Alteration of the Earth's climate by human activities. This can occur on various scales. For example, conventional agriculture alters the microclimate in the lowest few meters of air, causing changes in the evapotranspiration and local heating characteristics of the air-surface interface. These changes lead to different degrees of air turbulence over the plants and to different moisture and temperature distributions in the local air. An example at a larger scale is that the innermost parts of cities are several degrees warmer than the surrounding countryside, and have slightly more rainfall. These changes are brought about by the differing surface features of urban land versus natural countryside and the ways that cities dispose of water (for example, storm sewers). The urban environment prevents evaporation cooling of surfaces in the city. The modified surface texture of cities (horizontal and vertical planes of buildings and streets versus gently rolling surfaces over natural forest or grassland) leads to a more efficient trapping of solar heating of the near-surface air. The scales of buildings and other structures also lead to a different pattern of atmospheric boundary-layer turbulence, modifying the stirring efficiency of the atmosphere. See also Micrometeorology; Urban climatology.
At the next larger scale, human alteration of regional climates is caused by changes in the Earth's average reflectivity to sunlight. For example, the activities of building roads and highways and deforestation alter the amount of sunshine that is reflected to space as opposed to being absorbed by the surface and thereby heating the air through contact. Such contact heating leads to temperature increases and evaporation of liquid water at the surface. Vapor wakes from jet airplanes are known to block direct solar radiation near busy airports by up to 20%. Human activities also inject dust, smoke, and other aerosols into the air, causing sunlight to be scattered back to space. Dust particles screen out sunlight before it can enter the lower atmosphere and warm the near-surface air. See also Air pollution; Smog.
One of the most important and best understood features of the atmosphere is the process that keeps the Earth's surface much warmer than it would be with no atmosphere. This process involves several gases in the air that trap infrared radiation, or heat, emitted by the surface and reradiate it in all directions, including back to the surface. The heat-trapping gases include water vapor, carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). These gases constitute only a small fraction of the atmosphere, but their heat-trapping properties raise the surface temperature of the Earth by a large amount, estimated to be more than 55°F (30°C). Human activities, however, are increasing the concentrations of CO2, CH4, and N2O in the atmosphere, and in addition industrially synthesized chemicals—chlorofluorocarbons (CFCs) and related compounds—are being released to the air, where they add to the trapping of infrared radiation. Carbon dioxide is released to the air mainly from fossil fuel use, which also contributes to the emissions of CH4 and N2O. Agricultural and industrial processes add to the emissions of these gases. These concentration increases add to the already powerful heat-trapping capability of the atmosphere, raising the possibility that the surface will warm above its past temperatures, which have remained roughly constant, within about 3°F (1.7°C) for the past 10,000 years. Treaties are in existence that control internationally the production and use of many of the CFCs and related compounds, so their rate of growth has slowed, and for some a small decrease in atmospheric concentration has been observed. Large emissions of sulfur dioxide in industrial regions are thought to result in airborne sulfate particles that reflect sunlight and decrease the amount of heating in the Northern Hemisphere. See also Atmosphere; Greenhouse effect.
