Transcript CH 4 (Methane)

Greenhouse Gases-2:
CH4 (Methane)
CH4
• Comes primarily from biological sources,
although some enters the atmosphere
from fossil fuel sources.
• Several kinds of bacteria that are
particularly abundant in wetlands and
rice fields release methane into the
atmosphere.
• Methane releasing bacteria are also
found in large numbers in the guts of
ruminant animals such as cows.
• On a per molecule basis, an additional methane
molecule in the current atmosphere is about 21
times more effective at affecting climate than an
additional molecule of CO2 (IPCC, 1990).
• The Intergovernmental Panel on Climate Change
(IPCC) is the leading international body for the
assessment of climate change. It was established
by the United Nations Environment Programme
(UNEP) and the World Meteorological
Organization (WMO) in 1988 to provide the world
with a clear scientific view on the current state of
knowledge in climate change and its potential
environmental and socio-economic impacts.
Wet cultivation of rice: CH4
How rice produces methane?
• Methane (CH4) and carbon dioxide (CO2) are the end
products of carbon decomposition in rice fields and
other wetlands. Methane, a major greenhouse gas, is
the terminal step of the anaerobic breakdown of
organic matter in wetland soils. It is exclusively
produced by methanogenic bacteria that can
metabolize only in the absence of free oxygen and at
redox potentials below -150 mV (Neue et al. 1997).
• In rice field, methane production generally increases
during the cropping season. Easily degradable soil
carbon, plant litter, root exudates, decomposing roots
and aquatic biomass that are added to the anaerobic
zone of the paddy soil (this is the zone below the thin
oxidized or brown soil surface) are the major carbon
sources for methane production.
The general equation for anaerobic action
Anaerobic microbes
Organic matters + Combined oxygen
New
cells+ Energy for cells + CH4 + CO2 + Other end
products (such as H2S, H2, N2)
The sources of combined oxygen include the
radicals of CO3-2, SO4-2, NO3-1 and PO4-3.
How cows produce methane ?
• Cows, goats, sheep and several other animals
belong to a class of animals called ruminants.
Ruminants have four stomachs and digest
their food in their stomachs instead of in their
intestines, as humans do. Ruminants eat food,
regurgitate it as cud and eat it again. The
stomachs are filled with bacteria (methane
releasing bacteria) that aid in digestion, but
also produce methane.
COW
Goat
Sheep
How cows produce methane ?
• Cows emit a massive amount of methane
through belching, with a lesser amount through
flatulence.
• Statistics vary regarding how much methane the
average dairy cow expels. Some experts say 100
liters to 200 liters CH4 a day (or about 26
gallons to about 53 gallons), while others say it's
up to 500 liters (about 132 gallons) a day. In any
case, that's a lot of methane, an amount
comparable to the pollution produced by a car
in a day.
Methane is removed from the
atmosphere by oxidation.
CH4 + 2O2
CO2 + 2H2O
On average, a molecule of
methane is destroyed by this
reaction 10 years after it was
emitted.