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Anthropogenic greenhouse gases
Global anthropogenic greenhouse gas emissions broken down into 8 different
sectors for the year 2000.
The projected temperature increase for a range of greenhouse gas stabilization
scenarios (the coloured bands). The black line in middle of the shaded area
indicates 'best estimates'; the red and the blue lines the likely limits. From
the work of IPCC AR4, 2007.The concentrations of several greenhouse gases have
increased over time. Human activity increases the greenhouse effect primarily
through release of carbon dioxide, but human influences on other greenhouse
gases can also be important. Some of the main sources of greenhouse gases due to
human activity include:
burning of fossil fuels and deforestation leading to higher carbon dioxide
concentrations;
livestock and paddy rice farming, land use and wetland changes, pipeline losses,
and covered vented landfill emissions leading to higher methane atmospheric
concentrations. Many of the newer style fully vented septic systems that enhance
and target the fermentation process also are major sources of atmospheric
methane;
use of chlorofluorocarbons (CFCs) in refrigeration systems, and use of CFCs and
halons in fire suppression systems and manufacturing processes.
agricultural activities, including the use of fertilizers, that lead to higher
nitrous oxide concentrations.
The seven sources of CO2 from fossil fuel combustion are (with percentage
contributions for 2000-2004) :
Solid fuels (e.g. coal): 35%
Liquid fuels (e.g. petrol): 36%
Gaseous fuels (e.g. natural gas): 20%
Flaring gas industrially and at wells: <1%
Cement production: 3%
Non-fuel hydrocarbons: <1%
The "international bunkers" of shipping and air transport not included in
national inventories: 4%
Greenhouse gas emissions from industry, transportation (1/3 of total US global
warming pollution) and agriculture are very likely the main cause of recently
observed global warming. Major sources of an individual's GHG include home
heating and cooling, electricity consumption, and automobiles. Corresponding
conservation measures are improving home building insulation, cellular shades,
Compact fluorescent lamps, and choosing high miles per gallon vehicles.
Carbon dioxide, methane, nitrous oxide and three groups of fluorinated gases
(sulfur hexafluoride, HFCs, and PFCs) are the major greenhouse gases and the
subject of the Kyoto Protocol, which entered into force in 2005.
CFCs, although greenhouse gases, are regulated by the Montreal Protocol, which
was motivated by CFCs' contribution to ozone depletion rather than by their
contribution to global warming. Note that ozone depletion has only a minor role
in greenhouse warming though the two processes often are confused in the popular
media.
The role of water vapor
Increasing water vapor at Boulder, Colorado.Water vapor is a naturally occurring
greenhouse gas and accounts for the largest percentage of the greenhouse effect,
between 36% and 66% . Water vapor concentrations fluctuate regionally, but human
activity does not directly affect water vapor concentrations except at local
scales (for example, near irrigated fields).
Current state-of-the-art climate models include fully interactive clouds . They
show that an increase in atmospheric temperature caused by the greenhouse effect
due to anthropogenic gases will in turn lead to an increase in the water vapor
content of the troposphere, with approximately constant relative humidity. The
increased water vapor in turn leads to an increase in the greenhouse effect and
thus a further increase in temperature; the increase in temperature leads to
still further increase in atmospheric water vapor; and the feedback cycle
continues until equilibrium is reached. Thus water vapor acts as a positive
feedback to the forcing provided by human-released greenhouse gases such as CO2.
Increase of greenhouse gases
Measurements from Antarctic ice cores show that just before industrial emissions
began, atmospheric CO2 levels were about 280 parts per million by volume (ppm;
the units μL/L are occasionally used and are identical to parts per million by
volume). From the same ice cores it appears that CO2 concentrations stayed
between 260 and 280 ppm during the preceding 10,000 years. Studies using
evidence from stomata of fossilized leaves suggest greater variability, with CO2
levels above 300 ppm during the period 7,000-10,000 years ago, though others
have argued that these findings more likely reflect calibration/contamination
problems rather than actual CO2 variability.
Since the beginning of the Industrial Revolution, the concentrations of many of
the greenhouse gases have increased. The concentration of CO2 has increased by
about 100 ppm (i.e., from 280 ppm to 380 ppm). The first 50 ppm increase took
place in about 200 years, from the start of the Industrial Revolution to around
1973; the next 50 ppm increase took place in about 33 years, from 1973 to 2006.
PDF (96.8 KiB). Many observations are available on line in a variety of
Atmospheric Chemistry Observational Databases. The greenhouse gases with the
largest radiative forcing are:
Relevant to radiative forcing Gas Current (1998) Amount by volume Increase over
pre-industrial (1750) Percentage increase Radiative forcing (W/m2)
Carbon dioxide 365 ppm {383 ppm(2007.01)} 87 ppm {105 ppm(2007.01)} 31%
{37.77%(2007.01)} 1.46 {~1.532 (2007.01)}
Methane 1,745 ppb 1,045 ppb 150% 0.48
Nitrous oxide 314 ppb 44 ppb 16% 0.15
Global carbon dioxide emissions 1751–2000.Relevant to both radiative forcing and
ozone depletion; all of the following have no natural sources and hence zero
amounts pre-industrial Gas Current (1998)
Amount by volume Radiative forcing
(W/m2)
CFC-11 268 ppt 0.07
CFC-12 533 ppt 0.17
CFC-113 84 ppt 0.03
Carbon tetrachloride 102 ppt 0.01
HCFC-22 69 ppt 0.03
(Source: IPCC radiative forcing report 1994 updated (to 1998) by IPCC TAR table
6.1 ).
Recent rates of change and emission
The sharp acceleration in CO2 emissions since 2000 of >3% y-1 (>2 ppm y-1) from
1.1% y-1 during the 90's is attributable to the lapse of formerly declining
trends in carbon intensity of both developing and developed nations. Although
over 3/4 of cumulative anthropogenic CO2 is still attributable to the developed
world, China was responsible for most of global growth in emissions during this
period. All this indicates a global failure to decarbonise energy supply and an
underestimation of emissions growth on the part of the IPCC in their Special
Report on Emissions Scenarios. Localised plummeting emissions associated with
the collapse of the Soviet Union have been followed by slow emissions growth in
this region due to more efficient energy use, made necessary by the increasing
proportion of it that is exported. In comparison, methane has not increased
appreciably, and N2O by 0.25% y-1 .
The United States emitted 16.3% more GHG in 2005 than it did in 1990. According
to a preliminary estimate by the Netherlands Environmental Assessment Agency,
the largest national producer of CO2 emissions since 2006 has been China with an
estimated annual production of about 6200 megatonnes. It is followed by the
United States with about 5,800 megatonnes. Relative to 2005, China's fossil CO2
emissions of China increased in 2006 by 8.7%, while in the USA, comparable CO2
emissions decreased in 2006 by 1.4%. The agency notes that its estimates do not
include some CO2 sources of uncertain magnitude .
Removal from the atmosphere and global warming potential
Major greenhouse gas trendsAside from water vapor near the surface, which has a
residence time of days, most greenhouse gases take a very long time to leave the
atmosphere. Although it is not easy to know with precision how long, there are
estimates of the duration of stay, i.e., the time which is necessary so that the
gas disappears from the atmosphere, for the principal greenhouse gases. For the
first five years of this century, 48% of total anthropogenic CO2 emissions
remained in the atmosphere, a figure that is increasing and diagnostic of
weakening carbon sinks. Greenhouse gases can be removed from the atmosphere by
various processes:
as a consequence of a physical change (condensation and precipitation remove
water vapor from the atmosphere).
as a consequence of chemical reactions within the atmosphere. This is the case
for methane. It is oxidized by reaction with naturally occurring hydroxyl
radical, OH· and degraded to CO2 and water vapor at the end of a chain of
reactions (the contribution of the CO2 from the oxidation of methane is not
included in the methane Global warming potential). This also includes solution
and solid phase chemistry occurring in atmospheric aerosols.
as a consequence of a physical interchange at the interface between the
atmosphere and the other compartments of the planet. An example is the mixing of
atmospheric gases into the oceans at the boundary layer.
as a consequence of a chemical change at the interface between the atmosphere
and the other compartments of the planet. This is the case for CO2, which is
reduced by photosynthesis of plants, and which, after dissolving in the oceans,
reacts to form carbonic acid and bicarbonate and carbonate ions (see ocean
acidification).
as a consequence of a photochemical change. Halocarbons are dissociated by UV
light releasing Cl· and F· as free radicals in the stratosphere with harmful
effects on ozone (halocarbons are generally too stable to disappear by chemical
reaction in the atmosphere).
as a consequence of dissociative ionization caused by high energy cosmic rays or
lightning discharges, which break molecular bonds. For example, lightning forms
N anions from N2 which then react with O2 to form NO2.
Two scales can be used to describe the effect of different gases in the
atmosphere. The first, the atmospheric lifetime, describes how long it takes to
restore the system to equilibrium following a small increase in the
concentration of the gas in the atmosphere. Individual molecules may interchange
with other reservoirs such as soil, the oceans, and biological systems, but the
mean lifetime refers to the decaying away of the excess. It is sometimes
erroneously claimed that the atmospheric lifetime of CO2 is only a few years
because that is the average time for any CO2 molecule to stay in the atmosphere
before being removed by mixing into the ocean, uptake by photosynthesis, or
other processes. This ignores the balancing fluxes of CO2 into the atmosphere
from the other reservoirs. It is the net concentration changes of the various
greenhouse gases by all sources and sinks that determines atmospheric lifetime,
not just the removal processes.
The second scale is global warming potential (GWP). The GWP depends on both the
efficiency of the molecule as a greenhouse gas and its atmospheric lifetime. GWP
is measured relative to the same mass of CO2 and evaluated for a specific
timescale. Thus, if a molecule has a high GWP on a short time scale (say 20
years) but has only a short lifetime, it will have a large GWP on a 20 year
scale but a small one on a 100 year scale. Conversely, if a molecule has a
longer atmospheric lifetime than CO2 its GWP will increase with time.
Examples of the atmospheric lifetime and GWP for several greenhouse gases
include:
CO2 has a variable atmospheric lifetime (approximately 200-450 years for small
perturbations). Recent work indicates that recovery from a large input of
atmospheric CO2 from burning fossil fuels will result in an effective lifetime
of tens of thousands of years. Carbon dioxide is defined to have a GWP of 1 over
all time periods.
Methane has an atmospheric lifetime of 12 ± 3 years and a GWP of 62 over 20
years, 23 over 100 years and 7 over 500 years. The decrease in GWP associated
with longer times is associated with the fact that the methane is degraded to
water and CO2 by chemical reactions in the atmosphere.
Nitrous oxide has an atmospheric lifetime of 120 years and a GWP of 296 over 100
years.
CFC-12 has an atmospheric lifetime of 100 years and a GWP(100) of 10600.
HCFC-22 has an atmospheric lifetime of 12.1 years and a GWP(100) of 1700.
Tetrafluoromethane has an atmospheric lifetime of 50,000 years and a GWP(100) of
5700.
Sulfur hexafluoride has an atmospheric lifetime of 3,200 years and a GWP(100) of
22000.
Source : IPCC, table 6.7.
Related effects
MOPITT 2000 global carbon monoxideCarbon monoxide has an indirect radiative
effect by elevating concentrations of methane and tropospheric ozone through
scavenging of atmospheric constituents (e.g., the hydroxyl radical, OH) that
would otherwise destroy them. Carbon monoxide is created when carbon-containing
fuels are burned incompletely. Through natural processes in the atmosphere, it
is eventually oxidized to carbon dioxide. Carbon monoxide has an atmospheric
lifetime of only a few months and as a consequence is spatially more variable
than longer-lived gases.
Another potentially important indirect effect comes from methane, which in
addition to its direct radiative impact also contributes to ozone formation.
Shindell et al (2005) argue that the contribution to climate change from methane
is at least double previous estimates as a result of this effect.
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