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VACETS Regular Technical Column

"Science for Everyone"

"Science for Everyone" was a technical column posted regularly on the VACETS forum. The author of the following articles is Dr. Vo Ta Duc. For more publications produced by other VACETS  members, please visit the VACETS Member Publications page or Technical Columns page.

The VACETS Technical Column is contributed by various members , especially those of the VACETS Technical Affairs Committe. Articles are posted regulary on vacets@peak.org forum. Please send questions, comments and suggestions to vacets-ta@vacets.org

Mon, 21 Nov 1994

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).


Duc Ta Vo, Ph.D.
ducvo@lanl.gov

For discussion on this column, join vacets-tech@vacets.org


Copyright © 1996 by VACETS and Duc Ta Vo

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