Greenhouse gases (GHGs) are trace atmospheric gases that trap heat in the lower atmosphere, causing global warming. Such warming has been associated with droughts, tornadoes, ice melting, sea-level rise, saltwater intrusion, evaporation, and other climatic changes and effects.

Greenhouse gases have both natural and anthropogenic sources. They allow sunlight to pass through them and reach Earth's surface, but they trap the infrared radiation released by Earth's surface, preventing it from escaping into space. These trace atmospheric gases thus play an important role in the regulation of Earth's energy balance, raising the temperature of the lower atmosphere. GHG concentrations in the atmosphere have historically varied as a result of natural processes, such as volcanic activity. They have always been a small fraction of the overall atmosphere, however, exhibiting significant effects on the climate despite their low concentrations. Thus, small variations in GHG concentration may have disproportionate effects on Earth's climate. Since the Industrial Revolution, humans have added a significant amount of GHGs to the atmosphere by burning fossil fuels and cutting down trees. Scientists estimate that the Earth's average temperature has already increased by 0.3œ to 0.6œ Celsius since the beginning of the twentieth century.

The atmosphere comprises constant components and variable components. It is composed primarily of nitrogen (78 percent) and oxygen (21 percent). Its other constant components include argon, neon, krypton, and helium. Its variable components include carbon dioxide (CO2), water vapor (H2O), methane (CH4), sulfur dioxide (SO2), ozone (O3), and nitrous oxide (N2O). The variable components affect the weather and climate because they absorb heat emitted by Earth and thereby warm the atmosphere. In addition to the variable natural atmospheric GHGs, anthropogenic halocarbons, other chlorine- and bromine-containing substances, sulfur hexafluoride, hydrofluorocarbons, and perfluorocarbons contribute to the greenhouse effect.

CO2, composed of two oxygen atoms and one carbon atom, is a colorless, odorless gas deriving from carbon burning in the presence of sufficient oxygen. It is released to the atmosphere by forest fires, fossil fuel combustion, volcanic eruptions, plant and animal decomposition, oceanic evaporation, and respiration. It is removed from the atmosphere by CO2 sinks, seawater absorption, and photosynthesis.

Methane is a colorless, odorless, nontoxic gas consisting of four hydrogen atoms and one carbon atom. It is a constituent of natural gas and fossil fuel. It is released into the atmosphere when organic matter decomposes in oxygen-deficient environments. Natural sources include wetlands, swamps, marshes, termites, and oceans. Other sources are the mining and burning of fossil fuels, digestive processes in ruminant animals, and landfills. Methane reacts with hydroxyl radicals in the atmosphere, which break it down in the presence of sunlight, shortening its lifetime.

Nitrous oxide is a colorless, nonflammable gas with a sweetish odor. It is naturally produced by oceans and rain forests. Anthropogenic sources include nylon and nitric acid production, fertilizers, cars with catalytic converters, and the burning of organic matter. Nitrous oxide gas is consumed by microbial respiration in specific anoxic environments. Sulfur dioxide is released during volcanic activities, combustion of fossil fuel, transportation, and industrial metal processing. This gas is more reactive than is CO2, and it rapidly oxidizes to sulfate. It produces acidic gases and acid rain when it reacts with water and oxygen.

Ozone (triatomic oxygen) is a highly reactive, gaseous constituent of the atmosphere. A powerfully oxidizing, poisonous, blue gas with an unpleasant smell, it helps create smog. It is produced in chemical reactions of volatile organic compounds or nitrogen oxide with other atmospheric gases in the presence of sunlight. Oxygen and ozone absorb a critical range of the ultraviolet spectrum, preventing this dangerous radiation from reaching Earth's surface and making possible life on Earth.

Halocarbons have global warming potentials (GWPs) from three thousand to thirteen thousand times that of CO2; they remain in the atmosphere for hundreds of years. These compounds were commonly used in refrigeration, air conditioning, and electrical systems, but their use has been regulated as a result of their environmental and climatic effects.

The Working Groups of the Intergovernmental Panel on Climate Change (IPCC) presented a synthesis report in 2007, providing an integrated view of climate change from multiple perspectives. The report observed an increase of global air and ocean temperatures, melting of snows, and rising sea levels. The report estimated the one-hundred-year linear trend of Earth's average temperature between 1906 and 2005 at an increase of 0.74œ Celsius, significantly greater than the trend from 1901 to 2000 (0.6œ Celsius). The increase of temperature contributed to changes in wind patterns, affecting extratropical storm tracks and temperature patterns.

Global average sea level has risen between 1961 and 2001 at an average rate of 1.8 millimeters per year and between 1993 and 2008 at an average rate of 3.1 millimeters per year. The increase is due largely to melting glaciers and polar ice sheets. Satellite data between 1978 and 2008 show that average annual extent of Arctic sea ice shrank by an average of 2.7 percent per decade. The average summertime extent shrank far more, an average of 7.4 percent per decade.

Increases have been reported in the number and size of glacial lakes and the rate of change in some Arctic and Antarctic ecosystems. Runoff and earlier spring peak discharge in many glacier- and snow-fed rivers have also increased. These increases have in turn had effects on the thermal structure and water quality of the rivers and lakes fed by this runoff. Both marine and freshwater systems have been associated with rising water temperatures and with changes in ice cover, salinity, oxygen levels, and circulation patterns. These ecological changes have affected algal, plankton, and fish abundance.

Precipitation has increased in the eastern parts of North and South America, northern Europe, and northern and central Asia. It has decreased in the Mediterranean and southern Africa. These patterns also have affected algal, plankton, and fish abundance. Globally, since 1970, a greater area of Earth's surface has been affected by drought. Changes in atmospheric GHG and aerosol concentration, as well as solar radiation levels, affect the energy balance of Earth's climate system. Global GHG emissions increased by 70 percent over preindustrial levels between 1970 and 2004. CO2 emissions increased by 80 percent, but they began to decline after 2000. The global increase in CO2 and methane emissions is due to fossil fuel and land use, particularly agriculture.

Coastlines are particularly vulnerable to the consequences of climate change, such as sea-level rise and extreme weather. Around 120 million people on Earth are exposed to tropical cyclone hazards. During the twentieth century, global sea-level rises contributed to increased coastal inundation, erosion, ecosystem losses, loss of sea ice, thawing of permafrost, coastal retreat, and more frequent coral bleaching.

Anticipated future climate-related changes include a rise in sea level of up to 0.6 meter by 2100, a rise in sea surface temperatures by up to 3œ Celsius, an intensification of tropical cyclones, larger waves and storms, changes in precipitation and runoff patterns, and ocean acidification. These phenomena will vary on regional and local scales. Increased flooding and the degradation of freshwater, fisheries, and other resources could impact hundreds of millions of people, with significant socioeconomic costs. Degradation of coastal ecosystems, especially wetlands and coral reefs, affects the well-being of societies dependent on coastal ecosystems for goods and services.

In response to global warming, changes are being implemented to reduce GHG emissions. The United Nations Framework Convention on Climate Change prepared the 1997 Kyoto Protocol. Under the protocol, thirty-six states, including highly industrialized countries and countries undergoing transitions to a market economy, entered into legally binding agreements to limit and reduce GHG emissions. Developing countries assumed nonbinding obligations to limit their emissions as well.

In the energy sector, fuel use is slowly transitioning from coal to natural gas and renewable energy (hydropower, solar, wind, geothermal, tidal, wave, and bioenergy). In the transport sector, fuelefficient, hybrid, and fully electric vehicles are being designed and marketed, and governments are attempting to motivate commuters to use masstransit systems. More efficient uses of energy, including low-energy lightbulbs, day lighting, and efficient electrical, heating, and cooling appliances are being developed and deployed.

Industrial manufacturers have implemented electrical efficiency measures as well, and they have begun recycling, as well as capturing and storing CO2. Crop and land management techniques have also improved, leading to an increase in soil carbon storage and the restoration of peaty soils and degraded land. Rice cultivation techniques have been improved, and livestock management techniques are being developed to reduce methane and nitrogen emissions. More controversially, dedicated energy crops are being grown to replace fossil fuels.

Afforestation, reforestation, forest management, reduced deforestation, and harvested wood product management are also being geared toward reducing GHG emissions. Forestry products are in use for bioenenergy to replace fossil fuels. Improvements are being made in tree species, remote sensing for analyses of vegetation and soil carbon, and mapping of land use. In the waste industry, methane is being recovered from landfills and energy is being recovered from waste incineration. Organic waste is more widely used for composting, wastewater is minimized, and the wastewater produced is treated and recycled. Biocovers and biofilters are being developed to optimize methane oxidation.

Bibliography:

1) Dlugokencky, E. J., et al. "Continuing Decline in the Growth Rate of the Atmospheric Methane Burden." Nature 393 (1998): 447-450.

2) Gore, Al. An Inconvenient Truth: The Planetary Emergency of Global Warming and What We Can Do About It. Emmaus, Pa.: Rodale, 2006.

3) Le Treut, H., et al. "Historical Overview of Climate Change." In Climate Change, 2007--The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, edited by Susan Solomon et al. New York: Cambridge University Press, 2007.

4) Walker, Gabrielle. An Ocean of Air: Why the Wind Blows and Other Mysteries of the Atmosphere. Orlando, Fla.: Harcourt, 2007.

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