• What is the composition of the earth's atmosphere?
• What processes have influenced the atmosphere's composition?
• How does the earth's atmosphere interact with the ocean?
• What are greenhouse gases?
• What will be the impacts of global climate change associated with greenhouse gas increases?

Introduction
Earth’s atmosphere is a relatively thin shell. About 95% of it is contained within 14 km (8.6 mi) of the earth’s surface. All life on earth depends on the atmosphere, but despite our knowledge, humans are altering the atmosphere’s composition. Scientific evidence suggests that emissions from the burning of fossil fuels, from industrial sources such as cement manufacture, and from deforestation are changing the make-up of our atmosphere. In addition, trace gases such as methane and chlorofluorocarbons (CFCs and HCFCs) are having an impact on the atmosphere wholly out of proportion to their concentration.

The Development of the Current Atmosphere
The earth's third and current atmosphere was produced partly by the metabolism of living organisms. Photosynthetic organisms, the cyanobacteria, appeared about 3.8 billion years ago and began to produce oxygen. Over the next 2 billion years, atmospheric O₂ concentrations rose and CO₂ concentrations fell as a direct result of photosynthesis. This increase in atmospheric oxygen then set the stage for two major evolutionary events on the planet: the evolution of aerobic (oxygen-using) life forms and the establishment of the ozone layer.

Increases in atmospheric and seawater levels of oxygen poisoned sensitive microorganisms and led to the evolution of other microorganisms capable of using oxygen to liberate energy from organic compounds. This new aerobic metabolism was much more efficient than the anaerobic metabolism it replaced. Today, natural anaerobic microbial communities are restricted in their habitats to deep marginal or isolated seas, organic-rich sediments, and regions beneath the earth’s surface. The development of aerobic metabolism also permitted the evolution of multicellular organisms, which required more energy to support their increased biomass. All existing multicellular life forms employ aerobic metabolism.

Increases in atmospheric oxygen concentration eventually triggered the formation of the layer of ozone (O₃) that now exists in the upper atmosphere. Although considered a pollutant at ground-level, the ozone layer serves as an important filter of harmful ultraviolet radiation, which is responsible for sunburn and can cause skin cancer in humans.

Prior to the existence of the ozone layer, the earth’s terrestrial and ocean surface were exposed to extremely high intensities of ultraviolet radiation. The surface ocean waters filtered out some of this radiation, and thus provided some protection to organisms, but it is likely that ocean primary production (that is, production of high-energy compounds from photosynthesis) was still limited by the high ultraviolet light intensities. The terrestrial surface fared worse and was effectively sterilized by this radiation.

The Atmosphere’s Current Composition
The present-day atmosphere is composed primarily of N₂ gas (78.08% by volume), and oxygen (20.94% by volume). Also present, in quantities less than 1% by volume, are, in order: Ar, H₂O, CO₂, Ne, He, CH₄, NO₂, CO, NH₃, and O₃. The major controls upon the composition of the atmosphere and the cycling of these compounds are interactions with the Earth’s biosphere (living matter) and lithosphere (rock and geological processes such as volcanism). Presently, O₂ levels are stable, but CO₂ levels are not and display seasonal and longer-term trends. Seasonal changes in CO₂ concentration are related to primary production changes due to changing light durations. Longer-term (decade–century) increases in CO₂ are due to a variety of anthropogenic (human-caused) inputs as well as changes in land use that reduce the ability of terrestrial biota to absorb CO₂.

Because the amount of CO₂ in the atmosphere is very small, the concentration is easily changed by the addition of CO₂ from various sources.

Atmospheric Functions
In addition to providing the oxygen needed by most of earth’s life forms, the atmosphere provides a significant thermal insulation, preventing extreme changes in temperature over the daily light dark cycle. Unequal heating of the earth’s atmosphere and terrestrial surface create long-term climate and short-term weather patterns. The winds that result from these heating differences and resultant pressure differences also drive ocean currents. The atmosphere also transfers heat.

Changes Caused by Humans
After decades of research, scientists have finally concluded that humans have changed the composition of the earth's atmosphere. Since the beginning of the industrial revolution, atmospheric concentrations of the following greenhouse gases have changed: carbon dioxide has increased 30%; methane concentrations have more than doubled; and nitrous oxide concentrations have risen by about 15% (Figure 1). Greenhouse gases allow short wavelength radiation from the sun to pass through the atmosphere, but they absorb the longer wavelength radiation (i.e., heat) that is emitted by the earth. The need for energy to support industrial development, heat homes, cook food, watch television, and surf the Internet, as well as the increased use of automobiles, has resulted in the burning of great stores of fossil fuel.

Fossil fuels, including coal, oil, and natural gas, were formed by the preservation and slow anaerobic decomposition of ancient plant and phytoplankton deposits. These deposits took tens of millions of years to form but we are now utilizing them at a rapid rate. A key by-product of fossil fuel consumption is CO₂. Since the industrial revolution, we have added CO₂ to the atmosphere more rapidly than it can be absorbed by its variety of sinks. This has led to a slow but steady increase in CO₂ concentration that will result in at least a doubling of pre-1860 atmospheric CO₂ content by the year 2150, if present trends continue. The increase in atmospheric CO₂ has occurred not only because of increased inputs into the atmosphere, but also due to changes in the landscape, such as deforestation, that result in less removal of CO₂ from the atmosphere.

Methane is another greenhouse gas whose concentration has also become elevated because of human activities. It is emitted by cows and flooded farmlands (i.e., rice paddies), and both of these have increased dramatically within the last 2 centuries.

Global Warming Impacts
There is little controversy that human activities have caused these changes in the atmosphere, but there is intense debate regarding the magnitude of the problem that may result from these changes. However, knowledge that the 10 warmest years in this century all have occurred in the last 15 years has convinced most scientists that we are now seeing the direct response of the planet to the increase in greenhouse gases.

What will the impacts be of global climate change associated with greenhouse gas increases? Examine Figure 2 for an overview.

What will the impacts be of global climate change associated with greenhouse gas increases?

Human health will be directly affected by increases in the heat index, which can impact healthy people, but especially affects the elderly and those with heart and respiratory illnesses. Higher temperatures will also increase ozone pollution in the lower atmosphere, which also is a threat to people with respiratory illnesses. Global warming may also increase the incidence of some infectious diseases, particularly those that usually appear only in warm areas. Diseases that are spread by mosquitoes and other insects, including malaria, dengue fever, yellow fever, and encephalitis, could become more prevalent if warmer temperatures and wetter climates enable those insects to become established farther north.

Another important impact of global climate change will be alterations in precipitation patterns. Predicted changes in climate are expected to enhance both evaporation and precipitation in most areas of the United States. The net balance of these processes influences the availability and quality of water resources. In areas expected to become more arid, like California, lower river flows and lower lake levels could impair navigation, reduce hydroelectric power generation, decrease water quality, and reduce the supplies of water available for agricultural, residential, and industrial uses. In other areas, increased precipitation, which is expected to be more concentrated in large storms as temperatures rise, will increase the incidence of flooding.

Shifts northward in climatic regimes would also affect the types of crops that can be raised. Studies conducted in the 1980s generally concluded that climate change would have severe impacts on agriculture. More recent assessments have suggested that these might be partially offset, at least in the United States, by longer growing season and enhanced production from higher CO₂.

Climatic change will cause a shift in natural community composition as plant and animal species migrate to maintain their preferred habitats. For example, the projected 2°C (3.6°F) warming that will occur this next century could shift the ideal range for many North American forest species by about 300 km (200 mi.) to the north. Many plant species lack seed dispersion mechanisms that can adjust this rapidly, and thus these species may be susceptible to regional extinction. Coastal wetland plant communities may lose habitat because they may not be able to keep up with the predicted enhanced rate of sea level rise, or the transgression paths of these communities may be blocked by human development.

Among wildlife, certain birds and fishes are expected to be greatly impacted by predicted climate change. A large number of duck species are dependant upon "Prairie Potholes" found in the Northern Great Plains. A drier climate would decrease the amount of open water ponds in this region, with a commensurate reduction in duck populations. Among fish, those which inhabit inland aquatic environments are expected to be more vulnerable than coastal or marine species. Lake-locked fishes have little recourse in seeking cooler waters. Fish that inhabit rivers may be able to migrate northward to seek cooler water, but fish in east – west oriented rivers and lakes will not be able to escape warming impacts. The results of a 1995 EPA study, "Ecological Impacts From Climate Change: An Economic Analysis of Freshwater Recreational Fishing", suggest that the overall diversity of fishes in U.S. rivers and streams is likely to decline, because of the loss of cold water forms.

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