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V. INADVERTENT WEATHER MODIFICATION 617 St. Louis, Missouri, area in the 1960s under the name Metropolitan Meteorological Experiment (METROMEX). Data were collected from in- strumented aircraft, rawinsondes, radar sets, and surface weather stations. The data indicated that St. Louis was influencing the weather by introducing particulates and water vapor into the atmosphere, by changing the radiative prop- erties of the earth's surface, and by changing the turbulent mixing between the layers next to the ground and those hundreds of meters upward. A region extending downwind as far as 100 km was found to be characterized by an increased fre- quency of thunderstorms with a resultant in- crease in hail and summer rainfall. Studies are being conducted at present around many large cities to determine their specific ef- fects on the local climate. B. OTHER CHANGES RELATED TO LAND USE Urban effects on weather and climate are a special example of weather modification due to changes in land use. Other land use factors to be considered include deforestation, irrigation, and draining of swamps and marshlands. The intro- duction oflarge-scale irrigation in the arid south- western United States has been accompanied by an increase in the humidity in those areas, par- ticularly during the summer. However, there is little evidence that these changes have resulted in any substantial increases in rainfall because the occurrence of rainfall is generally controlled by large-scale motions in the atmosphere. One disastrous instance of inadvertent weather modification is desertification, or the in- advertent expansion of deserts. The problem is acute in several countries of Africa, notably Ethiopia and Mauritania. Extensive deforesta- tion and prolonged overgrazing have denuded the land. Such rains as do fall run off quickly, producing severe erosion rather than being held in the soil and returned to the local atmosphere through evapotranspiration. This change in run- off patterns is of itself very deleterious to crops and has been hypothesized to reduce the fre- quency of subsequent local showers, thereby causing additional damage over the longer term. C. THE GREENHOUSE EFFECT There is a serious possibility that the tempera- ture of the earth's atmosphere will rise by 1 to 20C over the next few decades as a result of increasing concentrations of carbon dioxide (C02). Monitoring of CO2 concentrations in the atmosphere shows a steady rise from about 290 ppm in the late 1800s to about 340 ppm today. The increase is usually ascribed to the consump- tion of fossil fuels. Because CO2 is one of the gases that intercepts outgoing infrared radiation from the earth, increases in its concentration could, in theory, lead to a warming of the entire earth. This action is analogous to one effect of the glass protecting plants in greenhouses, so the warming effect is called the greenhouse effect. In time the CO2 would leave the atmosphere and dissolve in the oceans. This buffering pro- cess has a time constant of several centuries and therefore cannot prevent a greenhouse effect lasting several generations. However, compen- sating mechanisms in the atmosphere itself may prevent the feared warming. For example, a slight increase in cloudiness over the earth would reflect more of the incoming solar radia- tion back into space and could conceivably com- pensate for the increased retention of infrared radiation. D. ACID RAIN Instances of acid rain due to human activities such as the burning of coal containing sulfur are a special case of inadvertent weather modifica- tion. [See AIR POLLUTION (METEOROLOGY).] E. NUCLEAR WINTER It has been suggested that a nuclear war would be followed by a "nuclear winter," which would be the most devastating of all possible cases of inadvertent weather modification. It is not certain that a nuclear winter would follow a nuclear war. However, numerical simulations indicate that nuclear explosions and subsequent fires would introduce sufficient particulate mat- ter, including elemental carbon, into the atmo- sphere which would absorb much of the incom- ing solar radiation. Some simulations indicate material would be injected into the stratosphere by tall convective storms set off or stimulated by the explosions and fires. While nuclear debris in the troposphere would fall out or wash out in a matter of a few days, the injection of nuclear debris into the stratosphere is particularly criti- cal because the smaller particles could reside in the stratosphere for several months. The general result would be a warming of the upper atmo- sphere and cooling of the surface and lower at-