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Climate vs. WeatherThe Greenhouse
Gases
GENEVA, Nov 23 Reuters - Concentrations of greenhouse
gases, the major cause of global warming, are at their highest levels ever
recorded and are still climbing, according to the U.N. World Meteorological
Organisation (WMO). The trend could be pushing the world towards the most
pessimistic assessments of the rise in temperatures expected in coming decades.
WMO underlined the need for urgent action.
Greenhouse effect is the warming effect on the
Earth due to solar radiations that are trapped within the Earth's
atmosphere. Trapping being carried out by the gases called the
greenhouse gases (GHG). The
insulating property of these GHG gases is
similar to that of the glass in a greenhouse: they allow solar
heat entering into the system, but discourage its escape.
The greenhouse gases present in the
atmosphere are:
- carbon dioxide,
- methane,
- nitrous oxide,
- water vapor which is the most abundant and thus most important;
- halocarbon gas,
- aerosols (airborne microscopic
particles or water droplets),
- sulphur
hexafluoride
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Each greenhouse gas is
characterized by its atmospheric lifetime and global warming
potential (GWP). GWP is a measure of the
relative radiative effect of a given mass of a substance compared to
CO2, integrated over a chosen time horizon. It is a way
of comparing the potential climate change associated with emissions
of different greenhouse gases.
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Global warming potential GWP
(Time horizon in years) |
|
GHG gases |
Lifetime |
20 years |
100
years |
500 years |
| carbon dioxide |
1 |
1 |
1 |
1 |
| methane |
12 |
72 |
21 |
7.6 |
| nitrous oxide |
114 |
289 |
310 |
153 |
Carbon dioxide (CO2) is the most important
greenhouse gas produced by human activities, primarily through the
combustion of fossil fuels. Its concentration in the Earth's
atmosphere has risen by more than 30% since the Industrial
Revolution.
Atmospheric concentration of carbon dioxide:
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From an analysis of samples of ancient ice, it is shown that carbon dioxide levels in the
atmosphere today are the highest they have been in 600,000 years.
More than 80% of carbon dioxide emission
comes from fossil fuel combustion.
(Source: NOAA) |
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Currently atmospheric carbon dioxide
concentration is at 380 ppm. It is rising alarmingly by 2 ppm
annually. It is calculated that if the carbon
dioxide concentration reaches 560 ppm, the world will be in great
danger. (Logging a source of carbon dioxide emissions) |
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On simple arithmetic, the danger level of
560 ppm would be surpassed by end of this century. The safety threshold
of carbon dioxide concentration in the atmosphere should be below 450
ppm to contain temperature rise within 2oC, or to
achieve 17 Gt or 28% of emissions reductions versus
BAU (
business as usual scenario whereby no special mitigation effort
is done ) by 2020.
350 is the new 450. Led by
the nation’s top climate scientist, James Hansen, a number of
leading scientists argued that the “old” target of 450 ppm scientists have been
arguing for isn’t enough. (Stabilize at 350 ppm or risk ice-free planet,
warns NASA, Yale, Sheffield, Versailles, Boston et al.
In December, America’s leading spokesman for climate action,
Al Gore, embraced the 350 ppm target).
Greenhouse gases - radiative forcing and
concentration:
|
GHG |
Radiative forcing
Wm-2 |
Conc. in 2005
ppm |
Notes |
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Carbon dioxide |
1.660 |
379 |
2ppm annual conc. inc (for the
past 10 years) |
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Methane |
0.480 |
1774 |
Conc. fluctuates (580-730 ppm for the past 10 000 years ) |
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Nitrous oxide |
0.160 |
319 |
18% increase |
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Montreal gases CFC, HCFC |
0.320 |
Phasing out |
Their emissions have decreased |
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Kyoto p. gases HFC, PFC, SF6 |
0.017 |
Small conc. |
Conc. increases rapidly |
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Stratospheric ozone |
0.050(-) |
Depleted |
Depletion has halted thanks to the Montreal Protocol |
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Aerosol |
0.500(-) |
Small |
Cooling and warming effects |
Table as summarized from data
from The Physical Science Basis AR4 - IPCC
(For a very loose understanding of radiative forcing, a
positive forcing (more incoming energy) tends to warm the
system, while a negative forcing (more outgoing energy)
tends to cool it.)
Methane (CH4)
| Human activities generate methane. Since 1750, atmospheric methane has
increased by about 150%. It represents 9% of total emissions, and accounts for 20% of the total radiative forcing. 350-500 million tons of methane
is added annually through human activities: in the process of livestock raising, coal mining,
biomass burning, drilling for oil and natural
gas, rice cultivation, and garbage sitting in landfills. |
| Natural methane is emitted from the
wetlands.
Bacteria that break down
organic matters in wetlands and bacteria that are found in cows,
sheep, goats, buffalo, termites, and camels produce methane
naturally. |
| From the ocean,
natural methane gas is being emitted through the
thawing of permafrost. |
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Methane is a much potent greenhouse gas
than carbon dioxide. Despite its shorter atmospheric lifetime of
about 8.4 years, it has a high global warming potential of 72
(averaged over 20 years) or 21 (averaged over 100 years). |
| A study released by the British Antarctic Survey found
that in the past 800,000 years methane had never tipped 750 ppb, but is now 1,780 ppb.
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Nitrous Oxide
Produced from fertilizer usage, cattle
rearing, biomass burning and others. It has a lifespan of above 100
years. Nitrous oxide (5% of total emissions), is emitted through the
use of nitrogen fertilizers, from burning fossil fuels and from
certain industrial and waste management processes.
Halocarbons
(CFC - chlorofluorocarbons, and others)
They are 'stubborn' greenhouse gases, with
a long lifespan, used extensively as refrigeration agents and in
other industrial purposes. They were found to have caused
stratospheric ozone depletion. Thanks to
the Montreal Protocol on Substances that Deplete the Ozone Layer and
its Amendments, their concentrations are regulated to be decreasing or stabilizing.
Concentrations of HCFCs, PFCs, and HFCs are currently increasing.
CFC and HCFC are GHG that are purely
anthropogenic. Their emissions have decreased due to their phase-out
under the Montreal Protocol.
HFC, PFC and SF6 are industrial
fluorinated gases covered by the Kyoto Protocol. Their
concentrations are
relatively small, though increasing rapidly.
Aerosols:
A total direct radiative forcing combined
across all aerosol types can be estimated as - 0.5. Different types
of aerosol may have net cooling or warming effect.
Nitrogen and oxygen, the two most abundant air components, do not exert greenhouse effects.
Tropospheric Ozone
In the atmosphere, ozone is continuously produced and destroyed by
chemical reactions arising from changes in land use and fuel
combustion. Lifespan is short ranging from days to weeks.
Ozone in the stratosphere was destroyed by industrially released
hydrocarbons causing the ozone hole
over Antarctica in 1980 and 1990.
Destruction of this layer
contributed to global warming as the ozone is capable of absorbing
the radiations from Earth. Depletion has halted thanks to the
Montreal Protocol. However, global concentration is still about 4%
below the 1980 level.
Water vapor
Water vapor is the most abundant greenhouse gas. It is 99.99% of
natural origin.
Increase in its concentration is more of indirect manner as due to
higher evaporation rate at sea due to global warming. Direct
emission contribution of water vapor as a radiative force is
negligible.
The greenhouse gases defined by
the Kyoto Protocol
subjective to emissions reduction targeting are carbon dioxide (CO2),
methane (CH4), nitrous
oxide (N2O), HFCs, PFCs,
and SF6. Water vapor is interestingly not included,
though most abundant.
From the view of the
critics:
Water vapor constitutes Earth's most significant greenhouse gas,
accounting for about 95% of Earth's greenhouse effect.
Interestingly, many "facts and figures' regarding global warming
completely ignore the powerful effects of water vapor in the
greenhouse system. By ignoring the water vapor effect, then human
impacts on global warming is over-estimated as much as 20-fold.
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