AN INTRODUCTION According to the National Academy of Sciences, the Earth's surface temperature has risen by about 1 degree Fahrenheit in the past century, with accelerated warming during the past two decades. There is new and stronger evidence that most of the warming over the last 50 years is attributable to human activities. Human activities have altered the chemical composition of the atmosphere through the buildup of greenhouse gases - primarily carbon dioxide, methane, and nitrous oxide. The heat-trapping property of these gases is undisputed although uncertainties exist about exactly how earth's climate responds to them. Go to the Emissions section for much more on greenhouse gases. Our Changing Atmosphere Energy from the sun drives the earth's weather and climate, and heats the earth's surface; in turn, the earth radiates energy back into space.

Atmospheric greenhouse gases (water vapor, carbon dioxide, and other gases) trap some of the outgoing energy, retaining heat somewhat like the glass panels of a greenhouse. Without this natural "greenhouse effect," temperatures would be much lower than they are now, and life as known today would not be possible. Instead, thanks to greenhouse gases, the earth's average temperature is a more hospitable 60^0 F. However, problems may arise when the atmospheric concentration of greenhouse gases increases.

Since the beginning of the industrial revolution, atmospheric concentrations of carbon dioxide have increased nearly 30%, methane concentrations have more than doubled, and nitrous oxide concentrations have risen by about 15%. These increases have enhanced the heat-trapping capability of the earth's atmosphere. Sulfate aerosols, a common air pollutant, cool the atmosphere by reflecting light back into space; however, sulfates are short-lived in the atmosphere and vary regionally. Why are greenhouse gas concentrations increasing? Scientists generally believe that the combustion of fossil fuels and other human activities are the primary reason for the increased concentration of carbon dioxide. Plant respiration and the decomposition of organic matter release more than 10 times the CO 2 released by human activities; but these releases have generally been in balance during the centuries leading up to the industrial revolution with carbon dioxide absorbed by terrestrial vegetation and the oceans. What has changed in the last few hundred years is the additional release of carbon dioxide by human activities.

Fossil fuels burned to run cars and trucks, heat homes and businesses, and power factories are responsible for about 98% of U. S. carbon dioxide emissions, 24% of methane emissions, and 18% of nitrous oxide emissions. Increased agriculture, deforestation, landfills, industrial production, and mining also contribute a significant share of emissions. In 1997, the United States emitted about one-fifth of total global greenhouse gases. Estimating future emissions is difficult, because it depends on demographic, economic, technological, policy, and institutional developments.

Several emissions scenarios have been developed based on differing projections of these underlying factors. For example, by 2100, in the absence of emissions control policies, carbon dioxide concentrations are projected to be 30-150% higher than today's levels. Changing Climate Global mean surface temperatures have increased 0. 5-1.

0^0 F since the late 19 th century. The 20 th century's 10 warmest years all occurred in the last 15 years of the century. Of these, 1998 was the warmest year on record. The snow cover in the Northern Hemisphere and floating ice in the Arctic Ocean have decreased. Globally, sea level has risen 4-8 inches over the past century. Worldwide precipitation over land has increased by about one percent.

The frequency of extreme rainfall events has increased throughout much of the United States. Increasing concentrations of greenhouse gases are likely to accelerate the rate of climate change. Scientists expect that the average global surface temperature could rise 1-4. 5^0 F (0. 6-2. 5^0 C) in the next fifty years, and 2.

2-10^0 F (1. 4-5. 8^0 C) in the next century, with significant regional variation. Evaporation will increase as the climate warms, which will increase average global precipitation.

Soil moisture is likely to decline in many regions, and intense rainstorms are likely to become more frequent. Sea level is likely to rise two feet along most of the U. S. coast. Calculations of climate change for specific areas are much less reliable than global ones, and it is unclear whether regional climate will become more variable. Trends Data on a wide variety of environmental indicators are consistent with the consequences that scientists generally expect to result from increasing concentrations of greenhouse gases.

Temperature Global temperatures are rising. Observations collected over the last century suggest that the average land surface temperature has risen 0. 45-0. 6^0 C (0. 8-1.

0^0 F) in the last century. Precipitation Precipitation has increased by about 1 percent over the world's continents in the last century. High latitude areas are tending to see more significant increases in rainfall, while precipitation has actually declined in many tropical areas. Sea Level Sea level has risen worldwide approximately 15-20 cm (6-8 inches) in the last century.

Approximately 2-5 cm (1-2 inches) of the rise has resulted from the melting of mountain glaciers. Another 2-7 cm has resulted from the expansion of ocean water that resulted from warmer ocean temperatures. Global surface temperatures have increased about 0. 6^0 C (plus or minus 0. 2^0 C) since the late-19 th century, and about one half degree F (0. 2 to 0.

3^0 C) over the past 25 years (the period with the most credible data). The warming has not been globally uniform. Some areas (including parts of the southeastern U. S. ) have cooled. The recent warmth has been greatest over N.

America and Eurasia between 40 and 70^0 N. Warming, assisted by the record El Ni~no of 1997-1998, has continued right up to the present. Linear trends can vary greatly depending on the period over which they are computed. Temperature trends in the lower troposphere (between about 2, 500 and 18, 000 ft. ) from 1979 to the present, the period for which Satellite Microwave Sounding Unit data exist, are small and may be unrepresentative of longer term trends and trends closer to the surface. Furthermore, there are small unresolved differences between radiosonde and satellite observations of tropospheric temperatures, though both data sources show slight warming trends.

If one calculates trends beginning with the commencement of radiosonde data in the 1950 s, there is a slight greater warming in the record due to increases in the 1970 s. There are statistical and physical reasons (e. g. , short record lengths, the transient differential effects of volcanic activity and El Ni~no, and boundary layer effects) for expecting differences between recent trends in surface and lower tropospheric temperatures, but the exact causes for the differences are still under investigation (see National Research Council report 'Reconciling Observations of Global Temperature Change'). An enhanced greenhouse effect is expected to cause cooling in higher parts of the atmosphere because the increased 'blanketing' effect in the lower atmosphere holds in more heat. Cooling of the lower stratosphere (about 30-35, 000 ft.

) since 1979 is shown by both satellite Microwave Sounding Unit and radiosonde data, but is larger in the radiosonde data. There has been a general, but not global, tendency toward reduced diurnal temperature range (the difference between high and low daily temperatures) over about 50% of the global land mass since the middle of the 20 th century. Cloud cover has increased in many of the areas with reduced diurnal temperature range. Relatively cool surface and tropospheric temperatures, and a relatively warmer lower stratosphere, were observed in 1992 and 1993, following the 1991 eruption of Mt. Pinatubo. The warming reappeared in 1994.

A dramatic global warming, at least partly associated with the record El Ni~no, took place in 1998. This warming episode is reflected from the surface to the top of the troposphere. Indirect indicators of warming such as borehole temperatures, snow cover, and glacier recession data, are in substantial agreement with the more direct indicators of recent warmth. Arctic sea ice has decreased since 1973, when satellite measurements began but Antarctic sea ice may have increased slightly. Human beings have in recent years discovered that they may have succeeded in achieving a momentous but rather unwanted accomplishment. Because of our numbers and our technology, it now seems likely that we have begun altering the climate of our planet.

Climatologists are confident that over the past century, the global average temperature has increased has increased by about half a degree Celsius. This warming is thought to be at least partly the result of human activity.