What are greenhouse gases?
Many chemical compounds present in Earth's atmosphere behave as
'greenhouse gases'. These are gases which allow direct sunlight (relative
shortwave energy) to reach the Earth's surface unimpeded. As the shortwave
energy (that in the visible and ultraviolet portion of the spectra) heats
the surface, longer-wave (infrared) energy (heat) is reradiated to the
atmosphere. Greenhouse gases absorb this energy, thereby allowing less heat
to escape back to space, and 'trapping' it in the lower atmosphere. Many
greenhouse gases occur naturally in the atmosphere, such as carbon dioxide,
methane, water vapor, and nitrous oxide, while others are synthetic. Those
that are man-made include the chlorofluorocarbons (CFCs), hydrofluorocarbons
(HFCs) and Perfluorocarbons (PFCs), as well as sulfur hexafluoride (SF6).
Atmospheric concentrations of both the natural and man-made gases have been
rising over the last few centuries due to the industrial revolution. As the
global population has increased and our reliance on fossil fuels (such as
coal, oil and natural gas) has been firmly solidified, so emissions of these
gases have risen. While gases such as carbon dioxide occur naturally in the
atmosphere, through our interference with the carbon cycle (through burning
forest lands, or mining and burning coal), we artificially move carbon from
solid storage to its gaseous state, thereby increasing atmospheric
concentrations.
Water Vapor
Water Vapor is the most abundant greenhouse gas in the atmosphere, which
is why it is addressed here first. However, changes in its concentration is
also considered to be a result of climate feedbacks related to the
warming of the atmosphere rather than a direct result of industrialization.
The feedback loop in which water is involved is critically important to
projecting future climate change, but as yet is still fairly poorly measured
and understood.
As the temperature of the atmosphere rises, more water is evaporated from
ground storage (rivers, oceans, reservoirs, soil). Because the air is
warmer, the relative humidity can be higher (in essence, the air is able to
'hold' more water when its warmer), leading to more water vapor in
the atmosphere. As a greenhouse gas, the higher concentration of water vapor
is then able to absorb more thermal IR energy radiated from the Earth, thus
further warming the atmosphere. The warmer atmosphere can then hold more
water vapor and so on and so on. This is referred to as a 'positive feedback
loop'. However, huge scientific uncertainty exists in defining the extent
and importance of this feedback loop. As water vapor increases in the
atmosphere, more of it will eventually also condense into clouds, which are
more able to reflect incoming solar radiation (thus allowing less energy to
reach the Earth's surface and heat it up). The future monitoring of
atmospheric processes involving water vapor will be critical to fully
understand the feedbacks in the climate system leading to global climate
change. As yet, though the basics of the hydrological cycle are fairly well
understood, we have very little comprehension of the complexity of the
feedback loops. Also, while we have good atmospheric measurements of other
key greenhouse gases such as carbon dioxide and methane, we have poor
measurements of global water vapor, so it is not certain by how much
atmospheric concentrations have risen in recent decades or centuries, though
satellite measurements, combined with balloon data and some in-situ ground
measurements indicate generally positive trends in global water vapor.
Carbon Dioxide
The natural production and absorption of carbon dioxide (CO2)
is achieved through the terrestrial biosphere and the ocean. However,
humankind has altered the natural carbon cycle by burning coal, oil, natural
gas and wood and since the industrial revolution began in the mid 1700s,
each of these actvities has increased in scale and distribution. Carbon
dioxide was the first greenhouse gas demonstrated to be increasing in
atmospheric concentration with the first conclusive
measurements being made in the last half of the 20th century. Prior to
the industrial revolution, concentrations were fairly stable at 280ppm.
Today, they are around 370ppm, an increase of well over 30%. The atmospheric
concentration has a marked seasonal oscillation that is mostly due to the
greater extent of landmass in the northern hemisphere (NH) and its
vegetation. A greater drawdown of CO2 occurs in the NH spring and
summer as plants convert CO2 to plant material through
photosynthesis. It is then released again in the fall and winter as the
plants decompose.
Methane
Methane is an extrememly effective absorber of radiation, though its
atmospheric concentration is less than CO2 and its lifetime in
the atmosphere is brief (10-12 years), compared to some other greenhouse
gases (such as CO2, N2O, CFCs). Methane(CH4)
has both natural and anthropogenic sources. It is released as part of the
biological processes in low oxygen environments, such as in swamplands or in
rice production (at the roots of the plants). Over the last 50 years, human
activities such as growing rice, raising cattle, using natural gas and
mining coal have added to the atmospheric concentration of methane. Direct
atmospheric measurement of atmospheric methane has been possible since the
late 1970s and its conentration rose from 1.52 ppmv in 1978 by around
1%/year to 1990, since when there has been little sustained increase. The
current atmospheric concentration is ~1.77 ppmv, and there is no scientific
consensus on why methane has not risen much since around 1990.
Tropospheric Ozone
Ultraviolet radiation and oxygen interact to form ozone in the
stratosphere. Existing in a broad band, commonly called the 'ozone layer', a
small fraction of this ozone naturally descends to the surface of the Earth.
However, during the 20th century, this tropospheric ozone has been
supplemented by ozone created by human processes. The exhaust emissions from
automobiles and pollution from factories (as well as burning vegetation)
leads to greater concentrations of carbon and nitrogen molecules in the
lower atmosphere which, when it they are acted on by sunlight, produce
ozone. Consequently, ozone has higher concentrations in and around cities
than in sparsely populated areas, though there is some transport of ozone
downwind of major urban areas. Ozone is an important contributor to
photochemical smog. Though the lifetime of ozone is short, and is therefore
not well-mixed through the atmosphere, there is a general band of higher
ozone concentration during NH spring and summer between 30�N and 50�N
resulting from the higher urbanization and industrial activity in this band.
Concentrations of ozone have risen by around 30% since the pre-industrial
era, and is now considered by the IPCC to be the third most important
greenhouse gas after carbon dioxide and methane. An additional complication
of ozone is that it also interacts with and is modulated by concentrations
of methane.
Nitrous Oxide
Concentrations of nitrous oxide also began to rise at the beginning of
the industrial revolution and is understood to be produced by microbial
processes in soil and water, including those reactions which occur in
fertilizer containing nitrogen. Increasing use of these fertilizers has been
made over the last century. Global concentration for N2O in 1998
was 314 ppb, and in addition to agricultural sources for the gas, some
industrial processes (fossil fuel-fired power plants, nylon production,
nitric acid production and vehicle emissions) also contribute to its
atmospheric load.
CFCs
etc.
CFCs (chlorofluorocarbons) have no natural source, but were entirely
synthesized for such diverse uses as refrigerants, aerosol propellants and
cleaning solvents. Their creation was in 1928 and since then concentrations
of CFCs in the atmosphere have been rising. Due to the discovery that they
are able to destroy stratospheric ozone, a global effort to halt their
production was undertaken and was extremely successful. So much so that
levels of the major CFCs are now remaining level or declining. However,
their long atmospheric lifetimes determine that some concentration of the
CFCs will remain in the atmosphere for over 100 years. Since they are also
greenhouse gas, along with such other long-lived synthesized gases as CF4
(carbontatrafuoride), SF6 (sulfurhexafluoride), they are of
concern. Another set of synthesized compounds called HFCs
(hydrofluorcarbons) are also greenhouse gases, though they are less stable
in the atmosphere and therefore have a shorter lifetime and less of an
impact as a greenhouse gas.
Carbon Monoxide and other reactive gases
Carbon monoxide (CO) is not considered a direct greenhouse gas, mostly
because it does not absorb terrestrial thermal IR energy strongly enough.
However, CO is able to modulate the production of methane and tropospheric
ozone. The Northern Hemisphere contains about twice as much CO as the
Southern Hemisphere because as much as half of the global burden of CO is
derived from human activity, which is predominantly located in the NH. Due
to the spatial variability of CO, it is difficult to ascertain global
concentrations, however, it appears as though they were generally increasing
until the late 1980s, and have since begun to decline somewhat. One possible
explanation is the reduction in vehicle emissions of CO since greater use of
catalytic converters has been made.
Volatile Organic Compounds (VOCs) also have a small direct impact as
greenhouse gases, as well being involved in chemical processes which
modulate ozone production. VOCs include non-methane hydrocarbons (NMHC), and
oxygenated NMHCs (eg. alcohols and organic acids), and their largest source
is natural emissions from vegetation. However, there are some anthropogenic
sources such as vehicle emissions, fuel production and biomass burning.
Though measurement of VOCs is extremely difficult, it is expected that most
anthropogenic emissions of these compounds have increased in recent decades.
Originally published at:
http://lwf.ncdc.noaa.gov/oa/climate/gases.html#wv |