Rainfall Patterns

A rainfall pattern describes the distribution of rain geographically, temporally, and seasonally. The tropics receive more rainfall than deserts on a more regular basis. Cooler places like the poles receive no rainfall as the moisture is converted to snow before it falls to the ground. Rainfall occurs more often during particular times of the year, creating rainy seasons. In other seasons, rainfall is scant. Worldwide, rain-fed agriculture is planned based on rainfall’s natural pattern. Water storage, irrigation networks, and urban water-supply systems are designed according to the average annual rainfall. A significant amount of rain on a continuous basis for a long time increases the possibility of flood and subsequent disaster to infrastructure.

No or little rainfall for a longer period (years) in an inhabited area could lead to drought and famine. Changing rainfall patterns are a consequence of global warming, which causes a change in ocean thermohaline circulation. The world’s agriculture, especially third-world agriculture, depends upon seasonal rainfall patterns. Recent erratic changes in rainfall patterns lead to low agriculture production, thus creating food insecurity for everincreasing world populations. Untimely floods, droughts, and famine are the consequences of these unassuming patterns.

Global Warming and Rainfall Patterns

Global warming leads to a near-term collapse of ocean thermohaline circulation, a global ocean circulation pattern that distributes water and heat both vertically through the water column and horizontally across the globe. Because of this collapse, 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 become warmer than average and some parts of Europe become rapidly cooler. El Niño and La Niña are examples of this phenomenon. The latest deviant trend is generating dramatic weather impacts such as rapid cooling in some parts of the world and greatly diminished rainfall in agricultural and urban areas. The United Nations Educational, Scientific and Cultural Organization and other studies found that changes in rainfall pattern can be attributed to shifts in the global wind pattern. These shifts are because of changes in ocean surface temperatures.

The effect of human activity on surface vegetation is also causing rainfall pattern variation. Widespread deforestation in parts of Africa and Asia is causing scarce rainfall and subsequent drought. Global warming affecting rainfall pattern variability is a commonly accepted phenomenon among the world’s scientific community. More precipitation is occurring 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 in 75 years (1925–99) of rainfall data analysis through 14 powerful computer models that the Northern Hemisphere’s midlatitude (a region 40–70 degrees north) received increased precipitation, which 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. Time and again, researchers worldwide have proven that natural pattern rainfall is good for plant growth, while variable rainfall patterns lead to lower amounts of water in the subsurface level of soil (in the upper 30 cm). Variable rainfall patterns also cause plant diversity in particular areas, indicating that weeds grow rapidly with variable rainfall. The significance of these changes is evident from recent large-scale, worldwide failure of crops, rangelands, and water-supply systems.

Mass starvation in the Sahel region of Africa is stark proof of this. Some argue that changes in rainfall patterns are unfounded because of a lack of instrumental records for a long duration. However, studies using indirect methods have proven that global warming, in fact, is causing serious variability in rainfall patterns. Tree-ring analysis for determining rainfall amounts in previous years (hundreds of years back) is one such study proving that rainfall pattern variability has been extensive in recent years.

If this trend continues, environmental managers will be pressed to make new decisions about the management of water and land. They will 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, and communities of vegetation and animals, and effect the viability of large water-resource developments. Variability in rainfall patterns would also cause mass human migration.