DR. S. M. ALAM
Mar 26 - Apr 1, 2012
Many chemical compounds found in the Earth's atmosphere act as greenhouse gases. These gases allow sunlight to enter the atmosphere freely. When sunlight strikes the Earth's surface, some of it is reflected back towards space as infrared radiation (heat).
Greenhouse gases absorb this infrared radiation and trap the heat in the atmosphere. Over time, the amount of energy sent from the sun to the Earth's surface should be about the same as the amount of energy radiated back into space, leaving the temperature of the Earth's surface roughly constant. Many gases exhibit these greenhouse properties. Some of them occur in nature (water vapor, carbon dioxide, methane, and nitrous oxide), while others are exclusively human-made (like gases used for aerosols).
A greenhouse gas is one of several gases that can absorb and emit long wave (infrared) radiation in a planetary atmosphere. This phenomenon is often termed the greenhouse effect. Of the sunlight that falls on the Earth's surface, approximately 40 per cent of that energy is reradiated upward into the atmosphere in the form of long wave radiation. Approximately 75 per cent of that upward radiated long wave energy is absorbed by water vapor, carbon dioxide, methane and other greenhouse gases. Since this absorption process is molecular in nature, the subsequent radiation of energy by these gases is multidirectional. As a result, about 50 per cent of the long wave emission is reradiated back toward the Earth where it is once again turned into heat energy. Through this process, greenhouse gases contribute to the amount of heat energy released at the Earth's surface and in the lower atmosphere.
Since the beginning of industrial revolution, concentrations of carbon dioxide, methane, and nitrous oxide have all risen dramatically because of human activities.
Fossil fuel combustion, land-use change, increasingly intensive agriculture, and an expanding global human population are the primary causes for these increases. Other greenhouse gases found in our planet's atmosphere include water vapor, ozone, sulfur hexafluoride, and chlorofluorocarbons.
Levels of several important greenhouse gases have increased by about 25 per cent since large-scale industrialization began around 150 years ago. During the past 20 years, about three-quarters of human-made carbon dioxide emissions were from burning fossil fuels. Concentrations of carbon dioxide in the atmosphere are naturally regulated by numerous processes collectively known as the carbon cycle.
The movement of carbon between the atmosphere and the land and oceans is dominated by natural processes, such as plant photosynthesis. While these natural processes can absorb some of the net 6.1 billion metric tons of anthropogenic carbon dioxide emissions produced each year (measured in carbon equivalent terms), an estimated 3.2 billion metric tons is added to the atmosphere annually. The Earth's positive imbalance between emissions and absorption result in the continuing growth in greenhouse gases in the atmosphere.
Given the natural variability of the Earth's climate, it is difficult to determine the extent of change that humans cause. In computer-based models, rising concentrations of greenhouse gases generally produce an increase in the average temperature of the Earth. Rising temperatures may, in turn, produce changes in weather, sea levels, and land use patterns, commonly referred to as climate change. Assessments generally suggest that the Earth's climate has warmed over the past century and that human activity affecting the atmosphere is likely an important driving factor. "Greenhouse gases are accumulating in Earth's atmosphere as a result of human activities, causing surface air temperatures and subsurface ocean temperatures to rise. Temperatures are, in fact, rising. The changes observed over the last several decades are likely mostly due to human activities, but we cannot rule out that some significant part of these changes is also a reflection of natural variability."
SOURCES OF GREENHOUSE GASES: Energy-related carbon dioxide emissions, resulting from petroleum and natural gas, represent 82 per cent of total U.S. human-made greenhouse gas emissions. The connection between energy use and carbon dioxide emissions is explored in the box on the reverse side. Another greenhouse gas, methane, comes from landfills, coalmines, oil and gas operations, and agriculture; it represents nine per cent of total emissions. Nitrous oxide (five per cent of total emissions), meanwhile, is emitted from burning fossil fuels and through the use of certain fertilizers and industrial processes. Human-made gases (two percent of total emissions) are released as byproducts of industrial processes and through leakage. World carbon dioxide emissions are expected to increase by 1.9 per cent annually between 2001 and 2025.
Much of the increase in these emissions is expected to occur in the developing world where emerging economies, such as China and India, fuel economic development with fossil energy. Developing countries' emissions are expected to grow above the world average at 2.7 annually between 2001 and 2025.
The U.S. produces about 25 per cent of global carbon dioxide emissions from burning fossil fuels; primarily because our economy is the largest in the world and we meet 85 per cent of our energy needs through burning fossil fuels. The U.S. is projected to lower its carbon intensity by 25 per cent from 2001 to 2025, and remain below the world average.
Energy from the sun in the form of some ultraviolet and visible light (short wavelength) passes through the glass of the greenhouse.
As the light strikes various surfaces in the greenhouse and they are heated, these surfaces in turn re-radiate the heat in the form of infrared radiation (long wavelength).
Human activities also add significantly to the level of naturally occurring greenhouse gases.
Carbon dioxide is released into the atmosphere by the burning of solid waste, wood, and wood products, and fossil fuels (oil, natural gas, and coal).
Nitrous oxide emissions occur during various agricultural and industrial processes, and when solid waste or fossil fuels are burned.
Methane is emitted when organic waste decomposes, whether in landfills or in connection with livestock farming.
HFCs and PFCs are the most heat-absorbent, but there are also wide differences between naturally occurring gases. For example, nitrous oxide absorbs 270 times more heat per molecule than carbon dioxide, and methane absorbs 21 times more heat per molecule than carbon dioxide.
CARBON DIOXIDE: Before 1700, levels of carbon dioxide were about 280 ppm. Concentrations of carbon dioxide in the atmosphere are now about 390 ppm. This increase in carbon dioxide in the atmosphere is mainly due to activities associated with the industrial revolution. Emissions from the combustion of fossil fuels account for about 65 per cent of the carbon dioxide added to the atmosphere. The remaining 35 per cent is derived from deforestation and the conversion of prairie, woodland, and forested ecosystems primarily into less productive agricultural systems.
Natural ecosystems can store 20 to 100 times more carbon dioxide per unit area than agricultural systems. Both deforestation and natural land-use change reduce the amount of standing plant mass or biomass found on the Earth's surface. This reduction causes a net export of carbon stored in biomass into the atmosphere through decomposition and burning.
Coal combustion for electricity generation represents the primary source, with oil and gas combustion also being significant contributors.
DEFORESTATION EMISSIONS: Deforestation includes tree removal, charcoal production, slash-and-burn practices, and forest degradation. In total, these activities contribute approximately 25 per cent of the carbon added to the atmosphere, largely as carbon dioxide. Deforestation and land-use change can have a double impact on greenhouse gas fluxes: carbon is released when forests are burned or logged, and the land-based 'sink' of carbon dioxide (the long term uptake and storage of carbon by plants and soils) is reduced when forest is replaced by cropland or rangeland. The majority of carbon dioxide emissions due to deforestation arise from activities in Africa, Asia, and South America according to UN FAO sources.
CARBON DIOXIDE SINKS: The oceans emit approximately 88 billion tons of carbon each year and absorb about 90 billion tons of carbon each year - a net sink of about two billion tons of carbon. Our planet's oceans and terrestrial ecosystems represent net sinks for carbon dioxide, together absorbing approximately three billion tons of carbon more from the atmosphere each year than they emit. Since the industrial revolution, these sinks have absorbed about 40 per cent of the carbon dioxide emissions released by human activities. Elevated temperatures, changes in rainfall patterns, and acidification of the oceans may serve to reduce the size of the land and ocean sinks - further increasing the concentration of carbon dioxide in the atmosphere.
Methane: Since 1750, atmospheric concentrations of the greenhouse gas methane have increased more than 150 per cent. The primary sources for the additional methane added to the atmosphere are rice cultivation, domestic grazing animals, deep seabed frozen methane clathrate thawing, termites, landfill out-gassing, oil and gas extraction, and coal mining.
Worldwide the countries responsible for generating significant amounts of methane include China, India, Brazil, Mexico, and Russia. These same nations are also projected to still be the greatest generators of methane in the year 2050. The USA and combined European Union countries provide lesser emissions, and the trends in those two advanced regions are flat to down.
RICE CULTIVATION: Rice cultivation presently accounts for about 20 per cent of worldwide methane emissions, including both natural and human-made sources.
Anaerobic conditions associated with rice paddy flooding results in the formation of methane gas. An accurate estimate of how much methane is being produced from rice paddies has been difficult to determine. More than 60 per cent of all rice paddies are found in India and China where scientific data concerning emission rates are hard to obtain. Nevertheless, scientists believe that the contribution of rice paddies is large because this type of crop production has more than doubled since 1950. Much of the recent expansion of rice paddy methane emissions is attributed to the need to feed an expanding human population by increasing crop yields through the cultivation of more land
RUMINANT LIVESTOCK EMISSIONS: Ruminant grazing is another important source of methane emissions. Grazing animals release methane to the environment as a result of herbaceous digestion. For example, one cow typically produces approximately 150 grams of methane per day. In the USA, the 2010 standing herd of bovines amounted to 106 million animals. These methane emissions are further magnified by the millions of sheep and goats kept worldwide for human use.
LANDFILLS: Landfills are the second biggest source of methane in the United States. Methane is created in landfills when organic waste decomposes without oxygen. The amount of methane produced depends on factors like the type of waste, moisture content of the waste, and the design and management practices at the landfill. Some landfills reduce methane emissions by burning this gas to produce electrical energy, which is sold to utility companies.
FOSSIL FUEL EXTRACTION: The process of extracting of natural gas, petroleum, and coal from the lithosphere is a significant source of methane emissions (called fugitive emissions) into the atmosphere. Methane emissions also occur during the processing, storage, and distribution of these fossil fuels. Fossil fuel use because of human activities is the fourth largest source of methane emissions in the United States.
NITROUS OXIDES: The average concentration of nitrous oxide is now increasing at a rate of 0.2 to 0.3 per cent per year. The role of nitrous oxide in the enhancement of the greenhouse effect is minor relative to the other greenhouse gases already discussed. Nitrous oxide also contributes to the artificial fertilization of ecosystems.
Sources for the increase of nitrous oxide in the atmosphere include land-use conversion, fossil fuel combustion, biomass burning, and soil fertilization. Most of the nitrous oxide added to the atmosphere each year comes from deforestation and the conversion of forest, savanna, and grassland ecosystems into agricultural fields and rangeland.
Both of these processes reduce the amount of nitrogen stored in living vegetation and soil through the decomposition of organic matter. Nitrous oxide is also released into the atmosphere when fossil fuels and biomass are burned. The use of nitrate and ammonium fertilizers to enhance plant growth is another important source of nitrous oxide. How much is released from this process has been difficult to quantify. Some estimates suggest that fertilizers may contribute as much as 50 per cent of nitrous oxide added to the atmosphere annually.
Chlorofluorocarbons are human-made chemicals that are highly persistent in the Earth's atmosphere. They are typically thousands of times more potent as greenhouse gases than carbon dioxide. While reductions in their use have been underway in Western Nations for over twenty years, the chemicals are growing in use in developing countries and other nations such as China and Brazil.
The Montreal Protocol, the international agreement that phases out ozone-depleting substances, requires the end of chlorodifluoromethane production by 2020 in developed countries and 2030 in developing countries.
CLIMATE FORCING EFFECTS: In climate science, the relative climate-forcing strength of different greenhouse gases is described relative to that of carbon dioxide. Methane is much more effective at absorbing infrared radiation (heat) and is thus a more powerful greenhouse gas. Yet its lifetime in the atmosphere is only about 10 years, compared to between 30 and 1000 years for a molecule of carbon dioxide. As such, the climate-forcing strength of a kilogram of methane on a 100 year time-horizon - its Global Warming Potential (GWP) - is 25. That is, every kilogram of methane in the atmosphere has the equivalent global warming potential of 25 kilograms of carbon dioxide. The GWP of nitrous oxide is 298.
Our impact on the global climate since the industrial revolution has been difficult to interpret. While emissions of greenhouse gases, like carbon dioxide and methane have had a net warming effect, emissions of sulphate aerosols have had a net cooling effect. The net effect is warmer global temperatures, but the complex interaction of these positive and negative influences on the Earth's climate system make predicting future effects difficult.
THE FUTURE: While many developed nations have had carbon emission reduction program underway for several years, often as part of the United Nations Framework Convention on Climate Change and its 'Kyoto Protocol', their combined emissions have continued to increase. In the coming decades, growth in carbon emissions is likely to be dominated by emissions from developing and transition economies like China and India.