Rainfall Patterns

Change iN RAiNFALL pattern is a consequence of global warming. The world's agriculture, especially third world agriculture, depends upon the seasonal rainfall pattern. Recent erratic changes in rainfall pattern lead toward low agriculture production, thus creating food insecurity for an ever-increasing world population. Flood, drought, and famine are the consequences of these changing patterns.

Rainfall pattern means the distribution of rain geographically, temporally, and seasonally. The tropics receive more rainfall than deserts. Cooler places like the poles receive no rainfall, as it is converted to snow before it falls to the ground. Rainfall happens more in a particular time of a year, during a rainy season. In other seasons, rainfall is scant. Therefore, agriculture (rain-fed), worldwide, is planned based on rainfall's natural pattern. Water storages, irrigation networks, and urban water supply systems are designed according to the average annual rainfall. If it rains a lot on a continuous basis for a longer time, there is possibility of flood and subsequent disaster to the infrastructure. No rainfall or little rainfall for a longer period (years) in an inhabited area could lead to drought and famine.

Global warming leads to a near-term collapse of the ocean's thermohaline circulation. Thermohaline circulation is a global ocean circulation pattern that distributes water and heat both vertically, through the water column, and horizontally across the globe. Due to this collapse of thermohaline circulation, warm surface waters move from the tropics to the North Atlantic and extra-warm water surfaces in the Pacific Ocean surrounding the equator. Thus, Western Europe, some parts of Asia, and many parts of the Americas get warmer than normal, and some parts of Europe get cooler rapidly. El Niño and La Niña are examples of this. This latest deviant trend generates dramatic weather impacts, such as rapid cooling in some parts of the world, and greatly diminished rainfall in agricultural and urban areas. UNESCO and other studies found that changes in rainfall pattern could be attributed to the shifts in global wind pattern. These shifts are due to the changes in the ocean surface temperature. Effect of human activity on the surface vegetation is also causing rainfall pattern variation. Widespread deforestation in parts of Africa and Asia is causing scarce rainfall and subsequent drought.

The world's scientific community commonly accepts that global warming has affected rainfall patterns. More precipitation is happening in northern Europe, Canada, and northern Russia, but less in swaths of sub-Saharan Africa, southern India, and southeast Asia. A Canadian research team found with 75 years (1925-99) of rainfall data analyzed through 14 powerful computer models that the Northern Hemisphere's midlatitude (a region of 40 to 70 degrees N) received increased precipitation over the years. It corroborates with the change in thermohaline circulation. The models also showed that in contrast, the Northern Hemisphere's tropics and subtropics (a region between the equator and 30 degrees N) became drier, while the Southern Hemisphere's similar region became wetter. This study was conducted for rainfall patterns over land.

Researchers claim that a natural pattern of rainfall is good for plant growth, as variable rainfall patterns lead to lower amounts of water in the subsurface level of the soil (in the upper 30 cm.). Variable rainfall patterns also cause plant diversity in a particular land. That means that weeds grow rapidly with variable rainfall. The significance of these changes is evident from recent large-scale failure of the crops, rangelands, and water-supply systems in the world. Mass starvation in Sahelian Africa is the stark proof of this.

Some might argue that changes in rainfall patterns are unfounded due to lack of instrumental records for a large time period. However, studies using indirect methods have proved that global warming, in fact, is causing serious rainfall pattern variability. Tree-ring analysis for predicting rainfall amounts in previous years (hundreds of years back) is one study, which proved that rainfall pattern variability is extensive in recent years.

If this trend continues, environmental managers need to make new decisions about the management of water and land. They need to accurately understand the interannual variability of rainfall and a possibility of runs of dry and wet years, which may cause important changes in runoff, sedimentation, soil erosion, or changes in communities of vegetation and animals, and of the viability of large water resource developments. Rainfall pattern variability would certainly cause mass human migration.

SEE ALSO: Rain; Thermohaline Circulation; Thunderstorms.

BIBILIOGRAPHY. Thomas Dunne and L.B. Leopold, Water in Environmental Planning (W.H. Freeman and Co., 1978); Cheryl Dybas, "Increase in Rainfall Variability Related to Global Climate Change," Earth Observatory, NASA (December 12, 2002); "Global Warming has Already Changed World's Rainfall Patterns," Com-monDreams.org (cited July 2007); UNESCO, "Changes in Climate," Arid Zone Research (v.20, 1963); Warren Viessman, Jr., and G.L. Lewis, Introduction to Hydrology (Prentice Hall, 2003).

SUDHANSHU SEKHAR PANDA Gainesville State College

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    What cause pattern of rainfall changes in a region?
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