More on "What can be done about climate change?"

There is no single decision maker who is responsible for either the choices that are likely to lead to climate change, or for the actions that people might take to respond to change. At the international level many countries add greenhouse gases to the atmosphere. Today's top emitters of carbon dioxide are:

Country	% of current human carbon dioxide emissions	% of world's population	Annual metric tons of carbon dioxide per person
United States	22.0	5.0	20.0
Former Soviet Union	18.0	6.0	13.5
Peoples Republic of China	10.0	22.0	2.3
Germany	4.6	1.5	14.0
Japan	4.5	2.5	8.5
India	2.7	17.0	.8
United Kingdom	2.5	1.0	10.0
Data are for 1988 and are drawn from"Trends '90: a compendium of data on global change." Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, 1990.

In the future, emissions from a number of developing countries such as China and India are likely to rise dramatically as these countries follow the example set by the developed countries and burn large amounts of coal and oil in order to raise their people's standards of living. The effect will be further amplified if the population in these countries continues to grow.

Any serious effort to control the future emissions of carbon dioxide and other greenhouse gases will require international cooperation.

In order to protect the ozone layer, an agreement to phase out most uses of CFCs was reached in 1987. In many ways, this agreement was much easier than the kinds of agreements that will be needed to control carbon dioxide emissions to slow climate change. CFCs play a key role in only a few parts of the economy. It has been relatively easy to develop affordable substitutes. The "world environment summit" held in Rio de Janeiro, Brazil in 1992 reached a much looser agreement on carbon dioxide and other greenhouse gases. Nations pledged to work to reduce their emissions. While the wording is vague, and there are no legally binding obligations, there is now an international framework for monitoring and reviewing progress towards reducing emissions.

The burning of coal and oil, which is the principle source of carbon dioxide, underlies almost all economic activities. So far, abundant low cost alternative sources of energy have not been identified. Developed countries could probably afford to reduce their energy consumption, and invest in more energy efficient technologies. Without help in such forms as economic assistance and technology transfer, developing countries may find the cost of such changes unacceptable. After all, in the name of some very general long term "common good" they are being asked to give up on some of the very real short-term economic benefits that their citizens will experience through development.

For this reason, many developing countries argue that the developed countries should take the lead and contribute the most to reducing emissions of greenhouse gases.

The same situation of "many decision makers" applies on the response side of the climate problem. Residents and regional managers in areas such as the low lying coastal resort of Hilton Head, South Carolina are likely to have very different views about climate change from the residents and regional managers of a city such as Fairbanks, Alaska. Wealthy Sierra Club members in Boulder, Colorado are likely to have very different views from the impoverished members of a rural economic development cooperative in central China as they struggle to build a stronger economy.

In short, the problem of climate change is not a single problem. It is a multitude of problems that will be faced by many different groups with different needs and different concerns. At the end of this details booklet we will elaborate on these issues. First, however, we need to explore some of the specific things that might be done.

What impact might specific policy options have? How much might they cost?

As we explained in Part 3 of the main brochure, there are three broad policy options for dealing with the climate problem. These are: abatement, adaptation, and geo- engineering. The next five pages explore these options in somewhat more detail and consider how effective they may be and how much they may cost. Of course, a number of other considerations will play an important role in determining the options a country, state or business chooses, including whether the policies are acceptable to the public, whether they are fair, and whether they are more important than other goals society might have. These considerations can be very important, but they are basically a matter of a choice among different values. There is no single right answer to such choices. Economists and engineers are still somewhat uncertain about how effective some policies might be and what they would cost, but rough estimates are possible. In the discussion that follows, we will focus only on how much each option can reduce U.S. carbon dioxide emissions and how much it will cost. We leave it to you to consider the other value choices each option involves.

Abatement options are strategies that reduce emissions. We will consider three kinds, improved energy efficiency, use of cleaner energy sources, and changes in agriculture and forestry.

Improving energy efficiency will reduce emissions of carbon dioxide, the most significant greenhouse gas. If it is pursued wisely, it should also improve economic performance. Here are examples of three strategies that the U.S. might pursue to improve its energy efficiency:

Reduce energy use in buildings. About 1/3 of all the energy used in the U.S., and 2/3 of all the electricity, goes into buildings. Most goes to heating, cooling and lighting. Researchers estimate that with improved insulation, glazing, weather-stripping, furnaces and air conditioners, and lighting in residential and commercial buildings, U.S. carbon dioxide emissions could be reduced by about 360 million tons per year, about 5% of total U.S. emissions. They also estimate that such changes would lead to reduced energy use, and actually save money, between $25 and $75 per ton of carbon dioxide saved.

Improve fuel efficiency of new cars. Currently the average mileage obtained by new cars in the U.S. is 27.5 mpg. If this were raised to 32.5 mpg, and held there, over time, U.S. emissions would decline by about 250 million tons per year, about 4% of U.S. emissions. Estimates of the costs of such a program range from a savings of $76 per ton of carbon dioxide removed to a cost of $16 per ton of carbon dioxide. The savings result if the reduced fuel cost outweighs other cost increases.

Make appliances more efficient. Currently available technology allows refrigerators, dishwashers, water heaters and other home appliances to be substantially more efficient than they are. If this technology were used in place of older, less efficient technology, the U.S. could reduce carbon dioxide emissions by about 75 million tons per year (1.3% of U.S. emissions) while at the same time saving $35 to $44 per ton.

What is energy efficiency? When we drive a car chemical energy stored in gasoline is converted into mechanical energy and used to create motion. When we use an electric stove, a power plant first converts chemical energy stored in coal to electrical energy which is carried through the electrical system. Then the stove converts it to heat energy. The proportion of the original energy which ends up being used for the final purpose (motion, cooking) measures the energy efficiency. Nature sets some basic limits on how efficiently energy can be used, but in most cases our products and manufacturing processes are still a long way from operating at this theoretical limit. Why does energy efficiency matter? If we can make things like cars and appliances do their job just as well while using less energy, then we do not need to burn as much coal and oil. Burning fossil fuels like coal and oil produces carbon dioxide. Increasing the energy efficiency in transportation, homes, offices, and factories is the best way we have to reduce carbon dioxide emissions without lowering our standard of living.

Replacing coal, oil and gasoline with cleaner energy sources and technologies would reduce carbon dioxide emissions and improve efficiency. The main issue for this strategy is whether there are enough abundant, low cost alternatives to coal, oil, and gasoline.

Instead of gasoline, use ethanol, hydrogen or electricity in cars and trucks. Technology currently exists to allow cars to run on these and other alternative fuels. Ethanol is a kind of alcohol made from corn. If ethanol were made from sustainable agriculture, or if hydrogen or electricity were generated by renewable means, converting all vehicles would eventually reduce carbon dioxide emissions by over 1000 million tons per year (17% of U.S. emissions). However, the technology for some alternative fuel options (i.e., electric and hydrogen powered cars) is presently too expensive to be widely adopted by consumers, and researchers do not know whether farmers can produce enough corn for ethanol to replace gasoline. Such changes would cost between $50 and $177 per ton of carbon dioxide saved.

Switch 10% of building electricity use from electric resistance heat to natural gas heating. Natural gas, whether it is used to warm rooms or heat water, is more efficient than electric heat. As a result, it is also cheaper and releases far less carbon dioxide than the coal burned to make electricity. By switching only 10% of commercial and residential electricity use to natural gas heating systems, U.S. carbon dioxide emissions could be reduced by about 75 million tons per year (1.3% of U.S. emissions) at an estimated savings of $90 per ton.

Replace all existing coal and oil fired electric power plants with new high efficiency plants that use natural gas. The combustion of natural gas emits less carbon dioxide than the combustion of coal. If all existing coal and oil power plants were replaced by modern high efficiency natural gas systems, the U.S. would reduce its greenhouse gas emissions about 1000 million tons per year (17% of U.S. emissions). Some scientists doubt that there is enough natural gas to make this possible. The cost of such a plan, though uncertain, is estimated between $0 and $177 per ton of carbon dioxide.

Replace half of the existing oil and coal fired power plants with solar power plants. The amount of solar energy reaching the earth's surface each year is enormous, thousands of times greater than worldwide annual fossil fuel use. While costs are still high, technology currently exists to use this solar energy to provide elec- tricity, light, heat, and steam for buildings and industry. If it were used wherever possible, it could reduce greenhouse gas emissions by about 1000 million tons per year (17% of U.S. emissions). However, substantial progress is necessary before solar technology is affordable as a basic source of electricity. The cost of reducing emissions through this program is estimated to be between $76 to $177 per ton of carbon dioxide.

Where possible, replace all fossil fuel plants with nuclear power plants. Nuclear power currently provides about 7% of electricity in the U.S., but concerns over the safety, cost, and environmental impacts of nuclear energy have halted development. Improvements in nuclear power might allow it to be considered as an option for reducing carbon dioxide emissions. If nuclear power were widely adopted in the U.S., the reduction in carbon dioxide emissions could reach as high as 1500 million tons per year (25% of U.S. emissions). Estimates of the cost of this policy range from $0 to $51 per ton of carbon dioxide saved.

Agriculture, deforestation and other human activities are also responsible for significant quantities of greenhouse gas emissions. Reductions can be achieved through improved waste management, altered use and formulation of fertilizers, and changes in land use.

Establish an international "forestry fund" to prevent deforestation. Deforestation in the developing world accounts for over 20% of the man-made greenhouse effect. The U.S. can play a role in policies to limit deforestation. Because deforestation is due largely to population and economic pressures, tropical rain forests will be preserved only if they have more value standing than cut down. One idea is an international "forestry fund," an endowment, funded by the developed world, which places $80 per acre ($200 per hectare) of protected forest into an investment account. The interest from the account is given to people living near or in the protected forests, to help them develop sustainable forestry practices, and to support them during the transition away from "slash and burn" agriculture. Residents would receive the interest as long as they practiced sustainable forestry. Fully implemented, the program could reduce global carbon dioxide emissions by 7000 million tons annually, at a cost of about $0.40 per ton of carbon dioxide saved.

Reduce methane emissions by improving waste management practices and changing agricultural techniques. Though it accounts for only a small share of the man-made greenhouse effect, methane is a powerful greenhouse gas. Emissions come from rice paddies, cows and other "ruminant" animals, and from decomposing waste. Emissions can be reduced by cultivating fast-growing rice or high-density paddies, by placing ruminant animals on diets that reduce the amount of methane they emit as a byproduct of digestion, and by handling plant and animal wastes in a manner that reduces the amount of methane produced as they decompose. Such actions could reduce greenhouse gas emissions by over 200 million tons of carbon dioxide equivalent per year, at a cost of $0 to $5 per ton.

Relocation of people, agriculture and industry is one way to adapt to the changes in temperature, sea level and water distribution that might result from climate change. For example, state and federal governments often subsidize the rebuilding of homes and replenishment of beaches in areas that have experienced severe storms or floods. If sea level rise makes devastating storms and floods more common in certain regions, government could use these subsidies to help people relocate to less vulnerable areas, instead of rebuilding in the same spot. Banks and insurance companies may begin to influence building choices if they believe climate change may affect the properties they finance or insure. In the U.S. people migrate all the time for a variety of reasons. For this reason, it is difficult to say which portion of the costs of relocation should be assigned to climate change.

Improving irrigation and developing new crop strains would allow agriculture to adapt to moderate climate change. The efficiency of irrigation systems improved 35% between 1950 and 1980, and some researchers believe efficiency can be improved substantially more by making some relatively cheap changes to existing technology. As for crops, state, federal, and private labs today cultivate and test thousands of strains of agricultural plants. There are, for example, about 450 different strains of corn in commercial use. The costs of adapting to modest climate change would probably be a few percent or less of the overall costs of agriculture. Maintaining funding for research on crop varieties is a good way to prepare for the possible impacts of global warming on agriculture.

Migration corridors for plants and animals in the natural environment might help the re-establishment of ecosystems in new locations as a response to climate change. As discussed in Details Booklet Part 2, gradual change would allow many natural ecosystems to migrate with the climate. However, natural migration of ecosystems can be blocked by human development, such as cities, highways, and farms. One way to help these ecosystems adapt to global warming might be to provide them with "corridors" of undeveloped land through which ecosystems can migrate as necessary. The costs are uncertain partly because such corridors might have other "open space" benefits and partly because it is unclear how many would be needed to be effective.

Geo-engineering options are potentially powerful, but as yet untested, ways either to stop the accumulation of carbon dioxide in the atmosphere, or to counteract its effects on our climate.

Global reforestation programs could be designed to plant large numbers of trees to extract carbon dioxide and store it. A global reforestation program could remove 250 million tons of carbon dioxide per year (4% of U.S. emissions) at a cost of $3 to $10 per ton.

Adding iron to fertilize the ocean may cause phytoplankton in the top layers of the ocean to absorb more carbon dioxide. While not all scientists agree that this strategy is safe, and recent tests in the ocean suggest it might not work, the absorption potential is very large, from 600 million to as high as 3000 million tons of carbon dioxide per year (10% to 50% of U.S. emissions). If it turns out to be a safe, viable option, it would probably cost somewhere between 10¢ and $15 dollars per ton of carbon dioxide removed.

Screen out sunlight to counteract the effects of increased concentrations of greenhouse gases. Either large thin screens in low orbit or small particles of dust placed very high in the atmosphere could be used to reduce the amount of sunlight striking the earth. Thus, as the earth's atmosphere trapped more heat, less heat energy would be put into the earth's system by the sun, maintaining basically the same temperature. While untested, this strategy has the potential to counteract the warming effect of large amounts of carbon dioxide, at a cost of between 3¢ and $2.5 dollars per ton of carbon dioxide.

How can people decide for themselves what should be done about climate change?

The climate problem affects everyone, and everyone has a stake in deciding what should be done. It is for you to decide what actions you should take as an individual (in your home, your car, and so forth). Equally important, as a citizen you must decide which policies to support or oppose. It may be tempting to decide that the climate problem is just too complicated to deal with. Without telling you what to choose, we can offer some advice on how to organize the choices and make decisions.

We will simplify the problem by reducing it to a choice between three broad policy alternatives:

POLICY 1: no abatement;

POLICY 2: moderate abatement;

POLICY 3: stringent abatement.

First, we will discuss the goal of each of these policies. Then we will suggest some of the factors that might be important to you in making a choice. Finally, we will give examples of the sorts of people who might choose each one.

As its name suggests, the no abatement policy takes no immediate action on climate change or greenhouse gas emissions. Proponents of this policy may view the science of global warming as too uncertain to justify action, may believe that the impacts of global warming will be very small, or may think that abatement actions are too expensive.

The goal of moderate abatement is to slow greenhouse gas emissions and give society more time to solve the problem. One version of this policy would commit every economically developed nation to reduce greenhouse gas emissions to 20% below 1988 levels per dollar of gross domestic product (GDP) by the year 2000. Because this emissions limit is stated in terms of GDP, total emissions could still grow as the population or economy expands. Moderate abatement would not prevent climate change. But, if all the world did it, it would reduce the impacts of climate change by about 25% by the middle of the next century.

Stringent abatement is the most ambitious climate change policy. By reducing total greenhouse gas emissions worldwide to 60% below 1988 levels and holding them there permanently, it aims to prevent climate change altogether. Unlike moderate abatement, which allows emissions to rise as the economy grows, moderate abatement does not allow any emissions growth beyond 60% below 1988 levels. Thus, in order for the world economy to grow and for living standards to rise in the developing world, ever more innovative ways to prevent emissions from growing would have to be found. If all the world did it, stringent abatement would prevent global climate change.

Suppose you or a friend wants to decide which of these three policies to support. Your decision should depend on at least two considerations:

1. What do you think the impacts of climate change are likely to be? That is, how much do you think climate will change, and what impact do you believe that change will have on the things you care about? Again to make things simple, assume that there are only three possible beliefs about the impacts of climate change: it can be not bad, moderately bad, or very bad. Of course your judgment not only depends on what you believe about climate change, but also on what you value. For example, two people might agree that climate change will destroy many of the world's most sensitive ecosystems, but disagree about how much they value those ecosystems. These people would rate the impact of climate change differently. The person who values them highly will probably rate the impacts of climate change as moderately bad or very bad. The other person, who is perhaps mainly concerned with the economic impacts of climate change and doesn't think sensitive ecosystems are of great importance, might rate the impact of climate change as not bad. In short, how you rate the impacts of climate change depends on what you value.

2. How much do you think abatement will cost? Again, for the sake of simplicity assume that there are only three possible beliefs about the cost of abatement: low, medium, or high. Unlike the case above, where we were dealing with values which are very difficult, if not impossible, to measure in dollars, here we are dealing with costs that can be quantified. By low cost we mean that moderate abatement can be achieved in the U.S. at a net savings of $50 billion per year (because the energy conservation involved would save money), while stringent abatement would cost about $250 billion per year (or about $1000 per person per year). By medium cost we mean that moderate abatement will cost nothing in the long-term, while stringent abatement will cost about $500 billion per year. By high cost we mean that moderate abatement will cost about $60 billion per year, while stringent abatement would cost as much as $3 trillion per year (about 60% of GDP).

To show how this might work, consider three friends named Ann, Sue, and Pat. Like you, each of these imaginary citizens must consider the two questions outlined above: (1) "What do you think the impacts of climate change are likely to be?" and (2) "How much do you think abatement measures will cost?" The two pages that follow give a summary of their positions.

ANN

Beliefs: She thinks climate change is a possibility, but isn't convinced it will occur. If climate change does occur, she believes it may damage the environment to some extent, but have only minor effects on the economy.

Values: She cares about the environment, but cares more about the economy and impacts on people's incomes and jobs.

How Ann reasons: Using the terms discussed above, Ann concludes that the impacts of climate change will be not bad. She acknowledges that there may be some environmental impact, but is concerned only if the environmental impact is really serious. It's the economy she cares about, and she thinks that won't suffer much. That's why she rates the impacts of climate change not bad.

As a result, the policy she chooses will depend on how she assesses their costs. If she expects abatement costs to be low or medium, she will support moderate abatement because this will either result in a net savings or have no cost.

On the other hand, if she expects abatement costs to be high, she will support the "no abatement" policy at least until environmental harm is shown to be important. This is because she cares more about the economy than the environment and does not want to spend a lot of money to reduce what she judges to be small and perhaps unlikely environmental damage.

SUE

Beliefs: She thinks climate change is very likely and believes the environmental consequences would be disastrous.

Values: She cares a great deal about the environment, and much less about the economy.

How Sue reasons: Sue thinks that the accumulation of greenhouse gases in the atmosphere will cause a great deal of environmental damage. She also thinks global warming is very likely to come about unless we dramatically reduce greenhouse gas emissions. Because she cares so much about protecting the environment, she rates the impacts of global warming as very bad. She would do this even if she believed the economic impacts would be high. Therefore, her top priority is preventing climate change altogether, not just slowing climate change with the policy of moderate abatement. If abatement costs are low or medium, she'll favor the "stringent abatement" policy. She is optimistic about the costs of energy efficiency and new energy technologies and doesn't believe that abatement costs will be high, but if she did, she would probably still favor stringent abatement.

PAT

Beliefs: She is uncertain about the possibility of climate change and worried about the unlikely but catastrophic consequences.

Values: She wants to avoid huge impacts on both the environment and the economy.

How Pat reasons: Pat rates the impacts of climate change as moderate. Neither the environment nor the economy is more important to her, and she wants to protect both against large losses.

If she thinks abatement costs will be low, she supports "stringent abatement." She reasons that $250 billion is a reasonable price for the U.S. to pay to completely prevent global warming.

If she expects abatement costs to be moderate, she supports moderate abatement. At moderate cost, stringent abatement costs $500 billion, which she thinks is too expensive given the uncertainties associated with the issue. Moderate abatement, which under this scenario costs nothing in the long-term, seems reasonable to her because it provides protection against the most catastrophic consequences of global warming.

If she believes abatement costs are high, she supports the "no abatement" policy. Under the high cost scenario, stringent abatement costs $3 trillion, enough to cripple the economy. "Moderate abatement" under this high cost scenario is a lot cheaper, only $60 billion, but it also provides a lot less protection. In Pat's opinion, too little protection for the money.

While these examples are for the U.S., they assume that the rest of the world does the same thing. Readers who would like to see the more detailed technical analysis on which these examples are based, are referred to: L.B. Lave and H. Dowlatabadi, "Climate Change: The effects of personal beliefs and scientific uncertainty," Environmental Science & Technology, Vol. 27, No. 10, pp. 1962-1972, 1993.

Glossary

aerosols: Extremely small particles of liquid or dust in the atmosphere. Burning coal releases sulfur dioxide which in the atmosphere is transformed into sulfate aerosols. One geo-engineering strategy would put more aerosols into the atmosphere to reflect sunlight back to space.

afforestation: Establishing new forests on unforested land. Afforresting large areas of land so that trees will absorb and store carbon from the atmosphere could slow carbon dioxide buildup.

albedo: The fraction of sunlight that is reflected by earth, ice, and clouds back into space. The value for today's earth is about one- third (i.e., two-thirds of the sunlight is absorbed).

biodiversity: The number of different kinds of plant and animal species that live in a region. On land, tropical rain forests have the highest biodiversity.

biomass: The amount of living matter in a particular region, usually expressed as weight (mass) per unit area (e.g., tons per acre).

carbon cycle: The processes by which carbon is cycled through the environment. Carbon, in the form of carbon dioxide, is absorbed from the atmosphere and used by plants in the process of photosynthesis to store energy. Plants and animals then return carbon dioxide to the atmosphere through respiration when they consume this entergy. On a much long time-scale, carbon is also cycled into and out of rocks.

carbon dioxide: A gas made up of two atoms of carbon and one atom of oxygen which is produced whenever carbon-based fuels are burned (or oxidized more slowly in plants and animals). Carbon dioxide is the most important "greenhouse gas" which may cause climate change. Human sources of carbon dioxide include burning fossil fuels for electricity, transportation, heating, cooling, and manufacturing. Burning trees in the process of deforestation also produces carbon dioxide. Abbreviated CO₂.

chlorofluorocarbons: A family of greenhouse gases used in air conditioning, as industrial solvents and in other commercial applications. Abbreviated CFCs. CFCs destroy ozone in the stratosphere (see ozone). CFCs were once widely used in spray cans but in the U.S. this use has now been banned. Other uses are also being eliminated under an international agreement negotiated in Montreal in 1987.

climate: The average pattern of weather in a place. While weather may change substantially from day-to-day, when changes in climate occur, they usually happen gradually over many years.

deforestation: Cutting most or all of the trees in a forested area. Deforestation contributes to warming by releasing carbon dioxide, changing the albedo (amount of sunlight reflected from the surface) and reducing the amount of carbon dioxide taken out of the atmosphere by trees. Today, deforestation may contribute about 20% of possible warming.

discount rate: A measure of how cost and benefits that will happen in the future compare to cost and benefits today.

energy intensity: The amount of energy used by an appliance or an industry to produce a product or service. For example, a fluorescent light requires only 20 watts to produce the same amount of light as a regular 100 watt light bulb, so its energy intensity is 5 times lower. Reducing energy intensity is one way to increase energy efficiency and emit less carbon dioxide.

feedback: The mechanism by which changes in one part of the earth-atmosphere system affect future changes in other parts of that system. Feedbacks come in two kinds. In climate change, negative feedbacks work to slow down or offset warming while positive feedbacks work to speed up or amplify warming.

fossil fuel: Coal, oil (from which gasoline is make), and natural gas are called fossil fuels because the chemical energy they contained is left over from plants and animals that lived long ago.

greenhouse effect: The process by which energy from the sun is trapped under the atmosphere to cause warming. Light energy can easily pass in through the atmosphere. Once some of this light is absorbed by dark surfaces, the resulting heat energy has greater difficulty getting back out. Through the naturally occurring greenhouse effect, water vapor, ozone and carbon dioxide have kept temperatures on the earth moderate for several billions years. Today, people are adding more gases which might increase the temperature.

greenhouse gas: Any gas in the atmosphere that contributes to the greenhouse effect. These include carbon dioxide, methane, ozone, nitrous oxide, CFCs, and water vapor. Most occur naturally as well as being created by people.

methane: A greenhouse gas consisting of one molecule of carbon and four molecules of hydrogen. Pound-for-pound it produces between 5 to 10 times more warming than carbon dioxide. Methane is produced naturally from rotting organic matter. Human sources of methane include agricultural activities such as growing rice and raising live stock, land-fills, coal mines, and natural gas systems. Abbreviated CH₄.

Montreal protocol: An international treaty signed in 1987 that limits production of chlorofluorocarbons.

natural gas: Gas obtained from wells used as a fuel. While it contains many chemicals the principle component of natural gas is methane.

nitrous oxide: A greenhouse gas consisting of two molecules of nitrogen and one molecule of oxygen. Pound-for-pound it produces about 300 times more warming than carbon dioxide. Nitrous oxide is created when fuels are burned and is also released during the use of nitrogen-based crop fertilizers. Abbreviated N₂O.

ozone: An unstable gas in which three molecules of oxygen occur together. Ozone is a greenhouse gas. In the atmosphere ozone occurs at two different altitudes. Low altitude tropospheric ozone is a form of air pollution (part of smog) produced by the emissions from cars and trucks. High in the atmosphere a thin layer of stratospheric ozone is naturally created by sunlight. This ozone layer shields the earth from dangerous (cancer-causing) ultraviolet radiation from the sun. Chlorine gas from chlorofluorocarbons speeds the breakdown of ozone in the ozone layer. While important, this is largely a different problem from the problem of global warming. Abbreviated O₃.

sea level rise: An increase in the average level of the ocean caused by expansion when water is warmed and by addition of more water when ice caps melt.

sequester: To remove or segregate. Scientists sometimes say that activities, such as planting trees, which remove carbon dioxide from the atmosphere, sequester carbon dioxide.

sink: A place where material is removed or stored. For example, the oceans absorb about 50% of the carbon dioxide released into the atmosphere. Scientists refer to the oceans as a carbon dioxide sink.

stratosphere: The upper part of the earth's atmosphere, above about seven miles.

sustainable development: Economic activities which can meet the needs of the present without compromising the ability of future generations to meet their own needs.

troposphere: The lower portion of the atmosphere in which we live.

weather: The condition of the atmosphere at a particular place and time measured in terms in wind, temperature, humidity, atmospheric pressure, and precipitation (rain, snow, etc.). In most places, weather can change from hour-to-hour, day-to-day, and season-to-season.

How to learn more.

For a comprehensive summary of the issues discussed in these brochures, see:

• U.S. Congress, Office of Technology Assessment, Changing by Degrees: Steps to Reduce Greenhouse Gases, OTA-O-482 (Washington, DC: U.S. Government Printing Office, 1991).
• Policy Implications of Greenhouse Warming -- Mitigation, Adaptation and the Science Base, National Academy of Sciences, National Academy Press, Washington, DC, 1992.