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Greenhouse Gases (corrected proofs 19.7.18)
Richard Tuckett (E) r.p.tuckett@bham.ac.uk
School of Chemistry, University of Birmingham, Birmingham B15 2TT, United Kingdom
DOI : 10.1016/B978-0-12-409547-2.14031-4
Abstract The subjects of what constitutes a greenhouse gas, and what the term greenhouse effect means are reviewed.
The greenhouse effect comprises two parts; the primary effect which has been in existence for thousands of years and
gives Planet Earth its hospitable average temperature of c. 17 oC and the smaller secondary effect which has been in
existence for only years and is caused by an increase in concentration of greenhouse gases. Much of this
increase is probably due to mankind’s activities on the planet. It is the latter effect that has caused the temperature of
Planet Earth to increase by c. 1 oC since the middle of the 19th century and, following the Paris 2015 COP21 convention, it
is hoped the increase can be limited to 1.52.0 oC by the end of this century. The two most significant secondary
greenhouse gases are CO2 and CH4, and together they contribute c. 80-85% of the secondary effect. This percentage has
not changed for the last 2030 years, but the total radiative forcing which causes the increase in the planet’s temperature
has increased consistently over this time window. Despite a few unexplained observations and inconsistencies, the huge
majority of the world’s scientists now accept that the increase in Planet Earth’s temperature, or global warming, is real,
and will have a disastrous impact on our ecosystem and environment unless everyone can adapt their lifestyles.
Keywords Greenhouse gas CO2 and CH4 Primary greenhouse effect
Secondary greenhouse effect Microscopic radiative efficiency Total radiative forcing
Lifetimes of greenhouse gases Global warming potential IPCC Assessment Reports
Introduction
This is the first article written about the science of Greenhouse Gases for the Encyclopedia of Analytical
Sciences. Articles have been written about specific examples of greenhouse gases, for example, the
chlorofluorohydrocarbons in the first edition, and chlorofluorocarbons and other halocarbons in the second edition,
but this is the first generic article. The section titled “The Science of Greenhouse Gases and the Greenhouse Effect”
concentrates on the science of greenhouse gases and what is now called the “greenhouse effect”. The section
titled “The Physical Properties of Secondary Greenhouse Gases” discusses the physical properties of greenhouse
gases that cause such a gas to be “effective,” that is, may contribute significantly to potential global warming; this
section is written both in a qualitative then quantitative manner, the former being addressed to the intelligent
nonexpert. This section concludes with personal ideas how the data should be interpreted, and general comments
about very long-lived (greater than c. 100 years) greenhouse gases. Finally, the methodology that is used to measure,
from an analytical viewpoint, the important properties of a greenhouse gas are described in the section
titled “Methodology of How Important Physical Properties of Greenhouse Gases Are Measured”.
Many articles/books, especially in the last ten years, have been written about whether potential global warming is
real, and if it is what we can and should do. A parallel paper, published in the Elsevier new online reference
database Earth Systems and Environmental Sciences, addresses and summarizes the different positions the world's
population seem to have taken on this contentious issue, and makes some suggestions for individuals and
Governments to consider.
The Science of Greenhouse Gases and the Greenhouse Effect
The author's academic career spans over 40 years working mostly in high-resolution gas-phase molecular
spectroscopy, and he came into Atmospheric Sciences almost by accident only in c. 2000. He therefore believes he
can write as a neutral person on this subject, but with the training of a spectroscopist at heart. And essentially the
term “greenhouse gas” is a spectroscopic issue; a greenhouse gas is one that absorbs infra-red radiation in the
region of the electromagnetic spectrum where the planet (i.e., Earth, in our case) is a source of infra-red radiation.
This begs the obvious question of why Planet Earth emits infra-red radiation (which our eyes do not see), and in
what wavelength or energy regions of the infra-red.
Planet Earth is in dynamic equilibrium, in that it continually absorbs and emits electromagnetic radiation. It receives
ultra-violet and visible radiation from the Sun, it emits infra-red radiation, and for the temperature of the planet to
remain constant, “energy in” must equal “energy out.” This equality can be used to determine what the average
temperature of the planet should be. Both the Sun and the Earth are black-body emitters of electromagnetic
radiation. That is, they are masses capable of emitting and absorbing all frequencies (or wavelengths) of
electromagnetic radiation uniformly. The distribution curve of energy emitted per unit time per unit area versus
wavelength for a black body was worked out c. one hundred years ago by Planck, and is shown pictorially in Fig. 1.
Without mathematical detail, two points are relevant. First, the total energy emitted integrated over all wavelengths
is proportional to T4. Second, the wavelength of the maximum in the distribution curve varies inversely with T, that
is, λmaxα(T)− 1. These are Stefan's and Wien's Laws, respectively. Comparing the black-body curves of the Sun and the
Earth, the sun emits UV/visible radiation with a peak at c. 500 nm characteristic of Tsun = 5780K. The temperature of
the Earth is about 20 times lower, so the Earth's black-body emission curve peaks at a wavelength 20 times longer or
c. 10 μm. Thus the earth emits infra-red radiation with the total range of wavelengths spanning c. 4–50 μm, the
majority of the emission being in the range 6–25 μm (or 400–1700 cm− 1).
Fig. 1 Black-body emission curves from the sun (T ~ 5780 K) and the earth (T ~ 290 K), showing the operation of Wien's Law
that λmax α(1/T). The two graphs are not to scale.
The solar flux energy intercepted per second by the Earth's surface from the Sun's emission can be written as
, where Fs is the solar flux constant outside the Earth's atmosphere (1368 J s− 1 m− 2), Re is the radius of
the Earth (6.38 × 106m), and A is the Earth's albedo, corresponding to the reduction of incoming solar flux by
absorption and scattering of radiation by aerosol particles (average value 0.28). The infra-red energy emitted per
second from the Earth's surface is , where s is Stefan's constant (5.67 × 10− 8 J s− 1 m− 2 K− 4) and is
the surface area of the earth. By equating these two terms, at equilibrium, therefore, the temperature of the
Earth, Te, can be written:
(1)
Using the data above yields a value for Te of c. 256 K. It is worth noting that, due to the ¼ power of the large bracket
in Eq. (1), any error in the absolute values of Fs, A and s are significantly reduced or damped out in the value
determined for Te. Mercifully, the average temperature of the Earth is not a Siberian − 17°C, otherwise life would be
a very unpleasant experience for the majority of humans on the planet. The reason why the Earth has a hospitable
higher average value of c. 290 K (+ 17°C) is the greenhouse effect. For thousands of years, absorption of some of the
emitted infrared radiation by molecules in the Earth's atmosphere (mostly CO2, O3, CH4, and H2O) has trapped this
radiation from escaping out of the Earth's atmosphere (just as a garden greenhouse operates), some is re-radiated
back towards the Earth's surface, thereby causing an elevation of the temperature of the surface of the Earth. Thus,
it is the greenhouse effect that has maintained our planet at this average temperature, and for this fact we should all
be very grateful. This phenomenon is often called the primary greenhouse effect, it is a huge effect, and the
dominant primary greenhouse gas is H2O, not CO2, O3 or CH4. It is therefore a myth to portray all aspects of the
greenhouse effect as bad news because it is the reverse that is true. This fact may explain the huge confusion there
is worldwide in the general public about what the greenhouse effect actually is.
And confusion there certainly is. If greenhouse gases such as CO2, H2O, CH4, and O3 are really such good news, what
is the problem? Yet, it seems that statements about the greenhouse effect these days by scientists are often
immediately refuted by others, with both stating their claims with almost religious fervor. Politicians and the media
have not helped. In the UK, it is only c. 20 years that the newly-appointed Secretary of State for the Environment
made the cardinal sin of confusing the greenhouse effect with ozone depletion by saying they had the same scientific
causes. (In retrospect John Gummer was closer to the truth than he realized, in that one class of chemicals, the
chlorofluorocarbons, are both the principal cause of ozone depletion and are major but secondary greenhouse gases.
However, these two facts are scientifically unrelated.) Finally and with some justification, many scientists, notably
Cheng et al., are querying whether the absolute Tearth is the most reliable guide to what is happening to our climate,
and perhaps we should concentrate more on other observables such as the temperature or acidity of our oceans.
Yet, there has been strong evidence for the presence of greenhouse gases absorbing infra-red radiation for many
decades, i.e. as long as satellites have been orbiting Planet Earth. Fig. 2 shows data collected by the Nimbus 4
satellite circum-navigating the Earth as far back as 1979, at an altitude outside the troposphere
(0 < altitude, h < 10 km) and stratosphere (10 < h < 50 km). The infra-red emission spectrum in the range 6–25 μm
escaping from Earth represents a black-body emitter with a temperature of c. 290 K, with absorptions (i.e., dips)
between 12 and 17 μm, around 9.6 μm, and λ < 8 μm. These wavelengths correspond to infra-red absorption bands
of CO2, O3 and H2O respectively, three atmospheric gases that contribute to the primary greenhouse effect.
Fig. 2 Infrared emission spectrum escaping to space as observed in 1979 by the Nimbus 4 satellite outside the earth's
atmosphere. Absorptions due to CO2 between 12 and 17 μm, O3 (around 9.6 μm) and H2O (λ < 8 μm) are shown.
With permission from Dickinson, R. E.; Clark, W. C. (eds), Carbon Dioxide Review (1982) OUP.
That said, the argument that the primary greenhouse gases have maintained Planet Earth at a constant temperature
of c. 290 K presupposes that their concentrations have remained approximately constant over very long periods of
time. This has not happened with CO2, CH4, and to a lesser extent O3 over c. 270 years since the start of the Industrial
Revolution. It is changes in the concentrations of these gases and newer anthropogenic (i.e., man-made) greenhouse
gases that have caused a secondary greenhouse effect to occur over this time window, leading to the temperature
rises that we are all experiencing today. That, at least, is the argument of the proponents of the “greenhouse gases,
mostly CO2, equals global warming” school of thought. There is no doubt that the concentration of CO2 in our
atmosphere has risen 43% from c. 280 ppm by volume (ppmv) to current levels of c. 400 ppmv over the last
270 years. (1 ppmv is equivalent to a number density of 2.46 × 1013 molecules cm− 3 for a pressure of 1 bar and a
temperature of 298 K.) It is also not in doubt that the average temperature of our planet has risen by c. 0.8–
1.1 K over this same time window (Fig. 3). In the author's opinion, however, what is not proven is that there is a
cause-and-effect correlation between these two facts, the main problem being that there is not sufficient structure
or resolution with time in either the CO2 concentration or the temperature data. Even more recent data of the last
100 years (Fig. 4), where the correlation seems to be better established, will not convince the skeptic. Indeed, being
devil's advocate, if the axes labels in Fig. 4 were removed and the graphs only were displayed, most scientists would
surely say that the two datasets displayed on the y-axis in red/yellow and blue might be correlated as whatever
the x-axis represents changes, but they could not prove it. History will perhaps prove that this is yet another example
of a common misconception of science, that it confirms certainty on any issue; this is very rarely the case.
Fig. 3 The average temperature of Planet Earth and the concentration level of CO2 in the Earth's atmosphere during the last
1000 years.
With permission from
www.env.gov.bc.ca/air/climate/indicat/images/appendnhtemp.gif and www.env.gov.bc.ca/air/climate/indicat/images/appendC
O2.gif.
Fig. 4 The average temperature of Planet Earth and the concentration level of CO2 in the Earth's atmosphere during
the “recent” history of the last 100 years.
With permission from the web sites shown in the figure.
As demonstrated most clearly by the recent IPCC 2013 report, however, the consensus of world scientists, and
certainly physical scientists, is that a strong correlation does exist and the number of such scientists grows rapidly
every year. It is important, however, to remember that the temperature rises we are now experiencing due
to secondary greenhouse gases post the Industrial Revolution, c. 0.8–1.1 K, is small compared to that established
over thousands of years before the Industrial Revolution, c. 34 K, due to primary greenhouse gases. If nothing else,
this fact shows how fragile the climate of Planet Earth is to relatively small changes in concentrations of gas-phase
molecules in our atmosphere.
By contrast, an excellent example in atmospheric science of sufficient resolution being present to confirm very
strongly a correlation between two sets of data occurred in 1989. The concentrations of O3 and the ClO free radical
in the stratosphere were shown to have a strong anticorrelation effect when data were collected by an aircraft as a
function of latitude in the Antarctic (Fig. 5). There was not only the general observation that a decrease of
O3 concentration correlated with an increase in ClO concentration, but much more important the resolution was
sufficient to show that at certain latitudes dips in O3 concentration corresponded exactly with rises in ClO
concentration. Even the most doubting scientist accepted that the decrease in O3 concentration in the Antarctic
Spring was related somehow to the increase in ClO concentration, and this result led to an understanding over the
next 10–15 years of the heterogeneous chemistry of chlorine-containing compounds on polar stratospheric clouds.
Fig. 5 Anticorrelation between the concentrations of ozone, O3, and the chlorine monoxide radical, ClO, in the stratosphere
above the Antarctic during their Spring season of 1987.
With permission from Anderson et al., J. Geophys. Res. D., 1989, 94, 11465. https://doi.org/10.1029/JD094iD09p11465.
It would be very surprising if there was not some relationship between increases in CO2 concentration and the
temperature of Planet Earth over the last 270 years. Nevertheless, there are two aspects of Fig. 3 in particular that
remain unanswered by proponents of such a simple theory. First, the data suggests that Tearth actually decreased
between 1750 and c. 1920 whilst the CO2 concentration increased from c. 280 to 310 ppmv over this time window.
Second, the drop in temperature around the year 1480, often called the “little ice age”, was not mirrored by a similar
drop in CO2 concentration. All that said, however, the apparent “agreement” between rises of both CO2 levels
and Te over the last 50 years is very striking. The most likely explanation may be that there are multitude of effects,
the largest of which is the concentrations of secondary greenhouse gases in the atmosphere, contributing to the
increase in temperature of Planet Earth. At certain times of history, these effects have been “in phase” (as now), at
other times they may have been in “antiphase” and working against each other.
The Physical Properties of Secondary Greenhouse Gases
In this section, we address only secondary greenhouse gases because they are currently causing the huge majority of
problems to our climate. Notice that the word climate is used, and not weather. The latter describes a very short-
term effect, for example, will there be a frost tonight, will it rain tomorrow? This is completely different
from climate where data and trends from the past are used to predict the future, often on a timescale of tens to
hundreds of years. It appears that even the current President of the United States makes this elementary mistake.
Qualitative Description
The fundamental physical property of a greenhouse gas is that it must absorb infrared radiation via one or more of
its vibrational modes in the infra-red range of 6–25 μm. Furthermore, since the dominant primary greenhouse gases
of CO2, O3, and H2O absorb in the range 12–17 μm (or 590–830 cm− 1), 9.6 μm (1040 cm− 1), and λ < 8 μm
(> 1250 cm− 1), an effective secondary greenhouse gas is one which absorbs infra-red radiation strongly outside these
ranges of wavelengths. A molecular vibrational mode is only infrared active if the motion of the atoms generates a
dipole moment. That is, dμ/dQ must be nonzero, where μ is an instantaneous dipole moment and Q a displacement
coordinate representing the vibration of interest. It is worth stating the obvious straightaway; N2 and O2, being
homonuclear diatomic molecules which constitute over 99% of the Earth's atmosphere, do not absorb infrared
radiation because their sole vibrational mode does not generate a dipole moment, so they play no part in the
greenhouse effect and global warming. It is only trace gases in the atmosphere (Table 1) such as CO2 (0.04%),
CH4 (0.0002%), O3 (3 × 10− 6%) and chlorofluorocarbons such as CF2Cl2 (5 × 10− 8%) which contribute to the
(secondary) greenhouse effect. Using different words to those used in the earlier section on “The Science of
Greenhouse Gases and the Greenhouse Effect”, it appears that the Earth's atmosphere is particularly fragile if less
than 1% of the molecules present can have such a major effect on life on the planet. Furthermore, the most
important molecular trace gas, CO2, absorbs via its ν2 bending vibrational mode at 667 cm− 1 or 15.0 μm, which
coincidentally is very close to the peak of the Earth's black-body curve; the spectroscopic properties of CO2 have not
been kind to the environment. Thus, infra-red spectroscopy of gas-phase molecules, in particular at what
wavelengths and how strongly a molecule absorbs such radiation, will be important properties to determine how
effective a trace pollutant will be to the greenhouse effect. It is stressed that there is no new science here, this is
basic undergraduate physical sciences. Thus, writing as a spectroscopist, it is my opinion that there is little new in the
last one hundred years about the science of greenhouse gases.
Table 1 Main constituents of ground-level clean air in the Earth's atmosphere
Molecule
Mole fraction
ppmva (2018)
ppmv (1748)
N2
0.78 or 78%
780,900
780,900
O2
0.21 or 21%
209,400
209,400
H2O
0.03 (100% humidity, 298 K)
30,000
31,000
H2O
0.01 (50% humidity, 298 K)
10,000
16,000
Molecule
Mole fraction
ppmva (2018)
ppmv (1748)
Ar
0.01 or 1%
9300
9300
CO2
4.0 × 10− 4 or 0.040%
400
280
Ne
1.8 × 10− 5 or 0.002%
18
18
CH4
1.8 × 10− 6 or 0.0002%
1.80
0.72
N2O
3.2 × 10− 7 or 0.00003%
0.32
0.27
O3b
3.4 × 10− 8 or 0.000003%
0.034
0.025
All CFCsc
8.7 × 10− 10 or 8.7 × 10− 8%
0.0009
0
All HCFCsd
1.9 × 10− 10 or 1.9 × 10− 8%
0.0002
0
All PFCse
8.3 × 10− 11 or 8.3 × 10− 9%
0.00008
0
All HFCsf
6.1 × 10− 11 or 6.1 × 10− 9%
0.00006
0
a Parts per million by volume. 1 ppmv is equivalent to a number density of 2.46 × 1013 molecules cm− 3 for a pressure of 1 bar and
a temperature of 298 K.
b The concentration level of O3 is very difficult to determine because it is poorly mixed in the troposphere. It shows large
variation with both region and altitude.
c Chlorofluorocarbons (e.g., CF2Cl2).
d Hydrochlorofluorocarbons (e.g., CHClF2).
e Perfluorocarbons (e.g., CF4, C2F6, SF5CF3, SF6).
f Hydrofluorocarbons (e.g., CH3CF3).
The second property of interest is the lifetime of the pollutant in the Warth's atmosphere: the longer the lifetime,
the greater contribution a greenhouse gas will make to global warming. It is noted that the word lifetime can mean
different things to different scientists according to their discipline, and this issue is discussed further in the section
titled “Methodology of How Important Physical Properties of Greenhouse Gases Are Measured”. The main physical
or chemical processes removing pollutants from the troposphere and stratosphere are reactions with OH free
radicals and/or electronically-excited oxygen atoms, O*(1D), and photodissociation in the range 200–300 nm (in the
stratosphere) or 300–500 nm (in the troposphere). Thus, the reaction kinetics of pollutant gases with OH and O*(1D)
and their photochemical properties in the UV/visible will yield important parameters to determine their
effectiveness as greenhouse gases. All these data are incorporated into a dimensionless number, the global warming
potential (GWP), sometimes called the greenhouse potential, of a greenhouse gas. All values are calibrated with
respect to CO2 whose GWP value is 1. So, a molecule with a large GWP is one with strong infrared absorption in the
windows where the primary greenhouse gases such as CO2 etc. do not absorb, long lifetimes, and concentrations
rising rapidly due to human presence on the planet. GWP values of some of the most important secondary
greenhouse gases are given in the bottom row of Table 2. They range between 1 and 23,500, and CO2 has the lowest
GWP value of the eight greenhouse gases shown.
Table 2 Eight examples of secondary greenhouse gases and their contribution to global warming
Greenhouse gas
CO2
O3
CH4
N2O
NF3
CF2Cl2
[all CFCs]
SF6
SF5CF3
Concentration
(2018) / ppmv
400
0.034
1.80
0.32
c.1.0 x 106
0.0005
[0.0009]
5.6 x 106
1.2 x 107
Change in
concentration
(17482018) / ppmv
120
0.009
1.08
0.05
c.1.0 x 106
0.0005
[0.0009]
5.6 x 106
1.2 x 107
Radiative efficiency,
ao a / W m-2 ppbv-1
1.37 x
105
3.33 x 102
3.63 x
104
3.00 x
103
0.20
0.32
[0.200.32]
0.57
0.59
Total radiative
forcing a,b / W m-2
1.82
c. 0.35 c
0.48
0.17
2.0 x 104
0.17
[0.26]
4.1 x 103
7.2 x 105
Contribution from
long-lived secondary
greenhouse gases
excluding ozone to
overall greenhouse
effect a,d / %
64
(57)
0
(11)
17
(16)
6
(5)
0.01
(0.01)
6 [9]
(5 [8])
0.14
(0.13)
0.0025
(0.0023)
Lifetime,
a,e /
years
50200 f
days
weeks g
12
121
500
100
[451020]
3200
800
Global warming
potential (100 year
projection) a /
dimensionless
1
h
28
265
16100
10200
[4660
13900]
23500
17400
a Data from the latest IPCC AR5 report published in 2013, although the data quoted are only accurate up to 2011. Data for
NF3has since been updated by Totterdill et al. (2016). Despite all the detail, the important figure is unchanged over the last c.
30 years, that 80%–85% of the total radiative forcing, now 2.83 W m− 2, is due to CO2 and CH4.
b Due to change in concentration of long-lived greenhouse gas from the preindustrial era to the present time.
c An estimated positive radiative forcing of 0.40 W m− 2 in the troposphere is partially canceled by a negative forcing of
0.05 W m− 2 in the stratosphere.
d Assumes the latest value for the total radiative forcing of 2.83 ± 0.03 W m− 2 from the IPCC 2013 AR5 report. The values in
brackets show the percentage contributions when the estimated radiative forcing for ozone, 0.35 W m− 2, is included in the value
for the total radiative forcing.
e Assumes a single-exponential decay for removal of greenhouse gas from the atmosphere.
f CO2 does not show a single-exponential decay, see Shine et al. (2005).
g O3 is poorly mixed in the troposphere, so a single value for the lifetime is difficult to estimate. It is removed by the reaction
OH + O3 → HO2 + O2. Its concentration shows large variations both with region and altitude.
h GWP values are generally not applied to short-lived pollutants in the atmosphere, due to serious inhomogeneous changes in
their concentration.
Quantitative Description
Information in the “Qualitative Description” section above is described in descriptive terms. However, all the data
can be quantified. The term that characterizes the infrared absorption properties of a greenhouse gas is the radiative
efficiency, ao. It measures the strength of the absorption bands of the greenhouse gas, x, integrated over the infra-
red black-body region of c. 400–1700 cm− 1. It is a (per molecule) microscopic property and is usually expressed in the
slightly unusual units of W m− 2 ppbv− 1. If this value is multiplied by the change in concentration of pollutant over a
defined time window, usually the 270 years from the start of the Industrial Revolution to the current day, the
macroscopic radiative forcing in units of W m− 2 is obtained. (A pollutant whose concentration has not changed over
this long time window will therefore have a macroscopic radiative forcing of zero.) One may then compare the
radiative forcing of different pollutant molecules over this time window, showing the current contribution of
different greenhouse gases to the total greenhouse effect. Thus the IPCC 2013 report quotes the radiative forcing for
CO2 and CH4 as 1.82 and 0.48 W m− 2, respectively, out of a total for long-lived greenhouse gases of 2.83 W m− 2.
These two molecules therefore contribute 81% in total (64% and 17%, individually) to the global warming effect. In
simple language, the radiative forcing value gives a current-day estimate of how serious a greenhouse gas is to the
environment in the future, using concentration data from the past.
The overall effect of one molecule of pollutant on the Earth's climate is described by its GWP value. It measures the
radiative forcing, Ax, of a pulse emission of the greenhouse gas over a defined time period, t, usually 100 years,
relative to the time-integrated radiative forcing of a pulse emission of an equal mass of CO2:
(2)
The GWP value therefore informs how important one molecule of pollutant x is to global warming via the
(secondary) greenhouse effect compared to one molecule of CO2, which is defined to have a GWP value of unity; it
attempts to project into the future how serious the presence in the atmosphere of one molecule of a long-lived
greenhouse gas will be. For most greenhouse gases, the radiative forcing following an emission at t = 0 takes a
simple exponential form:
(3)
where τx is the lifetime for removal of species x from the atmosphere. For CO2, a single-exponential decay is not
appropriate because the lifetime ranges from 50 to 200 years, dependent on latitude and longitude. Eq. (3) is then
modified to:
(4)
where the response function, the bracket in the right-hand side of Eq. (4), is derived from more complete carbon
cycles. Values for bi (i = 0–4) and τi (i = 1–4) have been given by Shine et al. It is important to note that the radiative
forcing, Ao, in Eqs. (2)–(4) has units of W m− 2 kg− 1. For this reason, it is given a different symbol to the microscopic
radiative efficiency, ao, with units of W m− 2 ppbv− 1, but conversion between the two units is simple. The time
integral of the large bracket on the right-hand side of Eq. (4), defined KCO2, has dimensions of time, and takes values
of 13.4 and 45.7 years for a time period of 20 and 100 years, respectively, the values of t for which GWP values are
most often quoted. Within the approximation that the greenhouse gas, x, follows a single-exponential time decay in
the atmosphere, it is then possible to parameterize Eq. (2) to give an analytical expression for the GWP of x over a
time period t:
(5)
In this simple form, the GWP value only incorporates values for the radiative efficiency of greenhouse gases x and
CO2, ao,x and ao, CO2; the molecular weights of x and CO2; the lifetime of x in the atmosphere, τx; the time period into
the future over which the effect of the pollutant is determined; and the constant KCO2 which can easily be
determined for any value of t. Thus the GWP value is a microscopic property of one molecule of the greenhouse gas,
and it scales with both the lifetime and the microscopic radiative forcing of the pollutant. Note that its recent rate of
increase in concentration (e.g., the rise in concentration per annum over the last decade) does not contribute
directly to the GWP value, causing some criticism by Shine and others of the use of GWPs in policy formulation.
Interpretation of the Data
Despite negligible change in the science, this author believes that there has been major changes over the last c.
50 years in how the data is interpreted. Data for eight secondary greenhouse gases is shown in Table 2. CO2, O3, CH4
and N2O constitute naturally-occurring greenhouse gases whose concentration levels ideally would have remained
constant at preindustrial revolution levels. (Note that although H2O vapor is the most abundant primary greenhouse
gas in the atmosphere, it is neither long-lived nor well mixed: concentrations range from 0% to 3% (i.e., 0–
30,000 ppmv) over the planet, and the average lifetime is only a few days. Its average global concentration has not
changed significantly in the last 270 years, it therefore has zero radiative forcing and does not count as a secondary
greenhouse gas.) O3, CH4 and N2O are gases with larger ao values than that of CO2. The CH4 concentration, although
small, has increased by c. 150% since preindustrial times, a much greater percentage increase than even CO2, c. 43%.
It is the second most important secondary greenhouse gas after CO2, and its current total radiative forcing is c. 26%
that of CO2. O3 and N2O have smaller percentage increases, c. 36% and 16% respectively, over this same time period.
N2O has the fourth highest total radiative forcing of all these naturally-occurring gases, although the case of O3 with
the third highest total is anomalous (footnote c of Table 2). Dichloro-difluoromethane, CF2Cl2, is one of the most
common of chlorofluorocarbons. These man-made, anthropogenic chemicals have grown in concentration from zero
in preindustrial times to a current total concentration of 0.9 ppbv; 1 ppbv is equivalent to 1 part per 109 (billion) by
volume, or a number density of 2.46 × 1010 molecules cm− 3 at 1 bar pressure and 298 K. Their concentration is now
decreasing due to the 1987 Montreal and later International Protocols, introduced to halt stratospheric ozone
destruction. SF6 and CF3SF5 are two long-lived halocarbons with currently very low concentration levels, but with
high annual percentage increases and exceptionally long lifetimes in the atmosphere. They have very high ao and
GWP values, essentially because of their large number of strong infrared-active vibrational modes and their long
lifetimes. NF3 is probably the latest secondary greenhouse gas recorded in such Tables.
It is noted that CO2 and CH4 have the lowest GWP values of all greenhouse gases. It begs the obvious question; why
is there such concern about levels of CO2 in the atmosphere, and with the possible exception of CH4 no other
greenhouse gas is hardly ever mentioned in the media? The answer is that the overall contribution of a pollutant to
the greenhouse effect, present and future, involves a convolution of its change of concentration with the GWP value.
Thus CO2 and CH4 currently contribute most to the (secondary) greenhouse effect (third bottom row of Table 2)
simply due to their high change in atmospheric concentration since the Industrial Revolution. Note, however, that
the ao and GWP values of both gases are relatively low. Indeed, the ν2 bending mode of CO2 at 15.0 μm, which is the
vibrational mode most responsible for greenhouse activity in CO2, may be close to saturation. By contrast, CF3SF5 is a
perfluorocarbon molecule with the highest microscopic radiative forcing of any known greenhouse gas (earning it
the title “super”greenhouse gas), even higher than that of SF6. SF6 is an anthropogenic chemical used extensively as a
dielectric insulator in high-voltage industrial applications and, as noted first by Sturges et al., the variations of
concentration levels of SF6 and CF3SF5 with time in the last 50 years have tracked each other very closely. The GWP
values of these two molecules are very high, SF6 being slightly higher because its atmospheric lifetime, c. 3200 years,
is about four times greater than that of CF3SF5. However, the contribution of these two molecules, especially CF3SF5,
to the overall greenhouse effect is still small because their atmospheric concentrations, despite rising rapidly at the
rate of c. 6%–7% per annum, are still very low, at the level of only a few parts per 1012 (trillion) by volume.
What is perhaps most revealing about Table 2 is that nothing much has changed in the last c. 50 years. It is now
accepted that there are gases realized to be long-lived greenhouse gases (e.g., NF3, CF3SF5) which was not
appreciated five decades ago. Furthermore, some of the finer detail of the Table (e.g., accuracy of the radiative
efficiency, a0, values, and atmospheric lifetimes of secondary greenhouse gases) plus improved data for
photodissociation cross sections of greenhouse gases as a function of both wavelength and temperature have
improved hugely. The important figure, however, has not changed; 80%–85% of contribution from long-lived
greenhouse gases to the overall secondary greenhouse effect is due to two gases only, CO2 and CH4, with the
former's contribution now being approximately three and a half times that of the latter. It is accepted that 15%–20%
is due to other long-lived species, but it is surely imperative that the public concentrates on what really counts, that
is, CO2 and CH4. That said, what has changed is the total radiative forcing and concentration of each gas. These
values have increased year by year, data (which surely cannot be challenged) published in the World Meteorological
Organization Greenhouse Gas Bulletins, the UN Intergovernmental Panel on Climate Change (IPCC) Assessment
Reports, and the US Environmental Protection Agency Climate Change Indicators reports, to name but three. The
accuracy and precision of the data for CO2 over the last 60 years is particularly striking (Fig. 6).
Fig. 6 Increasing levels of CO2 in the Earth's atmosphere over the last 60 years, recorded at the Mauna Loa Observatory in
Hawaii. There is a relentless increase in concentration, year by year (with permission fromwww.esrl.noaa.gov). The small
oscillations every 6 months are caused by seasonal changes due to photosynthetic activity of vegetation which consumes CO 2.
The oscillations reduce when data are recorded in regions with smaller amounts of vegetation, such as Antarctica.
To give some numbers, the total radiative forcing was 1.17 W m− 2 in 1960, 1.48 W m− 2 in 1970, 1.91 W m− 2 in 1980,
and 2.45 W m− 2 in 1990 (the first IPCC Assessment Report 1 (AR1)). It then increased to 2.45 W m− 2 in 1995 (AR2,
data from 1992), 2.43 W m− 2 in 2001 (AR3, data from 1998), 2.63 W m− 2 in 2007 (AR4, data from 2005) and 2.83
(± 0.03) W m− 2 in the latest report of 2013 (AR5, data from 2011). Slightly different figures were tabulated by the US
Environmental Protection Agency in 2016; they reported a continuous increase with no discontinuities over the last
36 years from 1979 to 2015, but the differences from the IPCC reports are small. If we concentrate on the most
damaging long-lived greenhouse gas, CO2, the current level is c. 400 ppmv. At an International Symposium in 2005
on the stabilization of greenhouse gases, the view of most climate scientists then was that if this value exceeded
550 ppmv the temperature of the Earth's atmosphere would likely rise by over 2°C since the preindustrial era, c.
1.0°C over present levels. (It should be noted that the 550 ppmv figure is decreasing as the modeling improves, with
much depending on how the word “likely” is interpreted.) With the atmosphere retaining c. 7% more water vapor for
every 1°C rise in temperature (UN IPCC report 2013), the primary greenhouse effect will increase, and the point of
no return may be reached because it will be almost impossible to stabilize Tearth; this could be the start of what
scientists call the runaway greenhouse effect. A recent article by Victor et al. has even suggested that the 2°C target
by the end of this century, only 82 years away, is nonscientific, and probably was set at this low level solely for
expedient political purposes. In 2014, a combined Royal Society and National Academy of Sciences report suggested
that, without drastic action and a “business as usual” model of carbon emissions, the most likely rise would be 3.4 to
5.6°C above preindustrial levels. Finally, the COP21 Paris Convention in 2015 hoped that all countries would aim
much further, and achieve a target as low as 1.5°C; this will surely be challenging given that we are already c. 1.0°C
there. Many books have already predicted what might happen to the world's civilization and ecosystem if the
temperature rise is above 3°C.
General Comments on Very Long-Lived Secondary Greenhouse Gases
Even in 1994, Ravishankara et al. wrote that that the release of any long-lived species into the atmosphere should be
viewed with great concern. They noted that the chlorofluorocarbons, with relatively “short” lifetimes of c. 100 years,
have had a disastrous effect over a relatively short period of time, c. 30–50 years, on the ozone layer in the
stratosphere that protects humans from harmful UV radiation. However, following implementation of international
treaties (e.g., Montreal, 1987) it is now expected that the ozone layer will recover within 50–100 years and probably
by the end of this century: a wonderful example of successful collaboration between lab-based scientists and
international politicians. At present, there are no known undesired chemical effects of low concentrations of
perfluorocarbons such as CF4, SF6, and CF3SF5 in the atmosphere. However, their rapidly-increasing concentrations (c.
7% per annum for SF6) and their exceptionally long lifetimes (thousands, not hundreds of years) means that life on
Earth may not be able to adapt to any changes these gases may cause in the future. Ravishankara et al. suggested
that all such long-lived molecules should be considered guilty, unless proven otherwise. The simplest and probably
naïve suggestion remains that humans should not emit any more pollutants into the atmosphere than are absolutely
necessary. The worldwide debate just starting, probably 50 years too late, is what constitutes “absolutely
necessary.”Many scientists are increasingly coming to the view that it would be wonderful if the greenhouse gas
problem was as easy to solve as the stratospheric ozone problem had been 30 years ago.
Nobody should believe that the land temperature is the only criterion for measuring the effect of global warming.
The rising of sea levels, and their increasing temperature and decreasing pH are thought by Cheng et al. to be just as,
if not more significant, and the analogy of the health of a human being dependent on many factors (e.g., blood
pressure, weight and heart rate, to name but three) is now used by many, including Victor et al. Gore, a contender
for the US 2000 Presidency, is perhaps the only world political leader to have been making the same points with
vehemence for many years, especially now that he has left the political world. So despite uncertainties in a very
small number of aspects of the scientific data, this author suggests that if the above accumulated evidence and
predictions are shown to be wrong, then history will judge Climate (Non-) Change to be the biggest scientific hoax of
all time.
In conclusion to this section, the macroscopic properties of greenhouse gases, such as their method of production,
their concentration and their annual rate of increase or decrease, are therefore mainly controlled by environmental
and sociological factors, such as industrial and agricultural methods, and ultimately population levels on Planet
Earth. The microscopic properties of these compounds, however, are controlled by factors that undergraduates
world-wide learn about in science degree courses: infrared spectroscopy, reaction kinetics and UV/VIS absorption
cross sections, which are all aspects of Analytical Sciences.
Methodology of How Important Physical Properties of Greenhouse Gases Are Measured
Table 2 shows that there are three important molecular properties that determine the severity or otherwise of a
greenhouse gas: the infra-red radiative efficiency over the range c. 400–1700 cm− 1, the concentration and the
lifetime. From an analytical point of view, these values are determined from a combination of laboratory and field
measurements, or modeling of the chemical composition of the atmosphere.
Infra-red (IR) spectra of greenhouse gases are laboratory-based measurements, usually made with Fourier Transform
interferometers. To put measurements onto an absolute scale, calibration gases of known radiative efficiencies with
IR absorptions in the range c. 400–1700 cm− 1 are measured simultaneously on the same instrumentation. The IR
spectrum is convoluted with the absorption curve of a universal black body at c. 290 K (or + 17°C), giving the total
radiative efficiency. The results are presented in the slightly unusual units of W m− 2 ppbv− 1 used by atmospheric
climate scientists. This methodology has been described recently and extensively by Hodnebrog et al. in 2013.
Measurements of the atmospheric concentration of greenhouse gases are usually field measurements, that is, made
outside the laboratory which are common to research institutes or universities. Measurements
of current concentrations are either in situ experiments using stationary instruments at clean air stations often in
remote coastal situations, for example, Mauna Loa Hawaii (at 3400 m above sea level), Baring Head New Zealand,
Jungfraujoch Switzerland (at 3460 m above sea level), Mace Head Ireland, or air samples are taken in glass flasks to
the laboratory for analysis. There is now a worldwide array of such instrumentation, the results are shared between
the communities, and the results are remarkably consistent. The US Environmental Protection Agency (EPA) collates
all the data, updated annually. Data are analyzed in a slightly different way by the UN Intergovernmental Panel for
Climate Change Assessment Reports every c. 5 years, but the main results are not essentially different.
Measurements on concentration of greenhouse gases going back in time (which can be hundreds of thousands of
years) come from ice core or firn samples, often taken in Greenland or the Antarctic. This is only relevant for
biogenic greenhouse gases such as CO2, CH4 and N2O, that is, those occurring naturally, because anthropogenic
greenhouse gases, by definition, are person-made in the last century. The subject was reviewed in 1990 by Jasper et
al. and in 1996 by Battle et al. Isotope fingerprinting comparing the 12C to 13C ratio in CO2 and CH4 can be an accurate
indicator of concentrations of these gases stretching back in time. The techniques used in the laboratory to measure
atmospheric samples from gas flasks or ice cores are infra-red spectroscopy, gas chromatography and mass
spectroscopy, the CO2 samples coming (amazingly) from combustion of algae frozen into the ice! The EPA
conclusions are emphatic; concentrations of CO2 and CH4, especially, are now at an unprecedented high level which
have not occurred in the last 800,000 years.
The lifetime of a greenhouse gas in the atmosphere is more problematic, because this word means different things
to different scientists. To a physical chemist, the lifetime generally means the inverse of the pseudo-first-order rate
constant of the dominant chemical or physical process that removes the greenhouse gas from the atmosphere.
Taking CH4 as an example, it is removed from the troposphere via oxidation of the OH free radical by the reaction
OH + CH4 → H2O + CH3. The rate coefficient for this reaction at 290 K is well-known from laboratory gas kinetics, so
the lifetime is approximately equal to (k290[OH])− 1. Assuming the average tropospheric OH concentration to be
0.05 pptv or 1.2 × 106 molecules cm− 3, the lifetime of CH4 is calculated to be c. 4 years. This is within a factor of three
of the accepted value of 12 years (Table 2), and for most scientists this factor is significant. The difference arises
because CH4 is not emitted uniformly from the earth's surface, a finite time is needed to transport CH4 via convection
and diffusion into the troposphere, and oxidation occurs at different altitudes in the troposphere where the OH
concentration varies from its average value. In grossly simple language, this is as an example of a two-step kinetic
process,
(6)
with first-order rate constants k1 and k2. The first step, A → B, represents the transport of the pollutant into the
atmosphere, whilst the second step, B → C, represents the chemical or photolytic process, for example, reaction
with an OH radical in the troposphere, removing the pollutant from the atmosphere. In general, the overall rate of
the process (whose inverse is called the lifetime) will be a function of both k1 and k2, but its value will be dominated
by the slower of the two steps. Thus, in calculating the lifetime of CH4 simply as (k290[OH])− 1, we are assuming that
the first step, transport into the region of the atmosphere where chemical removal occurs, is infinitely fast.
The exceptionally long-lived greenhouse gases in Table 2 (e.g., NF3, SF6 and SF5CF3) behave in the opposite sense.
Now, the slow, rate-determining process is the first step, that is, transport of the greenhouse gas from the Earth's
surface into the region of the atmosphere where chemical or photolytic removal occurs. These processes that
ultimately remove NF3 etc. will have very little influence on the lifetime, that is, k1 « k2 in Eq. (6). These molecules do
not react with OH or O* (1D) to any significant extent, and are not photolysed by visible or UV radiation in the
troposphere or stratosphere. They therefore rise higher into the mesosphere (altitude > 60 km) where the dominant
processes that can remove pollutants are electron attachment and vacuum-UV photodissociation at the Lyman-
α wavelength of 121.6 nm. We can define a chemical lifetime, τchemical, for such species as:
chemical
(7)
ke is the electron attachment rate coefficient, σ121.6 is the absorption cross-section at this wavelength, [e−] is the
average number density of electrons in the mesosphere, J121.6 is the mesospheric solar flux and Φ121.6 the quantum
yield for dissociation at 121.6 nm. Often, the photolysis term is much smaller than the electron-attachment term,
and the second term inside the squared bracket is ignored. However, the value of τchemical is a function of position,
particularly altitude, in the atmosphere. In the troposphere, τchemical will be infinite because both the concentration of
electrons and J121.6 are effectively zero, but in the mesosphere τchemical will be much less. However, multiplication
of ke for NF3 etc. by a typical electron density in the mesosphere, c. 104 cm− 3, yields a chemical lifetime which is far
too small and bears no relation to the true atmospheric lifetime, simply because most of the long-lived greenhouse
gas does not reside in the mesosphere.
One may therefore ask where the quoted lifetimes for NF3, SF6 and SF5CF3 of 500, 3200 and 800 years (Table 2),
respectively, come from. There are indeed major uncertainties in these values, but most people would say that,
unlike short-lived greenhouse gases, a factor of c. three, for example, between the lower and upper limits of 1000
and 9000 years for SF6, is not important. Lifetimes of such long-lived greenhouse gas can only be obtained from
globally-averaged loss frequencies. The pseudo-first-order destruction rate coefficient for each region of the
atmosphere is weighted according to the number of molecules of compound in that region,
global
(8)
where i is a region, ki is the pseudo-first-order removal rate coefficient for region i, Vi is the volume of region i,
and ni is the number density of the greenhouse gas under study in region i. The lifetime is then the inverse of
< k > global. This subject was reviewed and applied to CF2Cl2, lifetime c. 100 years, in 2002 by Limao-Vieira et al. The
averaging process, however, needs input from a 2- or 3-dimensional model of the atmosphere to supply values for ni.
This then becomes essentially a meteorological, not a chemical problem. It may explain the different interpretations
of what the lifetime of a greenhouse gas means.
Conclusion
The sub-section titled “General Comments on Very Long-Lived Secondary Greenhouse Gases” made the assumption
that we have an enormous environmental problem that could potentially affect every person on the planet. The
latest predictions are that we have c. 30–50 years to make some major changes in lifestyle, otherwise the increase in
Planet Earth's temperature will be too great to reverse. For this runaway greenhouse effect to start, the experts only
differ in what this increase in temperature needs to be, values ranging from + 3°C to + 6°C above preindustrial
temperatures. Note that we are already c. + 1.0°C there. In a parallel article being published in Earth Systems and
Environmental Sciences, this author makes some suggestions how this huge global problem should be addressed.
Further Reading
1. M. Battle, M. Bender, T. Sowers, P.P. Tans, J.H. Butler, J.W. Elkins, J.T. Ellis, T. Conway, N. Zhang, P.Lang and A.D. Cl
arke, Atmospheric Gas Concentrations Over the Last Century Measured in Air From Firn at the South
Pole, Nature 383, 1996, 231–235.
2. L. Cheng, J. Zhu and J. Abraham, Global Upper Ocean Heat Content Estimation: Recent Progress and the Remaining
Challenges, Atmos. Oceanic Sci. Letts. 8, 2015, 333–338.
3. Climate Change Evidence and Causes: an Overview from the Royal Society and the National Academy of
Sciences, 2014.
4. Gore A (2006). An Inconvenient Truth. http://en.wikipedia.org/wiki/An_Inconvenient_Truth.
5. O. Hodnebrog, M. Etminan, J.S. Fuglestvedt, G. Marston, G. Myhre, C.J. Nielsen, K.P. Shine and T.J.Wallington, Glo
bal Warming Potentials and Radiative Efficiencies of Halocarbons and Related Compounds: A Comprehensive
Review, Rev. Geophys. 51, 2013, 300–378.
6. Intergovernmental Panel on Climate Change 5th Assessment Report, 2013, Cambridge University Press, Working
group 1, chapters 1, 2 and 8.
7. International Symposium on the Stabilization of Greenhouse Gases, 2005.
http://www.g8.utoronto.ca/enviroment/2005steeringcommittee.pdf.
8. J.P. Jasper and J.M. Hayes, A Carbon Isotope Record of CO2 Levels During the Last Quaternary. Nature
347, 1990, 462–464.
9. R. Krska and R. Kellner, Chlorofluorohydrocarbons, In: Encyclopedia of Analytical Science, 1st ed., 1995, 700–707.
10. P. Limao-Vieira, S. Eden, P.A. Kendall, N.J. Mason and S.V. Hoffmann, VUV Photoabsorption Cross Sections for
CCl2F2, Chem. Phys. Lett. 364, 2002, 535–541.
11. D.E. Oram, Chlorofluorocarbons and Other Halocarbons, In: Encyclopedia of Analytical Science, 2nd ed.,
2005, 80–89.
12. A.R. Ravishankara and E.R. Lovejoy, Atmospheric Lifetime, Its Application and its Determination: CFC-Substitutes
as a Case Study, J. Chem. Soc. Farad. Trans. 90, 1994, 2159–2169.
13. K.P. Shine, J.S. Fuglestvedt, K. Hailemariam and N. Stuber, Alternatives to the Global Warming Potential for
Comparing Climate Impacts of Emissions of Greenhouse Gases, Climate Change 68, 2005, 281–302.
14. W.T. Sturges, T.J. Wallington, M.D. Hurley, K.P. Shine, K. Sihra, A. Engel, D.E. Oram, S.A. Penkett, R.Mulvaney and
C.A.M. Brenninkmeijer, A Potent Greenhouse Gas Identified in the Atmosphere: SF5CF3, Science 289, 2000, 611–613.
15. A. Totterdill, T. Kovacs, W. Feng, S. Dhomse, C.J. Smith, J.C. Gomez-
Martin, M.P. Chipperfield, P.M.Forster and J.M.C. Plane, Atmospheric lifetimes, infrared absorption spectra, radiative
forcings and global warming potentials of NF3 and CFC-115, Atmos. Chem. Phys. Discuss. 2016,
https://doi.org/10.5194/acp-2016-231.
16. R.P. Tuckett, The Role of Atmospheric Gases in Global Warming, In: T.M. Letcher, (Ed), Climate Change: Observed
Impacts on Planet Earth, 1st ed., 2009, 3–19, Chapter 1. https://doi.org/10.1016/B978-0-444-53301-2.00001-4.
17. US Environmental Protection Agency, Climate change indicators and climate forcing, 2016, 16.
https://www.epa.gov/climate-indicators.
18. D.G. Victor and C.F. Kennel, Ditch the 2°C Warming Goal, Nature 514, 2015, 30–31.
19. World Meteorological Organization Greenhouse Gas Bulletins, 1970.
