Last revised 1 May 1993 by the Information Unit on Climate Change (IUCC), UNEP, P.O. Box 356, CH-1219 Châtelaine, Switzerland. Tel. (41 22) 979 9111. Fax (41 22) 797 3464. E-mail


1 Energy and greenhouse gas emissions

The energy sector is the biggest contributor to man-made climate change. Energy use is responsible for about three-quarters of mankind’s carbon dioxide (CO2) emissions, one-fifth of our methane (CH4), and a significant quantity of our nitrous oxide (N2O). It also produces nitrogen oxides (NOx) hydro-carbons (HCs), and carbon monoxide (CO), which, though not greenhouse gases (GHGs) themselves, influence chemical cycles in the atmosphere that produce or destroy GHGs, such as tropospheric ozone. Most GHGs are released during the burning of fossil fuels. Oil, coal, and natural gas supply the energy needed to run automobiles, heat houses, and power factories. In addition to energy, however, these fuels also produce various by-products. Carbon and hydrogen in the burning fuel combine with oxygen (O2) in the atmosphere to yield heat (which can be converted into other forms of useful energy) as well as water vapor and carbon dioxide. If the fuel burned completely, the only by-product containing carbon would be carbon dioxide. However, since combustion is often incomplete, other carbon-containing gases are also produced, including carbon monoxide, methane, and other hydrocarbons. In addition, nitrous oxide and other nitrogen oxides are produced as by-products when fuel combustion causes nitrogen from the fuel or the air to combine with oxygen from the air. Increases in tropospheric ozone are indirectly caused by fuel combustion as a result of reactions between pollutants caused by combustion and other gases in the atmosphere. Extracting, processing, transporting, and distributing fossil fuels can also release greenhouse gases. These releases can be deliberate, as when natural gas is flared or vented from oil wells, emitting mostly methane and carbon dioxide, respectively. Releases can also result from accidents, poor maintenance, or small leaks in well heads and pipe fittings. Methane, which appears naturally in coal seams as pockets of gas or "dissolved" in the coal itself, is released when coal is mined or pulverized.Methane, hydrocarbons, and nitrogen oxides are emitted when oil and natural gas are refined into end products and when coal is processed (which involves crushing and washing) to remove ash, sulfur, and other impurities. Methane and smaller quantities of carbon dioxide and hydrocarbons are released from leaks in  natural gas pipelines. Hydrocarbons are also released during the transport and distribution of liquid fuels in the form of oil spills from tanker ships, small losses during the routine fueling of motor vehicles, and so on.Some fuels produce more carbon dioxide per unit of energy than do others. The amount of carbon dioxide emitted per unit of energy depends on the fuel’s carbon and energy content. The figures below give representative values for coal, refined oil products, natural gas, and wood. Figure A shows for each fuel the percentage by weight that is elemental carbon. Figure B shows how many gigajoules (GJ) of energy are released when a tonne of fuel is burned. Figure C indicates how many kilograms of carbon are created (in the form of carbon dioxide) when each fuel is burned to yield a gigajoule of energy. According to Figure C,coal emits around 1.7 times as much carbon per unit of energy when burned as does natural gas and 1.25 times as much as oil.  Although it produces a large amount of carbon dioxide, burning wood (and other biomass)contributes less to climate change than does burning fossil fuel. In Figure C, wood appears to have the highest emission coefficient. However, while the carbon contained in fossil fuels has been stored in the earth for hundreds of millions of years and is now being rapidly released over mere decades, this is not the case with plants. When plants are burned as fuel, their carbon is recycled back into the atmosphere at roughly the same rate at which it was removed, and thus makes no net contribution to the pool of carbon dioxide in the air. Of course, when biomass is removed but is not allowed to grow back - as in the case of massive deforestation - the use of biomass fuels use can yield net carbon dioxide emissions.  It is difficult to make precise calculations of the energy sector’s greenhouse gas emissions.Estimates of greenhouse gas emissions depend on the accuracy of the available energy statistics and on estimates of "emission factors", which attempt to describe how much of a gas is emitted per unit of fuel burned. Emission factors for carbon dioxide are well known, and the level of uncertainty in national CO2 emissions estimates are thus fairly low, probably around 10 percent. For the other gases, however, the emission factors are not so well understood, and estimates of national emissions may deviate from reality by a factor of two or more. Estimates of emissions from extracting, processing, transport, and so on are similarly uncertain.See also Fact Sheet 240: "Reducing greenhouse gas emissions from the energy sector".For further reading:Grubb, M., 1989. "On Coefficients for Determining Greenhouse Gas Emissions from Fossil Fuel Production and Consumption". P. 537 in Energy Technologies for Reducing Emissions of Greenhouse Gases. Proceedings of an Experts’ Seminar, Volume 1, OECD, Paris, 1989.ORNL, 1989. Estimates of CO2 Emissions from Fossil Fuel Burning and Cement Manufacturing. Based on the United Nations Energy Statistics and the U.S. Bureau of Mines Cement Manufacturing Data. G. Marland et al, Oak Ridge National Laboratory, May 1989. ORNL/CDIAC-25. This is a useful source for data.

2 Fuel and the carbon cycle

Most of the combustible fuels in common use contain carbon. Coal, oil, natural gas, and biomassfuels such as wood are all ultimately derived from the biological carbon cycle (see diagram below). The exceptions are hydrogen gas (H2), which is currently in limited use as a fuel, and exotic fuels (such as hydrazine, which contains only nitrogen and hydrogen) used for aerospace and other special purposes. Burning these carbon-based fuels to release useful energy also yields carbon dioxide (the most important greenhouse gas) as a by-product. The carbon contained in the fuel combines with oxygen (O2) in the air to yield heat, water vapor (H2O), and CO2. This reaction is described in chemical terms as:CH2 + 3O2 -> heat + 2H2O + CO2, where "CH2" represents about one carbon unit in the fossil fuel. Other by-products, such as methane (CH4), can also result when fuels are not completely burned.Carbon cycles back and forth between the atmosphere and the earth (the oceans also play a critical role in the carbon cycle). Plants absorb CO2 from the air and from water and use it to create plant cells, or biomass. This reaction is powered by sunlight and is often characterized in a simplified manner as:CO2 + (solar energy) + H2O -> O2 + CH2O,where "CH2O" roughly represents one new carbon unit in the biomass. Plants then release carbon back into the atmosphere when they are burned in fires or as fuel or when they die and decompose naturally. The carbon absorbed by plants is also returned to the air via animals, which exhale carbon dioxide whenthey breath and release it when they decompose. This biological carbon cycle has a natural balance, so that over time there is no net contribution to the "pool" of CO2 present in the atmosphere. One way of visualizing this process is to consider a hectare of sugar cane plants that is harvested to make ethanol fuel. The production and combustion of the ethanol temporarily transfers CO2 from the terrestrial carbon pool (carbon present in various forms on and under the earth’s surface) to the atmospheric pool. A year or two later, as the hectare of cane grows back to maturity, the CO2 emitted earlier is recaptured in plant biomass. Mankind’s reliance on fossil fuels has upset the natural balance of the carbon cycle. Biomass sometimes becomes buried in ocean sediment, swamps, or bogs and thus escapes the usual process of decomposition. Buried for hundreds of millions of years - typically at high temperatures and intense pressures - this dead organic matter sometimes turns into coal, oil, or natural gas. The store of carbon is gradually liberated by natural processes such as rock weathering, which keeps the carbon cycle in balance. By extracting and burning these stores of fossil fuel at a rapid pace, however, humans have accelerated the release of the buried carbon. We are returning hundreds of millions of years worth of accumulated CO2 to the atmosphere within the space of a half-dozen generations. The difference between the rate at which carbon is stored in new fossil reserves and the rate at which it is released from old reserves has created an imbalance in the carbon cycle and caused carbon in the form of CO2 to accumulate in the atmosphere. With careful management, biomass fuels can be used without contributing to net CO2 emissions. This can only occur when the rate at which trees and other biomass are harvested for fuel is balanced by the rate at which new biomass is created. This is one reason why planting forests is often advocated as an important policy for addressing the problem of mankind’s emissions of greenhouse gases. In cases where biomass is removed but does not (or is not allowed to) grow back, the use of biomass fuels is likely to yield net CO2 emissions just as the use of fossil fuels does. This occurs in instances where fuel-wood is consumed faster than forests can grow back, or where carbon in the soil is depleted by sub-optimal forestry or agricultural practices.For further reading:Ehrlich, P.R, A.H. Ehrlich, and J.P. Holdren, 1977, ECOSCIENCE. W.H. Freeman and Co., San francisco.

3 How extracting and transporting fossil fuel releases greenhouse gases

Carbon dioxide and methane - the two most important greenhouse gases - are emitted duringthe extraction and distribution of fossil fuels. Fossil fuels surrender most of their carbon when burned,but GHGs are also emitted when coal is dug out of mines and when oil is pumped up from wells. Additional quantities escape into the atmosphere when fuel is transported, as in gas pipelines. Together, these activities account for about one percent of total annual man-made carbon dioxide emissions (CO2) and about one-quarter of methane emissions (CH4).CO2 is released into the atmosphere when natural gas is "flared" from petroleum reservoirs. Natural gas and oil often occur together in deposits. Oil drillers sometimes simply flare, or burn off, the gas or release it directly into the atmosphere, particularly if the well is too far from gas pipelines or potential gas users. Global emissions of CO2 from gas flaring reached a peak during the mid-1970s and have declined since. Gas that previously was flared is now increasingly captured for use as fuel due to higher prices and demand for gas, as well as improvements in production equipment. Current (1989) global emissions of CO2 from this source are estimated at 202 million tonnes, about 0.8 percent of total man-made CO2 emissions. Most emissions from gas flaring take place in the oil-producing countries of Africa and Asia, as well as in the former USSR. Methane is released when natural gas escapes from oil and gas wells and pipe fittings. Natural gas is typically 85 to 95 percent methane. Transporting this gas from underground reservoirs to end-users via pipes and containers leads to routine and unavoidable leaks. Accidents and poor maintenance andequipment operation cause additional leaks. Newer, well-sealed pipeline systems can have leakage rates of less than 0.1 percent, while very old and leaky systems may lose as much as 5% of the gas passing through. Few measurements have been made, but present estimates are that leaks from equipment at oil and gas wells total about 10 million tonnes of methane a year. Annual emissions from pipelines are thought to be about 10-20 million tonnes, representing some 2-5% of total man-made methane emissions.Methane is also released when coal is mined and processed. This accounts for most of the methane emitted during fossil fuel extraction. Coal seams contain pockets of methane gas, and methane molecules also become attached through pressure and chemical attraction to the microscopic internal surfaces of the coal itself. The methane is released into the atmosphere when coal miners break open gas pockets in the coal and in coal-bearing rock. (Coal miners once used canaries as indicators of the presence of the colorless, odorless gas; if the birds died, methane concentrations in the mine were at dangerous levels.) Crushing and  pulverizing the coal also breaks open tiny methane gas pockets and liberates the methane adsorbed in the coal. It can take days or even months for this absorbed methane to escape from the mined coal. The amount of methane released per unit of coal depends on the type of coal and how it is mined. Some coal seams contain more methane per unit of coal than do others. In general, lower quality coals, such as "brown" coal or lignite, have lower methane contents than higher quality coals such as bituminous and anthracite coal. In addition, coal that is surface-mined releases on average just 10% as much methane per unit mined as does coal removed from underground mines. Not only is coal buried under high pressure deep in the earth able to hold more methane, but underground mining techniques allow additional methane to escape from both the coal that is not removed and from the coal-bearing rock. The table below shows methane emissions from underground and surface mining for the ten countries with the highest emissions. These ten countries produce over 90 percent of both the world’s coal and coal-related methane emissions. Three countries - China, the (former) Soviet Union, and the United States - together produce two-thirds of the world’s methane emissions from coal. For further reading:Marland, G., T.A. Boden, R.C. Griffin, S.F. Huang, P. Kanciruk, and T.R. Nelson, 1989. Estimates of CO2 Emissions from Fossil Fuel Burning and Cement Manufacturing, Based on the United Nations Energy Statistics and the U.S. Bureau of Mines Cement Manufacturing Data, Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee, May 1989. Report # ORNL/CDIAC-25. Estimates of CO2 from gas flaring were taken from this source. United States Environmental Protection Agency (US/EPA), 1990. Methane Emissions from Coal Mining: Issues and Opportunities for Reduction, prepared by ICF Resources, Inc., for the Office of Air and Radiation of the US/EPA, Washington D.C.. US/EPA Report # EPA/400/9-90/008. Data from this source were used for the table in the text.Intergovernmental Panel on Climate Change (IPCC) , 1990. Methane Emissions and Opportunities for Control. Published by the US Environmental Protection Agency, US/EPA Report # 400/9-90/007

4. Global energy use during the industrial Age

Almost all of mankind’s fossil-fuel emissions of carbon dioxide have occurred over the last century. It was not until the 1800s that coal, oil, and natural gas were unearthed and burned in large quantities in the newly-invented factories and machines of the Industrial Revolution. Industrialization brought about profound changes in human well-being, particularly in Europe, North America, and Japan. It also created or worsened many environmental problems, including climate change. Fossil fuel use currently accounts for about three-quarters of mankind’s emissions of so-called greenhouse gases.Coal dominated the energy scene in Europe and North America during the 19th and early 20th centuries. Coal was found in large deposits near the early industrial centres of Europe and North America. Figure A below shows the trend of global fossil-fuel carbon dioxide (CO2) emissions over the last 130 years (note the "valley" around 1935 when the Great Depression lowered energy use, and the plateau around 1980 caused by higher international oil prices). In industrialized countries the fuel mix has now shifted towards oil, gas, and other energy sources. Although large petroleum deposits were located early in the 20th century, oil use did not expand greatly until the post-World War II economic take-off. Natural gas, in limited use since the 1800s, started to supply an increasing share of the world’s energy by the 1970s (see Figure B). Among non-fossil energy sources, hydroelectric power has been exploited for about 100 years, and nuclear power was introduced in the 1950s; together they now supply about 15 percent of the global demand for internationally traded energy. Solar and wind power are used in both traditional applications (such as wind-assisted pumping) and high-tech ones (solar photovoltaics), but they satisfy only a small fraction of overall fuel needs. The fuel mix in developing countries includes a higher percentage of biomass fuels and, in some cases, coal. Biomass fuels continue to be widely used in many countries, particularly in homes. As India, China, and other developing countries have industrialized over the past decade, coal’s share of global CO2 emissions has increased somewhat, reversing the pattern of previous years. Countries such as China and Mexico have benefited from large domestic supplies of coal or oil, but most other developing countries have had to turn to imported fuels, typically oil, to power their industries.Although CO2 emissions have generally followed an upward trend, the rate of increase has fluctuated. Changes in overall carbon dioxide emissions reflect population and economic growth rates, per-capita energy use, and changes in fuel quality and fuel mix. During the last four decades of the 1800s, fuel consumption rose six times faster than population growth as fossil fuels were substituted for traditional fuels (see Figure C). From 1900 to 1930 total fuel use expanded more slowly, but - driven by increased fuel use per person - it still grew faster than the rate of population growth. CO2 emissions rose only 1.5 times as fast as population between 1930 and 1950 due to the impact of the Great Depression and World War II on industrial production. The post-war period of 1950 to 1970 saw a rapid expansion of both population and total fuel emissions, with emissions growing more than twice as fast as population. Here again, increases in per-capita energy consumption made the difference. Since 1970, higher fuel prices, new technologies, and a shift to natural gas (which has a lower carbon content than oil and coal) have reduced the growth in emissions relative to population. During the 1980s, in fact, growth in population exceeded growth in emissions, meaning that average emissions per capita actually declined. Regional patterns of per-capita energy use will continue to change. Over the last 40 years, the strongest absolute growth in per-capita carbon dioxide emissions has been in the industrialized countries, while the developing countries have provided (and continue to provide) most of the world’s population increase. Large increases in per-capita CO2 emissions occurred between 1950 and 1970 in Eastern Europe and the (former) USSR, North America, Japan, Australia, and Western Europe. During the 1980s, however, per-capita emissions in these regions have grown relatively little or even declined. The strongest growth in per-capita emissions since 1980 has been in Centrally-Planned Asia (principally China), south and east Asia, and the Middle East. See also Fact Sheet 240: "Reducing greenhouse gas emissions from the energy sector"For further reading:Ehrlich, P.R, A.H. Ehrlich, and J.P. Holdren, 1977, ECOSCIENCE. W.H. Freeman and Co., SanFrancisco.Marland, G., T.A. Boden, R.C. Griffin, S.F. Huang, P. Kanciruk, and T.R. Nelson, 1989. Estimates of CO2Emissions from Fossil Fuel Burning and Cement Manufacturing, Based on the United Nations Energy Statistics and the U.S. Bureau of Mines Cement Manufacturing Data, Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee, May 1989. Report # ORNL/CDIAC-25.Data from this source were used for Figures A and C.Ogawa, Yoshiki, "Economic Activity ad the Greenhouse Effect", in "Energy Journal", Vol. 1, no. 1. (Jan.1991), pp. 23-26.United States Environmental Protection Agency (US/EPA), 1990. Policy Options For Stabilizing Global Climate, edited by D.A. Lashof and D. Tirpak. Report # 21P-2003.1, December, 1991. US/EPA Office of Planning and Evaluation, Washington D.C.. Data from this source were used for Figures A and B.

5 Why cement-making produces carbon dioxide

Cement manufacturing is the third largest cause of man-made carbon dioxide emissions. While fossil fuel combustion and deforestation each produce significantly more carbon dioxide (CO2), cement-making is responsible for approximately 2.5% of total worldwide emissions from industrial sources (energy plus manufacturing sectors). Cement is a major industrial commodity. Manufactured commercially in at least 120 countries, it is mixed with sand and gravel to make concrete. Concrete is used in the construction of buildings, roads, and other structures, as well as in other products and applications. Its use as a residential building material is particularly important in countries where wood is not traditionally used for building or is in short supply.Annual CO2 emissions from cement production in nine major regions of the world are shown in Figure A below.Large quantities of CO2 are emitted during the production of lime, the key ingredient in cement. Lime, or calcium oxide (CaO), is created by heating calcium carbonate (CaCO3) in large furnaces called kilns. Calcium carbonate is derived from limestone, chalk, and other calcium-rich materials. The process of heating calcium carbonate to yield lime is called calcination or calcining and is written chemically as: CaCO3 + Heat -> CaO + CO2Lime combines with other minerals in the hot kiln to form cement’s "active ingredients". Like the CO2 emitted during the combustion of coal, oil, and gas, the carbon dioxide released during cement production is of fossil origin. The limestone and other calcium-carbonate-containing minerals used in cement production were created ages ago primarily by the burial in ocean sediments of biomass (such as sea shells, which have a high calcium carbonate content). Liberation of this store of carbon is normally very slow, but it has been accelerated many times over by the use of carbonate minerals in cement manufacturing.The lime content of cement does not vary much. Most of the structural cement currently produced is of the "Portland" cement type, which contains 60 to 67 percent lime by weight. There are specialty cements that contain less lime, but they are typically used in small quantities. While research is underway into suitable cement mixtures that have less lime than does Portland cement, options for significantly reducing CO2 emissions from cement are currently limited. Carbon dioxide emissions from cement production are estimated at 560 million tonnes per year.This estimate is based on the amount of cement that is produced, multiplied by an average emission factor. By assuming that the average lime content of cement is 63.5%, researchers have calculated an emission factor of 0.498 tonnes of CO2 to one tonne of cement.1CO2 emissions from cement production have increased about eight-fold in the last 40 years.Figure B below shows the estimated global emissions from this source since the 1950s. Note that these figures do not include the CO2 emissions from fuels used in the manufacturing process. Cement production and related emissions of CO2 have risen at roughly three times the rate of population growth over the entire period, and at twice the rate of population growth since 1970. Cement-related CO2 emissions by region are shown in the right-hand figure. Emissions from the industrialized world and China dominate, but emissions from all regions are significant, reflecting the global nature of cement production.For further reading:Marland, G., T.A. Boden, R.C. Griffin, S.F. Huang, P. Kanciruk, and T.R. Nelson, 1989. Estimates of CO2 Emissions from Fossil Fuel Burning and Cement Manufacturing, Based on the United Nations Energy Statistics and the U.S. Bureau of Mines Cement Manufacturing Data, Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee, May 1989. Report # ORNL/CDIAC-25.Data from this source were used in preparing Figures A and B.Tresouthick, S.W., and A. Mishulovich, 1990. "Energy and Environment Considerations for the Cement Industry", pp. B-110 to B-123 in Energy and Environment in the 21st Century, proceedings of a conference held March 26-28, 1990 at Massachusetts Institute of Technology, Cambridge, Massachusetts. U.S. Department of the Interior, Bureau of Mines, 1992. Cement: Annual Report 1990, authored by Wilton Johnson. United States Department of the Interior, Washington D.C. The cement production figures in Figure A were derived from this source.Notes:1 Marland, et. al.

6.Chlorofluorocarbons (CFCs) and climate change

Chlorofluorocarbons (CFCs) and other halocarbons are extremely potent greenhouse gases . . .They are released in relatively small quantities, but one kilogram of the most commonly used CFCs may have a direct effect on climate thousands of times larger than that of one kilogram of carbon dioxide. In addition, over the last two decades the percentage increase in CFCs in the atmosphere has been higher than that of other greenhouse gases (GHG); by 1990 concentrations of the different varieties of CFC were increasing by 4-12 percent per year. but because CFCs also destroy ozone -- itself a greenhouse gas -- their net effect on the climate is unclear. The strength of this "indirect" effect of ozone depletion depends on variables such as the temperature of the upper atmosphere and cannot yet be measured with any confidence. According to new research, however, it is possible that the indirect effect of CFCs cancels out some or all of the direct effect of their being powerful GHGs.CFCs are a family of man-made gases used for various industrial purposes. First developed in the 1920s in the United States, CFCs have been used in large quantities only since about 1950. The industrialized countries still account for well over 80 percent of CFC use, although newly-industrializing and developing countries are rapidly increasing their consumption levels. CFC-11 is used principally as a propellant in aerosol cans, although this use has been phased out in many countries, and in the manufacture of plastic foams for cushions and other products. CFC-12 is also used for foam manufacturing as well as in the cooling coils of refrigerators and air conditioners. HCFC-22 was recently introduced as a replacement for CFC-12 because it has a shorter lifetime in the atmosphere and is thus a much less powerful ozone-depleting agent. Halons (or bromofluorocarbons) are used as fire extinguishing materials.CFC-113, methyl chloroform, and carbon tetrachloride are used as solvents for cleaning (carbon tetrachloride is also a feed stock for the production of CFC-11 and CFC-12). There are other types of halocarbons, but they are used in small quantities.CFCs are generally colorless, odorless, and non-toxic. They also do not react chemically with other materials, and as a result they remain in the atmosphere for a long time -- often 50 to 100 years -- before they are destroyed by reactions catalyzed by sunlight. CFCs are composed of carbon, chlorine, and fluorine. Together with other manufactured gases that contain either fluorine or chlorine, and with the bromine-containing Halons, CFCs are referred to collectively as halogenated compounds, or halocarbons.There is often a significant lag time between the production of CFCs and their escape into the atmosphere. Some CFCs, such as those used in spray cans or as solvents for washing electronic parts,are emitted within just a few months or years of being produced. Others, such as those contained in durable equipment such as air conditioners, refrigerators, and fire extinguishers, may not be released for decades. Consequently, the annual use figures in the table do not, for many compounds, reflect annual emissions. So even if the manufacture of CFCs were to stop today, it would take many years for emissions to fall to zero, unless stringent measures were adopted for the recycling or capture of CFCs in old equipment.Although they are important greenhouse gases, CFCs are better known for their role in damaging the earth's ozone layer. CFCs first came to public attention in the mid-1980s after an "ozone hole" was discovered over the Antarctic. Scientists now know that a complex series of chemical reactions involving  CFCs occurs during the Arctic and Antarctic springtimes and leads to the depletion of ozone (O3). Stratospheric ozone forms a shield that prevents most of the sun's ultraviolet (UV) light reaching the earth's surface and causing skin cancer and other cell damage. In response to the weakening of this shield, most of the world's CFC users adopted the "Montreal Protocol" in 1987. This treaty commits signatory nations to phase out their use of CFCs and some other halocarbons by the year 2000. In November, 1992, growing fears of ozone depletion led to the Copenhagen agreement, which commits governments to a total phase out of the most destructive CFCs by the year 1996. This will help to protect the ozone layer and reduce the role of CFCs in climate change -- although the benefits of these agreements will not be felt for several years due to the long life-span of CFCs. Alternatives are being developed to replace CFCs. Some of these substitutes are halocarbons, such as the compound HCFC-22, which can replace CFC-12 in refrigeration and air conditioning systems. These substitute halocarbons are also greenhouse gases, but because they are shorter-lived than the CFCs used now they will have a more limited long-term effect on the climate. Other substitutes that are less harmful than HCFC-22 have been developed and tested and are now being rapidly introduced for various applications. Additional solutions involve changes in industrial processes to avoid the need for halocarbons entirely. For example, later-based cleaners are increasingly being substituted for CFCs in the electronics industry, and non-pressurized, or "pump" spray bottles are being sold instead of CFC-driven spray cans.For further reading:IPCC, "Scientific Assessment of Climate Change", Cambridge University Press, 1990.IPCC, "The Supplementary Report to the IPCC Scientific Assessment", Cambridge University Press, 1992.US Environment Protection Agency, 1990, "Policy Options for Stabilizing Global Climate", eds. D.A.Lashof and D. Tirpak. Report no. 21P-2003.1, December 1991. EPA Office of Planning and Evaluation,Washington DC.WMO/UNEP/NASA, "Scientific Assessment of Ozone Depletion", 1991.

7. Emissions of methane from livestock

About one-quarter of the methane emissions caused by human activities comes from domesticated animals. The second-most important greenhouse gas after carbon dioxide, methane (CH4) is released by cattle, dairy cows, buffalo, goats, sheep, camels, pigs, and horses. It is also emitted by the wastes of these and other animals. Total annual methane emissions from domesticated animals are thought to be about 100 million tonnes. Animals produce methane through "enteric fermentation". In this process plant matter is converted by bacteria and other microbes in the animal’s digestive tract into nutrients such as sugars and organic acids. These nutrients are used by the animal for energy and growth. A number of by-products, including methane, are also produced, but they are not used by the animal; some are released as gas into the atmosphere. (Although carbon dioxide (CO2) is produced in similar quantities as methane, it is derived from sustainably produced plant matter and thus makes no net addition to the atmosphere.) The carbon in the plant manner is converted into methane through this general, overall chemical reaction: microbial Organic Plant Matter + H2O -----------> CO2 + CH4 + (nutrients and metabolism other products). The amount of methane that an individual animal produces depends on many factors. The key variables are the species, the animal’s age and weight, its health and living conditions, and the type of feed it eats. Ruminant animals - such as cows, sheep, buffalo, and goats - have the highest methane emissions per unit of energy in their feed, but emissions from some non-ruminant animals, such as horses and pigs, are also significant. National differences in animal-farming are particularly important. Dairy cows in developing nations, for example, produce about 35 kg of methane per head per year, while those in industrialized nations, where cows are typically fed a richer diet and are physically confined, produce about 2.5 times as much per head. There is a strong link between human diet and methane emissions from livestock. Nations where beef forms a large part of the diet, for example, tend to have large herds of cattle. As beef consumption rises or falls, the number of livestock will, in general, also rise or fall, as will the related methane emissions. Similarly, the consumption of dairy goods, pork, mutton, and other meats, as well non-food items such as wool and draft labor (by oxen, camels, and horses), also influences the size of herds and methane emissions. The figures below present recent estimates of methane emissions by type of animal and by region. Due to their large numbers, cattle and dairy cows produce the bulk of total emissions. In addition, certain regions - both developing and industrialized - produce significant percentages of the global total. Emissions in South and East Asia are high principally because of large human populations; emissions per-capita are slightly lower than the world average. Latin America has the highest regional emissions per capita, due primarily to large cattle populations in the beef-exporting countries (notablyBrazil and Argentina). Centrally-planned Asia (mainly China) has by far the lowest per-capita emissions due to a diet low in meat and dairy products. See also Fact Sheet 271: "Reducing methane emissions from livestock farming".For further reading:Intergovernmental Panel on Climate Change (IPCC), 1990. Greenhouse Gas Emissions from Agricultural systems. Proceedings of a workshop on greenhouse gas emissions from agricultural systems held inWashington D.C., December 12 - 14, 1989. Published as US/EPA report 20P-2005, September, 1990 (2volumes).United States Environmental Protection Agency (US/EPA), 1990. Policy Options For Stabilizing Global Climate, edited by D.A. Lashof and D. Tirpak. Report # 21P-2003.1, December, 1991. US/EPA Office of Planning and Evaluation, Washington D.C. Data from computer files used for this report were used to create the tables.

9 Methane emissions from the disposal of livestock waste

About one-quarter of the total methane emissions caused by human activities comes from domesticated animals. The animals that emit this methane (CH4) include cattle, dairy cows, buffalo, goats, sheep, camels, pigs, and horses. Most livestock-related methane is produced by "enteric fermentation" of food in the animals’ digestive tracts. About one-quarter to one-third of it, however, or a total of 25 million tonnes per year, is released later from decomposing manure. Decomposition occurs as organic wastes in moist, oxygen-free (anaerobic) environments are broken down by bacteria and other microbes into methane, carbon dioxide, and trace amounts of small organic molecules, nitrogen compounds, and other products. The amount of methane released from animal manure in a particular region depends on many variables. The key variables are the number and types of animals present, the amount of manure produced by each animal, the amount of moisture and fiber in the animals’ wastes, the waste management system used, and the local climate. Eastern Europe, Western Europe, and North America have the largest emissions, primarily due to their use of liquid waste storage systems and anaerobic lagoons for treating cattle and swine wastes. The figure below shows regional methane emissions from livestock wastes as a percentage of global emissions and on a per-person basis. Emissions per person in the industrialized regions are two to eight times those in developing regions. Larger animals, not surprisingly, produce more manure per individual. Most domestic animals produce between 7 and 11 kilograms of "volatile solids" (VS) per tonne of animal per day. VS is the amount of organic matter present in the manure after it has dried. Each type of animal waste has its characteristic content of degradable organic matter (material that can be readily decomposed), moisture, nitrogen, and other compounds. As a consequence, the maximum methane-producing potential of the different manures varies both across species and, in instances where feeding practices vary, within a single species.Dairy and non-dairy cattle account for the largest part of global methane emission from livestock manures. After cattle, swine wastes make the second largest contribution. Waste disposal methods help to determine how much methane is emitted. If manure is left to decompose on dry soil, as typically happens with free-roaming animals in developing countries, it will be exposed to oxygen in the atmosphere and probably decompose aerobically. Relatively little methane will be produced, perhaps just 5-10 percent of the maximum possible. (This is particularly true in dry climates, where the manure dries out before extensive methane production can take place; very low temperatures also inhibit fermentation and methane production.) However, when animal wastes are collected and dumped into artificial or natural lagoons or ponds - a common practice in developed countries - they lose contact with the air because, as wastes decompose in these small bodies of water, the oxygen is quickly depleted. As a result, most of the waste is likely to decompose anaerobically, producing a substantial amount of methane - as much as 90 percent of the theoretical maximum. Other waste disposal practices fall in between these two extremes.See also Fact Sheet 271: "Reducing methane emissions from livestock farming".For further reading:L.M. Safley, et. al, 1992. Global Methane Emissions from Livestock and Poultry. United States Environmental Protection Agency (US/EPA) Report # EPA/400/ 1-91/048, February, 1992. US/EPA, Washington, D.C..

10 Methane emissions from rice cultivation

Rice fields produce about 60 million tonnes of methane per year. This represents about 17% of total methane (CH4) emissions resulting from human activities. Virtually all of this methane comes from "wetland" rice farming. Rice can be produced either by wetland, paddy rice farming or by upland, dry rice farming. Wetland rice is grown in fields that are flooded for much of the growing season with natural flood- or tide-waters or through irrigation. Upland rice, which accounts for just 10 percent of global rice production, is not flooded, and it is not a significant source of methane.Methane is produced when organic matter in the flooded rice paddy is decomposed by bacteria and other micro-organisms. When soil is covered by water, it becomes anaerobic, or lacking in oxygen. Under these conditions, methane-producing bacteria and other organisms decompose organic matter in or on the soil, including rice straw, the cells of dead algae and other plants that grow in the paddy, and perhaps organic fertilizers such as manure. The result of this reaction is methane, carbon dioxide (CO2 -but not in quantities significant for climate change), and other products:microbial Plant Organic Matter + H2O -----------> CO2 + CH4 + (other products). metabolismMethane is transported from the paddy soil to the atmosphere in three different ways. The primary method is through the rice plant itself, with the stem and leaves of the plant acting rather like pipelines from the soil to the air. This mode of transport probably accounts for 90-95 percent of emissions from a typical field. Methane also bubbles up directly from the soil through the water or is released into the air after first becoming dissolved in the water. Calculating how much methane is released from a particular field or region is difficult. Important variables include the number of acres under cultivation, the number of days that the paddy is submerged under water each year, and the rate of methane emission per acre per day. The uncertainty is caused by this last variable, which is complex and poorly understood. The methane emission rate is determined by soil temperature, the type of rice grown, the soil type, the amount and type of fertilizer applied, the average depth of water in the paddy, and other site-specific variables. Measurements at a fairly limited number of paddy sites have yielded a wide range of methane production rates. As a result, estimates of global methane production from rice paddies are considered uncertain. One recent estimate gives a range of 20 - 150 million tonnes of methane per year.1Asia produces most of the world’s rice. Since rice is the staple food throughout much of Asia, nearly 90 percent of the world’s paddy area is found there. China and India together have nearly half of the world’s rice fields and probably contribute a similar fraction of the global methane emissions from rice production.The options for reducing methane emissions from rice cultivation are limited. Reducing the area of rice under cultivation is unlikely to happen given the already tenuous food supply in many rice-dependent countries. Other options include replacing paddy rice with upland rice, developing strains of rice plant that need less time in flooded fields, and using different techniques for applying fertilizers. Each of these options will require much more research to become widely practical.For further reading:Intergovernmental Panel on Climate Change (IPCC), 1990. Greenhouse Gas Emissions from Agricultural Systems. Proceedings of a workshop on greenhouse gas emissions from agricultural systems held in Washington D.C., December 12 - 14, 1989. Published as US/EPA report 20P-2005, September, 1990 (2volumes).The Organization for Economic Co-operation and Development (OECD), 1991. Estimation of Greenhouse Gas Emissions and Sinks. Final Report from the Expert’s Meeting, 18-21 February, 1991. Prepared for the Intergovernmental Panel on Climate Change. OECD, Paris, 1991.United States Environmental Protection Agency (US/EPA), 1990. Policy Options For Stabilizing Global Climate, Technical Appendices, edited by D.A. Lashof and D. Tirpak. Report # 21P-2003.3, December, 1991. US/EPA Office of Planning and Evaluation, Washington D.C.Notes:1 IPCC, 1992 Supplement.

12 The impact of climate change on agriculture

Climate change would strongly affect agriculture, but scientists still don’t know exactly how. Most agricultural impacts studies are based on the results of general circulation models (GCMs). These climate models indicate that rising levels of greenhouse gases are likely to increase the global average surface temperature by 1.5-4.5 C over the next 100 years, raise sea-levels (thus inundating farmland and making coastal groundwater saltier), amplify extreme weather events such as storms and hot spells, shift climate zones poleward, and reduce soil moisture. Impacts studies consider how these general trends would affect agricultural production in specific regions. To date, most studies have assumed that agricultural technology and management will not improve and adapt. New studies are becoming increasingly sophisticated, however, and "adjustments experiments" now incorporate assumptions about the human response to climate change.Increased concentrations of CO2 may boost crop productivity. In principle, higher levels of CO2 should stimulate photosynthesis in certain plants; a doubling of CO2 may increase photosynthesis rates by as much as 30-100%. Laboratory experiments confirm that when plants absorb more carbon they grow bigger and more quickly. This is particularly true for C3 plants (so called because the product of their first biochemical reactions during photosynthesis has three carbon atoms). Increased carbon dioxide tends to suppress photo-respiration in these plants, making them more water-efficient. C3 plants include such major mid-latitude food staples as wheat, rice, and soya bean. The response of C4 plants, on the other hand, would not be as dramatic (although at current CO2 levels these plants photosynthesize more efficiently than do C3 plants). C4 plants include such low-latitude crops as maize, sorghum, sugar-cane, and millet, plus many pasture and forage grasses.Climate and agricultural zones would tend to shift towards the poles. Because average temperatures are expected to increase more near the poles than near the equator, the shift in climate zones will be more pronounced in the higher latitudes. In the mid-latitude regions (45 to 60 latitude), the shift is expected to be about 200-300 kilometres for every degree Celsius of warming. Since today’s latitudinal climate belts are each optimal for particular crops, such shifts could have a powerful impact on agricultural and livestock production. Crops for which temperature is the limiting factor may experience longer growing seasons. For example, in the Canadian prairies the growing season might lengthen by 10 days for every 1 C increase in average annual temperature. While some species would benefit from higher temperatures, others might not. A warmer climate might, for example, interfere with germination or with other key stages in their life cycle. It might also reduce soil moisture; evaporation rates increase in mid-latitudes by about 5% for each 1 C rise in average annual temperature. Another potentially limiting factor is that soil types in a new climate zone may be unable to support intensive agriculture as practised today in the main producer countries. For example,even if sub-Arctic Canada experiences climatic conditions similar to those now existing in the country’s southern grain-producing regions, its poor soil may be unable to sustain crop growth.  Mid-latitude yields may be reduced by 10-30% due to increased summer dryness. Climate models suggest that today’s leading grain-producing areas - in particular the Great Plains of the US - may experience more frequent droughts and heat waves by the year 2030. Extended periods of extreme weather conditions would destroy certain crops, negating completely the potential for greater productivity through "CO2 fertilization". During the extended drought of 1988 in the US corn belt region, for example, corn yields dropped by 40% and, for the first time since 1930, US grain consumption exceeded production.The poleward edges of the mid-latitude agricultural zones - northern Canada, Scandinavia, Russia, and Japan in the northern hemisphere, and southern Chile and Argentina in the southern one - may benefit from the combined effects of higher temperatures and CO2 fertilization. But the problems of rugged terrain and poor soil suggest that this would not be enough to compensate for reduced yields in the more productive areas.The impact on yields of low-latitude crops is more difficult to predict. While scientists are relatively confident that climate change will lead to higher temperatures, they are less sure of how it will affect precipitation - the key constraint on low-latitude and tropical agriculture. Climate models do suggest, however, that the intertropical convergence zones may migrate poleward, bringing the monsoon rains with them. The greatest risks for low-latitude countries, then, are that reduced rainfall and soil moisture will damage crops in semi-arid regions, and that additional heat stress will damage crops and especially livestock in humid tropical regions.  The impact on net global agricultural productivity is also difficult to assess. Higher yields in some areas may compensate for decreases in others - but again they may not, particularly if today’s major food exporters suffer serious losses. In addition, it is difficult to forecast to what extent farmers and governments will be able to adopt new techniques and management approaches to compensate for the negative impacts of climate change. It is also hard to predict how relationships between crops and pests will evolve.For further reading:Martin Parry, "Climate Change and World Agriculture", Earthscan Publications, 1990.Intergovernmental Panel on Climate Change, "The IPCC Scientific Assessment" and "The IPCC Impacts Assessment", WMO/IPCC, 1990.