Velders et al., 2009; UBA, 2009). Although emissions from 

developed countries likely account for most of the HFCs present in today’s atmosphere, most of the projected 

increases in production and emissions of HFCs stem from developing countries as they phase out CFCs and 

HCFCs and further industrialize. Demand for CFCs and HCFCs in developing countries has increased rapidly 

since 1998 (~8% yr-1 through 2007) and is likely to continue increasing in the future (Velders et al., 2009; UBA, 

2009). Projections suggest that a large fraction of future demand in these countries will be met with HFCs, given 

that CFC production is phased out and production of HCFCs will be capped in 2013 and subsequently phased out 

gradually. 

In the absence of controls on high-GWP HFCs, this increased demand is projected to cause substantial 

increases in future radiative forcing from HFCs (Velders et al., 2009; UBA, 2009) (Figure 6). The actual climate 

impact arising from substantial future increases in HFC emissions, however, is not pre-determined. Instead, it 

depends on the magnitude of emissions over time and the GWPs of HFCs and other alternatives predominantly 

used globally. For example, if the mix of HFCs used during the next several decades has an average 100-year 

GWP similar to the HFCs in use today (i.e., 1370-4180 for HFC-134a, -125, and -143a), total radiative forcing from 

HFCs is expected to increase to between 7 and 16% of that from CO2 in 2050 (compared to 0.6% in 2011) (Figure 

6). In such a scenario, projected CO2-eq emissions of HFCs increase to between 3 and 8 GtCO2-eq by 2050 

(Velders et al., 2009; UBA, 2009). If, however, the mix of substitute chemicals had a substantially smaller average 

GWP, the increase in radiative forcing would be much less, even for the same total mass of emissions (Figure 6). Both 

non-HFC and HFC alternatives with smaller climate impacts per mass emission are being developed by industry 

and are beginning to receive use as substitutes for the long-lived, high-GWP HFCs. 

CONCLUSION 

HFCs are reliably measured in the remote atmosphere by independent sampling networks even at current 

concentrations (< 100 ppt). Observations show increasing concentrations of HFCs owing to industry production 

and societal use in multiple applications in place of CFCs and HCFCs that are being phased out by the Montreal 

Protocol. Direct climate forcing caused by HFCs in the atmosphere today (0.019 W m-2) is currently less than 1% 

of that caused by other long-lived GHGs. When considered on a CO2-eq basis, 2011 emissions of all HFCs 

amount to 0.6 Gt CO2-eq, or about 1% of summed CO2-eq emissions from all long-lived GHGs in that year. 

Concern over continued use of the current mix of HFCs stems from the possibility that their climate impacts 

become substantial in the coming decades. HFC emissions and contributions to climate change are projected to 

increase substantially in response to increased demand especially in developing countries. The projected climate 

influences are dramatically reduced if the HFCs and other alternatives used in the future have substantially lower 

average GWPs than the mix of HFCs in widespread use today. 

Acknowledgements: 

The author is part of a group of scientists measuring global concentrations of greenhouse gases and ozonedepleting substances at NOAA and at the Univ. of Colorado’s Cooperative Institute for Research in 

Environmental Sciences that includes C. Siso, B. Miller, G. Dutton, B. Hall, J.W. Elkins, D. Nance, T. Conway, E. 

Dlugokencky, P. Lang, P.P. Tans, J.H. Butler, and others. The author is indebted to their contributions and 

willingness to share updated results. The author also benefitted from input from J.S. Daniel, G.J.M. Velders, and 

D.W. Fahey. Funding of NOAA global atmospheric measurements is provided in part through NOAA's Climate 

Program Office under its Atmospheric Chemistry and Composition and Global Carbon Cycle programs. 
BOX: Deriving emissions from atmospheric data and trace‐gas lifetimes.

Measurements of long-lived halocarbons in the atmosphere can be used to derive information about their 

emission magnitudes on multiple spatial scales. If we consider the entire atmosphere as a single box, mass balance 

considerations can be used to derive global emission magnitudes of a long-lived trace gas from atmospheric 

measurements at remote locations and an understanding of the rate at which the trace gas is destroyed in the 

atmosphere: 

The rate of change of the atmospheric mass of a trace gas is derived from emissions minus losses as: 

dG/dt = E – kG (1) 

where G is the global burden (mass) derived from globally distributed measurements in remote locations, E is 

the magnitude of emissions (mass yr-1), and k is the first-order rate constant for loss (yr-1) of the chemical from all 

loss processes. The inverse of k has units of time and is known as the trace gas lifetime, which represents the 

timescale over which 63% (or 1-1/e) of the trace gas is removed from the atmosphere by all loss processes. For 

example, a gas with a 5-yr lifetime is destroyed at the rate of 18% yr-1 (i.e., 18% = 1-1/exp(1/5)). For atmospheric 

gases such as CFCs, HCFCs, and HFCs, this timescale for removal is first-order, i.e., it does not depend on the 

magnitude of emission or time since emission. Among long-lived GHGs, only CO2 has a timescale for removal 

that is not first-order. Instead, CO2 becomes removed from the atmosphere on multiple timescales that depend 

on the time elapsed since emission (Forster and Ramaswamy et al., 2007, and Archer and Brovkin, 2008). 

In general, trace gas lifetimes are not directly observable; however, if global emissions are known accurately 

enough or if they have become insignificant, one can use Eq. (1) to estimate a trace gas lifetime directly from 

measured global atmospheric concentrations and trends (Prinn et al., 2000; Montzka et al., 2011b). Conversely, 

lifetimes can be estimated from multiple sources of information, such as laboratory measurements of primary 

chemical reactions and physical processes, including rate constants for reactions that destroy the trace gas and 

photolysis rates. A quantitative understanding of oxidant concentrations and of the light flux capable of 

photolytically decomposing a trace gas throughout the global atmosphere over all seasons is also required. For 

most HFCs, the primary mechanism for atmospheric removal is reaction with the hydroxyl radical (OH). For 

example, for HFC-134a the reaction: 

CH2F-CF3 + ·OH  ·CHF2-CF3 + HOH (2) 

occurs in the atmosphere and the radical produced (·CHF2-CF3) generally reacts with oxygen to form polar 

molecules that are fairly rapidly scavenged by liquid aerosols and removed from the atmosphere. Based on the 

laboratory-measured rate of reaction (2) and our understanding of global OH concentrations, the lifetime of HFC134a is approximately 13 years (Montzka and Reimann et al., 2011). 

Lifetime estimates provide information about loss rates, and global emission magnitudes can be derived for 

long-lived trace gases from eq (1) with global atmospheric observations that define the global burden (G) and its 

change over time. Emissions are often derived with a simple 1-box approach (e.g., eq. (1)), but are also obtained 

with more sophisticated inverse models that attempt to derive more accurate emission magnitudes by extrapolating 

the available information (measured global mean surface concentration, the temperature-dependent loss rate 

constant, hydroxyl radical concentrations) to a time-varying, 3-dimensional field. For HCFCs and other gases with 

lifetimes and growth rates similar to those of most HFCs, the simplistic 1-box approach provides a fairly robust 

emission estimate (Montzka and Reimann et al., 2011). 
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