GPHY 314 – The “Greenhouse Effect” and the role of methane (CH4)
Morgan MacKay
5833788
Adam Collingwood
Friday, April 8, 2011
GPHY 314 – The “Greenhouse Effect” and the role of methane (CH4)
The notion that many of the Earth’s systems are being influenced by global
climate change is no longer a proposed idea but a scientific fact with many
serious short- and long-term implications. For millions of years the Earth’s
climate remained relatively stable, undergoing natural climatic shifts and
changes over large time scales of thousands even millions of years (Zachos et al.
2001). However, the beginning of the Industrial Revolution a few hundred years
ago allowed for anthropogenic advancements that overuse and misuse the
environment and have undoubtedly resulted in significant, and dramatic
changes to the Earth’s climate and systems (Vitousek et al. 1997). Scientific
evidence of global warming, increased frequency and duration of extreme events
such as hurricanes and droughts, as well as changes in sea level and salinity, are
only a few of the many real examples that confirm the actions of global climate
change (Easterling et al. 2000). Many scientists contribute these activities to the
anthropogenic overloading of radiatively active gases (RAG’s) into the
atmosphere, such as methane (CH4), which have lead to increased global
warming and an amplified “greenhouse effect” (GHE) (Mitchell 1989).
The GHE is hardly a new phenomenon having eternally been an essential
process in the moderation of global temperatures that has allowed for the
existence and sustainment of life on Earth. Fundamentally, the GHE refers to the
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reflectance and absorption of infrared radiation by RAGs in the atmosphere. This
trapping of solar energy ultimately produces a warming effect and influences the
Earth’s climate and temperature regimes (Solomon et al. 2007). The Earth’s
atmosphere is composed of four main layers: the troposphere, stratosphere,
mesosphere and thermosphere; with scientists dominate focus on changes in
tropospheric composition, and to a lesser extent the stratosphere, where the GHE
mainly occurs (Dickenson and Cicerone 1986). The atmosphere is primarily
composed of nitrogen (N2, 78.1%), oxygen (O2, 20.9%), and argon (Ar, 0.93%),
however, trace gases (0.1%) such as water vapour (H2Og) carbon dioxide (CO2),
methane (CH4), nitrous oxide (N2O), ozone (O3), and chlorofluorocarbons
(CFCs), are the greatest determinants of global climate (Hansen et al. 2007).
Collectively, these trace gases are known as greenhouse gases (GHGs) for their
role in the GHE and radiative forcing of Earth’s climate. Although these GHGs
have always been present in the Earth’s atmosphere, occurring, emitted and
removed naturally, recent increases in their atmospheric concentrations appear
wholly anthropogenic in origin (Solomon et al. 2007). With the continuous
addition of GHGs into the atmosphere from anthropogenic sources, the GHE is
dramatically amplified, leading to increased radiative forcing of the climate as
well as a potentially detrimental warming of the Earth.
Although shadowed by the attention of carbon dioxide (CO2) and it’s
dominating influence in the GHE and climate change in general, the contribution
of methane (CH4) to these processes is becoming more widely discussed and of
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equal importance. Methane (CH4) is both a naturally occurring atmospheric gas
as well as a by-product of anthropogenic activities. An important component of
the chemistry of both the troposphere and stratosphere, methane plays an active
role in tropospheric ozone and hydroxyl-radical budgets, as well as providing
one half of the stratosphere’s hydrogen and water budget through methane
oxidation (Badr et al. 1992b). Before the Industrial Era, atmospheric
concentrations of methane remained relatively stable but have since steadily
increased allowing it become a significant contributor in amplifying the GHE.
There are essentially three characteristics that allow methane to be classified as a
significant contributor to the GHE as well as to global warming: it’s present
concentration in the atmosphere, it’s atmospheric lifetime, and it’s ability to
absorb infrared radiation (Khalil 1999). Concentrations of atmospheric methane
have increased an astounding 1000 ppb over the last two centuries from preIndustrial concentrations (low of 400ppb, high of 770 ppb), reaching 1775 ppb by
2005 (Spahni et al. 2005). Although a dramatic increase, it is still the second most
abundant trace gas in the atmosphere, preceded only by carbon dioxide.
Methane can also classified as a long-lived greenhouse gas (LLGHG), meaning
that it is chemically stable and able to persist in the atmosphere over relatively
long time scales (Lelieveld et al. 1998). Because methane is able to persist in the
atmosphere for approximately 8-12 years after emission, its climatic implications
also persist for this period of time (Khalil and Rasmussen 1990). Although most
methane molecules are destroyed or removed within ten years of being emitted,
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the continual increase of atmospheric methane concentrations may overwhelm
the atmosphere’s reduction systems and effectively increase this lifetime up to 15
years (Reay 2006). Although the atmospheric concentration of methane is less
than that of carbon dioxide, methane is a more effective absorber of infrared
radiation. Methane has a global warming potential of 23, meaning that for every
kilogram of methane that is emitted into the atmosphere, it has the equivalent
radiative forcing effect of 23 kilograms of carbon dioxide (Reay 2006). In simpler
terms, this means that over a one-hundred year period methane gas is
approximately twenty times more effective at absorbing heat than carbon
dioxide, as well as assuming twenty times the contribution to radiative forcing
than carbon dioxide over this same time period. With its long atmospheric
lifetime and high global warming potential, increases of atmospheric methane
can be considered a significant threat to the global climate and the GHE. It is
imperative that its implications be fully understood, as well as monitored,
modeled and managed appropriately.
In order to properly monitor, model and manage methane emissions, it is
important to first understand where methane comes from. As previously
mentioned, methane has both natural and anthropogenic sources. Annually,
hundreds of millions of tonnes of natural methane are emitted through processes
that include the anaerobic decomposition of organic matter in waterlogged soils,
swamps and marshes, as well from marine sediments, agricultural ruminants,
and forest fires (Crutzen et al. 1986). Recent studies reveal, however, that more
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than 70% of annual methane emissions are of anthropogenic origins. Perhaps the
largest anthropogenic methane sources occur within the agricultural industry,
specifically in Asia. The cultivation of rice provides approximately 29% of annual
anthropogenic methane emissions, with the most significant emissions found
over Asia, where populations are large and the cultivation of rice is greatest
(Neue 1993). As rice is grown on wetland fields, the organic decomposition of
wetland soils releases methane. With large populations to feed, and rice being
relatively cheap there is an abundance of rice fields and therefore an abundance
of methane emissions in these areas. Also, the manure of agricultural ruminants
provides an excellent source of fertilizer to plants and crops (Neue 1993). As
manure is composed of decomposing organic materials it also contributes to a
large source of anthropogenic methane emissions. The coal and petroleum
industries also provide relatively significant sources of methane. Methane is
often released as a product from coal deposits during coal mining operations, as
well as from petroleum sources of natural gas, of which methane is the dominant
component (Ula 1991). Although the production and distribution of natural gas
emits relative amounts of methane into the atmosphere, the leaking of natural
gas pipes also contributes as an important source (Crutzen et al. 1986). The
abundance of landfills around the globe also provides a large anthropogenic
source of methane, as methane is released in the anaerobic process of
decomposing organic wastes found within garbage mounds (Boeckx et al 1996).
An often forgotten or neglected source of methane also occurs in the treatment of
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wastewater. If wastewater is treated under anaerobic conditions, the organic
material contained within can emit methane as it is removed (Boeckx et al. 1996).
However, there still remains great uncertainty in the significance of methane
emissions from each of the above sources, as emissions and their effects are
difficult to quantify (Soloman et. al 2007).
Figure 1: Present natural and anthropogenic sources and emissions of methane.
Measurements are in teragrams/year. (Khalil 1999)
It is evident from the numerous anthropogenic methane sources that
humans have undoubtedly had a significant impact on the Earth’s atmospheric
composition and have largely contributed to the amplification of the GHE,
through the addition of GHGs, specifically methane, into the atmosphere. As
natural methane sources are largely dependant on temperature and
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precipitation, there is little that can be done to change the addition and effect of
these sources into the atmosphere. However, the ability to control and monitor
anthropogenic sources of methane exists and should be established with the
necessary research and risk assessments.
Although, we may not be able to control the natural emission of methane
into the atmosphere, there are natural methane sinks that effectively remove both
natural and anthropogenic methane from the atmosphere and influence
atmospheric methane growth rates. Approximately 80-90% of annual methane
removal can be associated with hydroxyl radicals (OH), an important oxidizing
chemical, in the troposphere and on a smaller scale dry soils and interactions
with chlorine radicals (Cl) at the marine boundary layer (Khalil 1999). In the
atmosphere, hydroxyl radicals are formed when solar UV radiation splits ozone
(O3) molecules to create an oxygen (O) atom and oxygen gas (O2). From here, O
atoms may interact with water vapour (H20g) to form two hydroxyl (OH)
radicals. These radicals can then be removed from the atmosphere by chemically
interacting with methane or carbon monoxide. Methane and carbon monoxide
often compete with each other for available hydroxyl radicals. Removal of trace
gases by hydroxyl radicals occurs approximately 90% in the troposphere and has
been estimated to remove 3.7Gt of atmospheric trace gases, including methane
(Solomon et al. 2007, Badr et al. 2002b). Methane sinks also occur within the
stratosphere. When methane is emitted, approximately 10% makes it way
through the troposphere and into the stratosphere where it undergoes oxidation
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into carbon dioxide and water, contributing to a significant portion of the
stratosphere’s water budget (Badr et al. 1992b). As previously mentioned, dry
soils can also act as methane sinks, as soil dwelling methanotrophs consume
methane in order to sustain life, similarly to how humans require oxygen. These
dry soils act only as a small sink to methane, estimated at only 5%, but are
nonetheless an important (Boeckx et al. 1996).
It is interesting to note that although it appears that methane is being
emitted into the atmosphere from sources faster than it is being removed by
sinks, scientists have recorded a declining growth rate of atmospheric methane.
This is interesting because it was thought that major sources of methane, such as
rice fields, would increase with increasing populations (Khalil 1999). There are
several hypotheses that have been proposed to account for this. One hypothesis
proposes that increased uses of N-based soils instead of manure in rice
producing countries has allowed for decreased methane emissions from
agricultural sources, while another suggests rice fields are being managed with
less water contributing to drier soils, instead acting as sinks not sources (Khalil
1999). Another hypothesis suggests that the concentration of free hydroxyl
radicals in the atmosphere has increased as ozone continues to be destroyed by
chlorofluorocarbons (CFCs). This suggests that more hydroxyl radicals would be
available to interact with methane and carbon monoxide, ultimately removing
methane (Khalil 1999). Other ideas associated with the decline of methane
growth rate in the atmosphere include that increased global warming has
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allowed for the drying out wetlands, minimizing its methane emissions, while
another suggests the stabilization of methane emissions from human activities
(Khalil 1999, Solomon et al. 2007).
Regardless of the declining atmospheric growth rate of methane,
atmospheric methane concentrations continue to rise, signifying the need for
drastic intervention and mitigation of anthropogenic sources before their impacts
become too great (Solomon et al. 2007). Determining future trends of methane,
involves the consideration of a variety of scenarios and complicated modeling,
however, these predictions may create awareness of the issues at hand. For
example, the scenario referred to as “business as usual” entails the rapid and
continual increase of GHGs and continued enhancement of the GHE. This
scenario creates awareness as it presents and warns of the predicted future
trends that could occur without the employment of appropriate mitigation
techniques. There are several mitigation strategies that could be employed to
reduce anthropogenic methane sources including intermittent irrigation of rice
fields, proper management of natural gas pipelines, and the institution of
financial incentives and governmental policies regarding the reduction of
methane emissions, as at deterrence method (Hansen et al. 2002). Before any of
these strategies are utilized, however, it is important to first perform a risk
assessment, because it cannot be assumed that decreasing methane sources and
increasing methane sinks is the proper solution. Weighing the costs and benefits
of several solutions will be most beneficial action when searching for a solution.
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Through the examination of the GHE and the role of methane, it is
undeniably apparent that human’s have had a dramatic impact on the Earth’s
climate and systems through the addition of GHGs into the atmosphere.
Advancements in technology and industry, have allowed for the misuse and
overuse of our environment with scientific evidence of its detrimental effects
already apparent. Although it is the second greatest factor forcing the Earth’s
climate, methane should be considered just as much as a threat to climate change
as carbon dioxide, and in some cases even more so. The ability for increased
atmospheric methane concentrations to significantly alter the global climate
beyond that of carbon dioxide is real and must not be ignored. Although
atmospheric growth rates have decreased, society must realize the harmful
implications of the continued increase in methane concentrations and the
associated significant threats of global warming and global climate change that
are imminent without action.
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Works Cited
Badr, O., Probert, S.D., and P.W. O’Callaghan. “Methane: a greenhouse gas in the
Earth’s atmosphere” Applied Energy. 41.2 (1992): 95-113
Badr, O., Probert, S.D., and P.W. O’Callaghan. “Sinks for atmospheric methane”
Applied Energy. 41.2 (1992b): 137-147
Boeckx, P., van Cleemput, O., and I. Villariro. “Methane emissions from a landfill
and the methane oxidization capacity of it’s soils” Soil Biology and Soil
Biochemistry 28 (1996): 1397-1405
Crutzen, P.J. “Methane’s sinks and sources.” Nature. 350.6317 (1991): 380-382
Crutzen, P.J., Aselmann, I. and W. Seiler. “Methane production by domesticated
animals, wild runimants, and other herbivorous fauna and humans”
Tellus, 38B (1986): 271-284
Dickenson, R.E. and R.J. Cicerone. “Future global warming from atmospheric
gases” Nature (1986): 109-115
Easterling, D.R., Meenl, G.A., Parmesan C, Changnon, S.A., Karl, T.R., and L.O.
Mearns. “Climate extremes: Observed Modelling and Impacts. Science 289
(2000): 2068-2074
Hansen, J., Sato, M., Kharecha, P., Russell, G., Lea, D.W. and M. Siddall. “Climate
Change and Trace Gases” Phil Tran. R. Soc. A. 365 (2007): 1925-1954
Khalil, A.K.M. “Non-CO2 greenhouse gases in the atmosphere” Annu. Rev.
Energy Environment 24 (1999): 645-661
Khalil, A.K.M. and R.A. Rasmussen. “Constraints on the Global Sources of
methane, and an analysis of recent budgets” Tellus. 42B (1990): 229-236
Lelieveld, J., Crutzen, P.J., and F.J. Dentener. “Changing concentration, lifetime
and climate forcing of atmospheric methane” Tellus, 50B (1998) 128-150
12
Mitchell, J.F.B. “The Greenhouse Effect and Climate Change” Review of Geophysics
27 (1989): 115-139
Mooney, H.A. Vitousek, P.M. and P.A Matson “Exchange of material between
terrestrial ecosystems and the atmosphere” Science 238 (1987) 926-932
Neue, H.U. “Methane emission from rice fields” BioScience 43:7 (1993) 466-474
Reay, D. “Methane.” Encyclopedia of Earth. (2006)
Solomn, S, Qin, D., Manning, M., Marquis, M., Averyt, K., Tignor, M.M.B., and
H.L. Miller. Climate Change 2007: The Physical Science Basis,
Contribution of Working Group I to the Fourth Assessment Report of the
Intergovernmental Panel on Climate Change. Cambridge, England:
Cambridge University Press, 2007.
Spahni, R., J. Chappellaz, Th.F. Stocker, L. Loulergue, G. Hausammann, K.
Kawamura, J. Flückiger, J. Schwander, D. Raynaud, V. Masson-Delmotte,
and J. Jouzel: Atmospheric methane and nitrous oxide of the late
Pleistocene from Antarctic ice cores. Science, 310 (2005) 1317–1321
Ula, A.H.M.S. “Global Warming and electric power generation: What is the
connection?” Energy Conversion 64 (1991): 599-604
Vitousek, P.M., Mooney, H.A., Lubchenco, J. and J.M. Melillo “Human
domestication of Earth’s ecosystems” Science 277 (1997) 494-499
Zachos, J., Pagani, M., Thomas, F., and K. Billups “Trends, Rhythms and
Aberrations in Global Climate 65Ma to Present” Science 292 (2001) 686-693
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