Rainfall And Global Warming

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Rainfall is vital to the creation of food and water on Earth. With less than 1 percent of all the planet's fresh water available for human and animal consumption, the regular recharging of groundwater and reservoirs from rain and snow is critical to keep fresh water sources flowing, and all plant life requires at least some water to grow. Regular, moderate precipitation is critical to the production of most of the 2,000 food crops under cultivation. Because of the importance of rainfall to the environment, climate experts have been trying to determine the possible impact of global warming on rainfall distribution around the world.

In 2007, a group of researchers constructed a computer modeling program drawing on data collected 1925-99, and, taking into account potential future increases in levels of greenhouse gases and solar activities, made projections well into the next century. Their study found the precipitation in areas lying between 40 and 70 degrees N latitude is likely to increase at a rate of 2.44 in. (62 mm.) per decade, and in areas lying between zero and 30 degrees S latitude is likely to increase 3.23 in. (82 mm.) per decade. However, the area lying between zero and 30 degrees N latitude is likely to see precipitation decreases of 3.86 in. (98 mm.) per decade. This means that regions toward the poles, including Europe, Canada, and parts of Latin America, will become wetter, while already parched regions will become much dryer.

This could be catastrophic for the Sahel region in Africa. A 2,400-mi. (3862 km.) long belt stretching from Senegal on the west coast to Eritrea on the east, the Sahel is the boundary line between the Sahara Desert and the more fertile sub-Saharan region to the south. The area is home to an estimated 50 million people, 62 percent

A patch of rain moving through Tobago in the Caribbean. Climate models predict more heavy rain events, separated by prolonged periods of dry weather. In a global warming scenario, the warmer air in the atmosphere holds more water.

of whom live in poverty. Since the 1970s, the Sahel has been struck by longer and more frequent droughts, with overall precipitation decreasing by around 20 percent during the period. Modeling indicates that in the coming century, rainfall averages in this region could drop by another 30 percent. The death toll from previous droughts has numbered in the millions; in the future, the area may become uninhabitable.

Areas where rainfall is expected to increase will likely see new patterns of precipitation as well. The overall amounts of rain may not change dramatically from year to year, but climate models predict more heavy rain events, separated by prolonged periods of dry weather. Much of this will be due to the heating of the atmosphere: warmer air holds more water, raising the potential for a quick release of a large volume of water. Air pollution will also play a role, as more particulates in the atmosphere give this increased amount of water vapor more condensation nuclei around which to coalesce.

Heavy rains are a potential threat to human life and property, raising the probability of events such as flash floods and landslides. Research has also found that frequent heavy rains combined with long dry spells quickly erode the vitality of a region's biomass, and in the long term contribute to a loss of soil quality and overall biodiversity.

Climate experts predict there could be some initial benefits to plant life in areas of increased rainfall. Scientists have seen a rise in the amounts of reactive nitrogen in the atmosphere, which they believe stems from a combination of rising rates of nitrogen oxides from the burning of fossil fuels and ammonia (NH3) from agricultural sources. This reactive nitrogen is carried to the ground by rain, where it stimulates plant growth, particularly in the forests. Increasing the vitality of the forest will increase the terrestrial carbon sink, as trees and plants absorb harmful carbon dioxide from the atmosphere as a process of photosynthesis. The amount of reactive nitrogen has increased fourfold since the beginning of the Industrial Revolution, and will likely to continue to grow in coming years. Scientists are not sure at what point there is too much of this substance available, when its risks to the environment begin to outweigh its benefits.

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