There are many causes of environmental changes on Earth. Natural events cause changes in climate. For example, large volcanic eruptions release tiny particles into the atmosphere that block sunlight, resulting in surface cooling that lasts for a few years. Variations in ocean currents such as El Niño can also change the distribution of heat and precipitation. Over longer time spans, tens to hundreds of thousands of years, natural changes in the geographical distribution of energy from the Sun and in the amounts of greenhouse gases and dust in the atmosphere have caused the climate to shift from ice ages to relatively warmer periods. On a longer timescale the presence of life on Earth has changed the environment of the planet radically, transforming a predominantly reducing atmosphere made up of methane and ammonia to today's oxygen-rich gaseous envelope.
Human activities can also change the environment. Orbiting satellites have photographed the transformation of deserts into productive agricultural areas. Conversely, satellites have tracked the advance of deserts (desertification) and the loss of forests (deforestation) as a result of human activity. One root cause of desertification and deforestation is the use of wood as the basic source of energy, with the consequent loss of trees and degradation of the soil. The most obvious impact of desertification is the degradation of rangeland and irrigated cropland and the decline in soil fertility and soil structure. Desertification affects about one-sixth of the world's population and affects 70 percent of all dry lands, amounting to 3.6 billion hectares (8.9 billion acres), or one-quarter of the total land area of the world.
The Greenhouse Phenomenon
In addition to desertification, changes caused by human activities include recent increases in the atmospheric concentrations of both greenhouse gases and sulfate particles ("aerosols"). Greenhouse gases such as carbon dioxide cover the atmosphere's "infrared window," and trap heat. Data from satellites can trace changes in the globally averaged surface temperature of Earth and can be used to predict temperature changes in the future. According to some models, if current trends continue, the amount of carbon dioxide in the atmosphere will double during the twenty-first century, and the average rate of warming of Earth's surface over the next hundred years will probably be greater than it was at any time in the last 10,000 years. The current best estimate of the expected rise of globally averaged surface temperature relative to 1990 is 1°C to 3.5°C by the year 2100, with continued increases thereafter.
Because seawater expands when heated and some glacial ice will melt, the global sea level is expected to rise a further 15 to 95 centimeters (6 to 37.5 inches) by 2100 as a result of global warming. Since 1978 satellite technology has been used to monitor the vast Arctic Sea ice cover on a routine basis. More recently, the Topex/Poseidon satellite has been instrumental in observing the global climate interaction between the sea and the atmosphere. In 2001 a joint U.S.-French oceanography mission, Jason 1, was scheduled to be launched to monitor world ocean circulation, study interactions between the oceans and the atmosphere, improve climate predictions, and observe events such as El Niño.
Around 1985 scientists taking ozone (O3) measurements in the Antarctic detected an alarming decrease in stratospheric ozone concentrations over the South Pole. This decline in atmospheric ozone was verified by instruments aboard the National Aeronautics and Space Administration's (NASA)'s Nimbus-7 satellite. Under usual circumstances ultraviolet radiation helps create and destroy ozone molecules. It is strong enough to break both ozone and oxygen molecules into individual oxygen atoms. This destruction of molecules allows the free oxygen atoms to bond with other oxygen molecules and form more ozone. However, chlorofluorocarbon (CFC) compounds such as the freon used in refrigeration systems upset this balance and destroy ozone (CFCs also are greenhouse gases). The depletion of ozone caused by CFCs results in increased ultraviolet radiation at Earth's surface that could be highly damaging to sensitive Arctic life forms. Ozone losses over the Arctic could also reduce ozone levels over the middle latitudes as a result of the mixing of air masses.
Although some forms of ozone-destroying CFCs have been banned, Arctic ozone depletion might be increased over the next few decades by further accumulations of greenhouse gases in the atmosphere. By trapping more heat near Earth's surface, these gases cause the stratosphere to become cooler and produce more stratospheric clouds, which have been implicated in rapid ozone loss.
Colonization and Terraforming of Planets
Although human-induced changes to Earth's environment are increasingly apparent, humans have also altered the environment of the Moon and the neighboring planets in very small ways. The footprints left by Apollo astronauts and atmospheric gases released by their landing craft produced infinitesimal alterations in the Moon's environment. Similarly, tire tracks and shallow trenches left on the surface of Mars by landers, such as Pathfinder and Viking, have changed the environment of that planet on a minute scale. However, greater environmental changes are almost inevitable as humans venture into the solar system.
Colonization of other worlds will affect those environments, but humans may also undertake the premeditated terraforming of planets to deliberately make them more Earth-like. Making Mars habitable will in many ways restore that planet's climate of billions of years ago, creating a thick atmosphere and a warm surface with bodies of liquid water. Ironically, greenhouse gases such as carbon dioxide and CFCs, which have undesirable effects on Earth, could be instrumental in terraforming Mars. Some researchers have proposed melting the southern polar ice cap on Mars to release large quantities of carbon dioxide into the atmosphere to heat up the planet. Others have suggested the use of super greenhouse gases for that purpose. Warming the atmosphere by using specially designed CFCs would be desirable and would not cause adverse affects on ozone formation.
Over time, Earth's environment has been changed for the better (e.g., transforming deserts to agricultural areas) and the worse (e.g., the ozone hole, greenhouse warming, desertification, etc.). In the future, the challenge will be to remain aware of the accompanying changes to the environment and responsibly guide and monitor those changes on the home planet and beyond.
see also Asteroid Mining (volume 4); Living on Other Worlds (volume 4); Natural Resources (volume 4); Planetary Protection (volume 4); Resource Utilization (volume 4); Settlements (volume 4); Terraforming (volume 4).
John F. Kross
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McKay, Christopher P. "Bringing Life to Mars."Scientific American Presents 10 (1999):52-57.
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Shelton, William R. Man's Conquest of Space. Washington, DC: National Geographic Society, 1975.
"The Physics and Biology of Making Mars Habitable." Massachusetts Institute of Technology. <http://web.mit.edu/mmm/www/summary.html>.
U.S. Global Change Research Information Office. <http://www.gcrio.org>.