Global Warming
Global warming, as used in the popular context, is a scientifically controversial phenomenon that attributes an increase in the average annual surface temperature of Earth to increased atmospheric concentrations of carbon dioxide and other gases. Global warming describes only one of several components involved in climate change and specifically refers to a warming of Earth's surface outside of the range of normal fluctuations that have occurred throughout Earth's history.
Climate describes the long-term meteorological conditions or average weather for a region. Throughout Earth's history there have been dramatic and cyclic changes in climatic weather patterns corresponding to cycles of glacial advance and retreat that occur on the scale of 100,000 years. Within these larger cycles are shorter duration warming and cooling trends that last from 20,000 to 40,000 years. Scientists estimate that approximately 10,000 years have elapsed since the end of the last ice age, and examination of physical and biological processes establishes that since the end of the last ice age there have been fluctuating periods of global warming and cooling.
Measurements made of weather and climate trends during the last decades of the twentieth century raised concern that global temperatures are rising not in response to natural cyclic fluctuations, but rather in response to increasing concentrations of atmospheric gases that are critical to the natural and life-enabling greenhouse effect (infrared re-radiation, mostly from water vapor and clouds, that warms the earth's surface).
Observations collected over the last century indicate that the average land surface temperature increased by 0.8–1.0°F (0.45–0.6°C). The effects of temperature increase, however, cannot be fully isolated and many meteorological models suggest that such increases temperatures also result in increased precipitation and rising sea levels.
Measurements and estimates of global precipitation indicate that precipitation over the world's landmasses has increased by approximately 1% during the twentieth century. Further, as predicted by many global warming models, the increases in precipitation were not uniform. High latitude regions tended to experience greater increases in precipitation while precipitation declined in tropical areas.
Measurements and estimates of sea level show increases of 6–8 in (15–20 cm) during the twentieth century. Geologists and meteorologists estimate that approximately 25% of the sea level rise resulted from the melting of mountain glaciers. The remainder of the rise can be accounted for by the expansion of ocean water in response to higher atmospheric temperatures.
Many scientists express concern that the measured increases in global temperature are not natural cyclic fluctuations, but rather reflect human alteration of the natural phenomena known as the greenhouse effect by increasing concentrations of greenhouse-related atmospheric gases. Estimates of atmospheric greenhouse gases prior to the nineteenth century (extrapolated from measurements involving ice cores) indicate that of the last few million years the concentration of greenhouse gases remained relatively unchanged prior to the European and American industrial revolutions. During the last two centuries, however, increased emissions from internal combustion engines and the use of certain chemicals have measurably increased concentrations of greenhouse gases that might result in an abnormal amount of global warming.
Although most greenhouse gases occur naturally, the evolution of an industrial civilization has significantly increased levels of these naturally occurring gases. In addition, new gases have been put into the atmosphere that potentiate (i.e., increase) the greenhouse effect. Important greenhouse gases in the modern Earth atmosphere include water vapor and carbon dioxide, methane, nitrous oxides, ozone, halogens (bromine, chlorine, and fluorine), halocarbons, and other trace gases.
The sources of the greenhouse gases are both natural and man-made. For example, ozone is a naturally occurring greenhouse gas found in the atmosphere. Ozone is constantly produced and broken down in natural atmospheric processes. In contrast, halocarbons enter the atmosphere primarily as the result of human use of products such as chlorofluorocarbons (CFCs). Water vapor and carbon dioxide are natural components of respiration, transpiration, evaporation and decay processes. Carbon dioxide is also a by-product of combustion. Although occurring at lower levels than water vapor or carbon dioxide, methane is also a potent greenhouse gas. Nitrous oxides, enhanced by the use of nitrogen fertilizers, nylon production, and the combustion of organic material, including fossil fuels have also been identified as contributing to strong greenhouse effects.
Alterations in the concentrations of greenhouse gases results in a disruption of equilibrium processes. Both increased formation and retardation of destruction cause compensatory mechanisms to fail and result in an increased or potentiated greenhouse effect. For example, the amount of water vapor released through evaporation increases directly with increases in the surface temperature of Earth. Within normal limits, increased levels of water vapor are usually controlled by increased warming and precipitation. Likewise, within normal limits, concentrations of carbon dioxide and methane are usually maintained with specified limits by a variety of physical and chemical processes.
Measurements made late in the twentieth century showed that since 1800, methane concentrations have doubled and carbon dioxide concentrations measured at he highest values estimated to have existed during the last 160,000 years. In fact, increases in carbon dioxide over the last 200 years were exponential up until 1973 (the rate of increase has since slowed).
Although the effects of these increases in global greenhouse gases are debated among scientists, the correlation of the increased levels of greenhouse gases with a measured increase in global temperature during the twentieth century, have strengthened the arguments of models that predict pronounced global warming over the next few centuries. In the alternative, some scientists remain skeptical because the earth has not actually responded to the same extent as predicted by these models. For example, where many models based upon the rate of change of greenhouse gases predicted a global warming of .8°F to 2.5°F (0.44°C to 1.39°C) over the last century, the actual measured increase is significantly less with a mean increase generally measured at .9°F (.5°C) and that this amount of global warming is within the natural variation of global temperatures.
One problem in reaching a scientific consensus regarding global warming is that the data used in many models is neither global nor a result of high-reliance systematic scientific measurement (i.e., that it generally neglects oceans and vast uninhabited areas). Other problems involve forming an accurate articulation of the interplay of global surface warming phenomena that include thermal conduction, greenhouse radiation, and convective currents. Most scientists agree, however, that an enhanced greenhouse effect will result in some degree of global warming.
Acid Rain; Atmospheric Pollution
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