Greenhouse
Effect: Atmosperic Trends of Greenhouse Gases
THE SIGNIFICANCE
OF CLIMATE CHANGE
It is now
known that the presence of atmospheric greenhouse gases has a significant
effect on climate. However, many questions about how the changing composition
of the atmosphere will affect climate remain unanswered. Nonetheless, a
number of researchers are attempting to determine what sorts of impacts
on our environment may result from global warming. Computer models of the
atmosphere and climate generally agree that an overall global warming will
occur because of increasing concentrations of greenhouse gases, but they
are much less certain in their representation of the spatial distribution
of this warming. The implication of atmosphere/climate change for life
on earth are even less well understood. These depend critically on the
rate and timing of atmosphere/climate change as well as on the overall
severity. Consequences will be experienced differently by different regions
at different times, both because climate change will differ from region
to region and because the interaction of human and natural systems with
the atmosphere and climate will differ. For some regions, the indirect
effects (such as population migration) experienced as a consequence of
changes in human and natural systems in other regions may be even more
important than direct effects.
There are
numerous pathways through which atmosphere/climate change may be experienced
by human and natural systems. Potential consequences that could occur in
four important areas may be summarized as follows:
1/ Sea level
rise: Sea levels are expected to rise as a response to global warming,
but the rate and timing remain uncertain. Over the last century, the global-mean
sea level has risen about 10 to 20 cm. Over the next century, current models
project a further increase in global-mean sea level of 60+-30 cm. The prospect
of such an increase in the rate of sea level rise is of concern to low-lying
coasts.
2/ Effect
on human health: Direct effects on human health of the emitted greenhouse
gases are believed to be small. Stratospheric ozone depletion could affect
health because of the corresponding increase in UV radiation.
3/ Agriculture
and food supplies: Changing in climatic patterns could require changes
in cropping patterns and consequently in infrastructures and costs, perhaps
bringing benefits to some regions while negatively impacting others. However,
rapid changes in climate or more severe climate change could make adaptation
more difficult.
4/ Ecosystems:
Increasing concentrations of CO2 increase plant growth. However, rapid
changes in climate threaten a reduction in biodiversity. Some existing
species of plants and animals might be unable to adapt, being insufficiently
mobile to migrate at the rate required for survival.
TRENDS OF
THE GREENHOUSE GASES
Changes in
the concentrations of greenhouse gases such as carbon dioxide (CO2), methane
(CH4), nitrous oxide (N2O), and the chlorofluorocarbons (CFCs) hold the
potential for changing the earth's climate.
1/ carbon
dioxide CO2: Estimate of the pre-industrial concentration of CO2 made by
sampling air trapped in ice cores indicate that the concentration of today's
CO2 is about a quarter more than pre-industrial level.
CO2 is estimated
to contribute to about half the total global warming. CO2 is released principally
from two human activities: fossil-fuel use and land-use changes. Estimates
of CO2 emissions from fossil fuel are known with relatively good confidence.
The present atmospheric stock of carbon is large but the estimated resource
of fossil fuels is even much larger. Although the carbon content of conventional
oil and natural gas resources is small, only about slightly more than half
as large as the current atmospheric stock of carbon, coal resources are
an order of magnitude larger, about 55 times larger than the current atmospheric
stock of carbon. Due to the uncertainties surrounding major human activities,
the uncertainty in CO2 emissions regarding the future rate of fossil-fuel
use remains large. Improvements in the efficiency of energy conversion
technologies and employments of the CO2-free technologies such as hydroelectric
power, nuclear power, solar energy, ... will reduce the rate of emission
of CO2 from fossil fuel in the future.
CO2 emissions
from land-use are much more uncertain. Net emissions from land-use change
are dominated by tropical deforestation. Estimates of net CO2 emissions
from land-use change have increased for recent years. Estimates of deforestation
in the last decade are greatest for Brazil, Colombia, the Ivory Coast,
Indonesia, Laos, and Thailand.
2/ Methane
CH4: Ice core data indicate that concentrations of CH4 has increased more
than 100% since the beginning of the industrial time. It is not clear that
all of the major sources of CH4 have been identified, and the emissions
rates of those that have been identified are subject to significant uncertainty.
The three principal human activities that have been identified as emissions
sources are cattle raising, rice production, and energy production and
use. Roughly a quarter of the total atmospheric CH4 emission is attributable
to the production, transfer, conversion and consumption of energy.
Methane emissions
are generally expected to contribute about 10% to 20% to the future global
radiative forcing. Because the sources of CH4 are uncertain, forecasts
of emissions are also uncertain. Most forecasts simply project the rate
of accumulation in the atmosphere to continue.
3/ Nitrous
Oxide N2O: Ice core data indicate that the concentration of N2O was stable
for approximately 3000 years and began to increase slightly since the beginning
of the industrial period. The sources of N2O emissions are poorly known.
The dominant human activities associated with N2O emissions are agricultural
and energy use.
N2O is expected
to contribute about 5% of total radiative forcing. Forecasts of future
N2O emissions are very uncertain.
4/ Chlorofluorocarbons
CFCs: The term chlorofluorocarbons refers to a family of compounds derived
from the CH4 or higher carbon-content hydrocarbon molecule. A CFC is formed
by replacing all hydrogen molecules with the halogens chlorine (Cl) or
fluorine (F). When the bromine (Br) atom is also used as a replacement,
the compounds are referred to as halons.
CFCs, particularly
CFCl3 and CF2Cl2 (i.e. CFC-11 and CFC-12), are receiving the most attention
because of their large concentrations and potentially significant effects
on stratospheric ozone. These gases are destroyed in the stratosphere (primarily
by photolysis) which releases all of their chlorine atoms to act as catalysts
for O3 destruction. Other chlorinated halocarbons or halons also release
chlorine or bromine to the stratosphere. CFC gases are also strong IR absorbers
in the "atmospheric window". One molecule of CFC-11 has about
12,000 times and one CFC-12 has about 15,800 times the radiative forcing
impact of a CO2 molecule. CFC-11 and CFC-12 are thought to have contributed
about one-third of the radiative forcing of non-CO2 gases (that is about
16% of the total effect). Other important chlorinated compounds include
CFC-113 (CF2ClCFCl2), HCFC-22 (CHF2Cl), and methyl chloroform (CH3CCl3).
The ultimate impact of CFCs on greenhouse warming and on stratospheric
O3 depletion depends no only on the relative effectiveness in destroying
O3 and their IR absorption characteristics, but also on quantities produced
in the future.
Countries
representing most of the world's current production and consumption of
CFCs have agreed to curtail production and use of CFCs. The Montreal Protocol
was reached out of the desire to protect stratospheric ozone. Under the
June 1990 London Agreement, production and new uses of CFCs and halons
will end by the year 2000. Actual future levels of production of CFCs will
depend upon the number of countries that eventually join the Montreal Protocol
and London Agreement.
Several replacements
for CFCs are under consideration. Most of them are either hydrochlorofluorocarbons
(HCFCs that contain hydrogen but no chlorine). Although these compounds
have substantially shorter atmospheric lifetimes than CFCs, a greater mass
may be required to fulfill the same requirements. Most of these compounds
are also greenhouse gases and could affect climate if concentrations become
large. On balance, however, over the long term they will likely have a
smaller radiative forcing impact than CFCs.
THE IMPORTANCE
OF O3 IN CLIMATE CHANGE
Ozone plays
an important dual role in affecting climate. It is the major absorber of
UV radiation in the stratosphere and thus is important in preventing the
UV radiation from reaching the ground and in determining the temperature
structure in the stratosphere. O3 is also an important absorber of IR radiation
and is thus a greenhouse gas.
Approximately
90% of the O3 in the atmosphere is contained in the stratosphere. Ozone
is produced by the photolysis of molecular oxygen followed by reaction
of the resulting oxygen atom with another oxygen molecule. Stratospheric
O3 is destroyed primarily through catalytic reactions involving various
free radical species, including nitrogen oxides (NO, NO2), chlorine oxides
(Cl, ClO), and hydrogen oxides (OH, HO2). Because the catalytic reactions
result in a net destruction of O3, a small concentration of these catalysis
species can have a significant influence on the stratospheric O3 concentrations.
REDUCING FUTURE
GREENHOUSE GAS EMISSIONS
For some nations,
the cost of reducing future greenhouse gas emissions is not an issue. For
some other, the cost has become a matter of great concern. The Montreal
Protocol, which has as its objective the reduction of emissions of substances
that deplete the earth's ozone layer, is the first major agreement to reduce
emissions of a greenhouse gas. Increasing attention has been paid to the
analysis of technologies to reduce fossil-fuel CO2 emissions. The analysis
of technologies to reduce the emission of gases other than CO2 and other
than CFCs has received far less attention. The need for attention of the
non-CO2 and non-CFC greenhouse gases will be determined when the understanding
of the roles of these gases toward the global warming becomes more clear
in the future. ----------------------
Reference:
D.J. Wuebbles and J. Edmonds in "Primer on Greenhouse Gases"
(Lewis Publishers, 1991).
