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Removal
Removal from the atmosphere and global warming potential
Aside 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. 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 GWP). 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 atoms 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.
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