Table 7a-1 lists the eleven most abundant gases found in the Earth's lower atmosphere by volume. Of the gases listed, nitrogen, oxygen, water vapor, carbon dioxide, methane, nitrous oxide, and ozone are extremely important to the health of the Earth's biosphere.
The table indicates that nitrogen and oxygen are the main components of the atmosphere by volume. Together these two gases make up approximately 99 % of the dry atmosphere. Both of these gases have very important associations with life. Nitrogen is removed from the atmosphere and deposited at the Earth's surface mainly by specialized nitrogen fixing bacteria, and by way of lightning through precipitation. The addition of this nitrogen to the Earth's surface soils and various water bodies supplies much needed nutrition for plant growth. Nitrogen returns to the atmosphere primarily through biomass combustion and denitrification.
Oxygen is exchanged between the atmosphere and life through the processes of photosynthesis and respiration. Photosynthesis produces oxygen when carbon dioxide and water are chemically converted into glucose with the help of sunlight. Respiration is a the opposite process of photosynthesis. In respiration, oxygen is combined with glucose to chemically release energy for metabolism. The products of this reaction are water and carbon dioxide.
The next most abundant gas on the table is water vapor. Water vapor varies in concentration in the atmosphere both spatially and temporally. The highest concentrations of water vapor are found near the equator over the oceans and tropical rain forests. Cold polar areas and subtropical continental deserts are locations where the volume of water vapor can approach zero percent. Water vapor has several very important functional roles on our planet:
- It redistributes heat energy on the Earth through latent heat energy exchange.
- The condensation of water vapor creates precipitation that falls to the Earth's surface providing needed fresh water for plants and animals.
- It helps warm the Earth's atmosphere through the greenhouse effect.
The fifth most abundant gas in the atmosphere is carbon dioxide. The volume of this gas has increased by over 25 % in the last three hundred years (see Figure 7a-1). This increase is primarily due to human induced burning for fossil fuels, deforestation, and other forms of land-use change. Some scientists believe that this increase is causing global warming through an enhancement of the greenhouse effect. Carbon dioxide is also exchanged between the atmosphere and life through the processes of photosynthesis and respiration.
Methane is a very strong greenhouse gas. Since 1750,
methane concentrations in the atmosphere have increased by more
than 140 %. The primary sources for the additional methane added
to the atmosphere (in order of importance) are: rice
cultivation; domestic grazing animals; termites; landfills; coal
mining; and, oil and gas extraction. Anaerobic conditions
associated with rice paddy flooding results in the formation of
methane gas. However, an accurate estimate of how much methane
is being produced from rice paddies has been difficult to
ascertain. More than 60 % of all rice paddies are found in India
and China where scientific data concerning emission rates are
unavailable. Nevertheless, scientists believe that the
contribution of rice paddies is large because this form of crop
production has more than doubled since 1950. Grazing animals
release methane to the environment as a result of herbaceous
digestion. Some researchers believe the addition of methane from
this source has more than quadrupled over the last century.
Termites also release methane through similar processes.
Land-use change in the tropics, due to deforestation, ranching,
and farming, may be causing termite numbers to expand. If this
assumption is correct, the contribution from these insects may
be important. Methane is also released from landfills, coal
mines, and gas and oil drilling. Landfills produce methane as
organic wastes decompose over time. Coal, oil, and natural gas
deposits release methane to the atmosphere when these deposits
are excavated or drilled.
The average concentration of the greenhouse gas
nitrous oxide is now increasing at a rate of 0.2 to
0.3 % per year. Its part in the enhancement of the greenhouse
effect is minor relative to the other greenhouse gases already
mentioned. However, it does have an important role in the
artificial fertilization of ecosystems. In extreme cases, this
fertilization can lead to the death of forests, eutrophication
of aquatic habitats, and species exclusion. Sources for the
increase of nitrous oxide in the atmosphere include: land-use
conversion; fossil fuel combustion; biomass burning; and soil
fertilization. Most of the nitrous oxide added to the atmosphere
each year comes from deforestation and the conversion of forest,
savanna and grassland ecosystems into agricultural fields and
rangeland. Both of these processes reduce the amount of nitrogen
stored in living vegetation and soil through the decomposition
of organic matter. Nitrous oxide is also released into the
atmosphere when fossil fuels and biomass are burned. However,
the combined contribution to the increase of this gas in the
atmosphere is thought to be minor. The use of nitrate and
ammonium fertilizers to enhance plant growth is another source
of nitrous oxide. How much is released from this process has
been difficult to quantify. Estimates suggest that the
contribution from this source represents from 50 % to 0.2 % of
nitrous oxide added to the atmosphere annually.
Ozone's role in the enhancement of the greenhouse
effect has been difficult to determine. Accurate measurements of
past long-term (more than 25 years in the past) levels of this
gas in the atmosphere are currently unavailable. Moreover,
concentrations of ozone gas are found in two different regions
of the Earth's atmosphere. The majority of the ozone (about 97
%) found in the atmosphere is concentrated in the
stratosphere at an altitude of 15 to 55 kilometers
above the Earth's surface. This stratospheric ozone provides an
important service to life on the Earth as it absorbs harmful
ultraviolet radiation. In recent years, levels of
stratospheric ozone have been
decreasing due to the buildup of human created
chlorofluorocarbons in the atmosphere. Since the late
1970s, scientists have noticed the development of severe holes
in the ozone layer over Antarctica. Satellite measurements have
indicated that the zone from 65° North to 65° South latitude has
had a 3 % decrease in stratospheric ozone since 1978.
Ozone is also highly concentrated at the Earth's surface in and around cities. Most of this ozone is created as a by product of human created photochemical smog. This buildup of ozone is toxic to organisms living at the Earth's surface.
Table 7a-1: Average composition of the atmosphere up to an altitude of 25 km.
Gas Name |
Chemical Formula |
Percent Volume |
Nitrogen |
N2 |
78.08% |
Oxygen |
O2 |
20.95% |
*Water |
H2O |
0 to 4% |
Argon |
Ar |
0.93% |
*Carbon Dioxide |
CO2 |
0.0360% |
Neon |
Ne |
0.0018% |
Helium |
He |
0.0005% |
*Methane |
CH4 |
0.00017% |
Hydrogen |
H2 |
0.00005% |
*Nitrous Oxide |
N2O |
0.00003% |
*Ozone |
O3 |
0.000004% |
* variable gases
Created by Dr. Michael Pidwirny, University of British Columbia - Okanagan Email Corrections and Suggestions to: michael.pidwirny@ubc.ca Copyright © 1999-2005 Michael Pidwirny 08/22/2004 12:57 |