Climate models have, in essentially every case, correctly predicted the droughts that are occurring today. There is every reason to believe that future droughts will follow these patterns and that the multimodel predictions have considerable credibility.
Fresh water will be in ever-shorter supply in many places as the climate changes. While responding to increasing temperatures, river flows may increase overall, with some rivers becoming more swollen, but many of those that provide water for most of the world's people will begin to dry up.
Increases in precipitation will cause the discharge of fresh water from some rivers around the world to rise by almost 15%. However, at the same time, water stresses are predicted to increase significantly in regions that are already relatively dry. Evaporation will reduce the moisture content of soils in many semiarid parts of the world, including northeast China, the grasslands of Africa, the Mediterranean, and the southern and western coasts of Australia. Soil moisture will fall by up to 40% in southern states of the United States. The Amazon basin is also predicted to suffer increasing drought.
Some of these predicted changes are already happening. One study shows that temperature changes have affected the flow in many of the world's 200 largest rivers during the past century, with the flow of Africa's rivers declining during the past 10 years. Climate change during the past few decades has already caused discharge from some rivers in North and South America and Asia to increase. But some have decreased substantially (see below). Runoff in Europe has remained stable, but the flow of water from Africa's rivers has fallen.
Failing rains are already a major cause of hardship in Africa, as witnessed by the current drought in East Africa, and the results shown in Figure 5.3 certainly imply that global warming will change rainfall patterns across the continent.
Rainfall has a direct effect on the way water drains into streams and rivers and, therefore, on supply. When rainfall in an area is less than 400 mm/yr, there is virtually no drainage into rivers and streams. Therefore, a decrease in precipitation will amplify the loss of available water because evaporation is increased. A 10% decrease in rainfall could cut the available water by 50%. Semiarid regions in Africa, such as southern Africa and the Sahel, are extremely vulnerable to the spread of deserts.
With a long dry spell ruining crops in northwest India, conditions in the country's breadbasket are the worst in a decade. Among the affected areas are the major rice- and cereal-growing states of Haryana, Punjab, and Rajasthan. In India's largest state, Uttar Pradesh, state officials declared 21 districts as drought-hit and sent drinking water to the driest areas.
4) Pteefcitaiion b|> Soil mofsi jre
The monsoon season is crucial to India's farm output. About 80% of the country's rainfall occurs between June and September. Two thirds of India's 1 billion people depend directly on agriculture for their livelihood, and farming contributes about 25% to gross domestic product (GDP).
Although droughtlike conditions persisted in some areas, thousands of mud and thatch houses were washed away in the eastern state of Bihar and in northern Assam after heavy downpours. According to Figure 5.3, precipitation is set to increase, especially in northern India, whereas soil moisture continues to decrease.
5.3.3 SOUTH AMERICA
Widespread drought in 2005 in the Amazon basin was the worst in some areas since records began and prompted the Brazilian government to declare a state of emergency. There is broad consensus that the 2005 drought was linked not to El Niño, the periodic phenomenon that begins with a warming of Pacific waters off the coast of South America, as was the case with most previous droughts in the Amazon, but to warming sea surface temperatures in the tropical North Atlantic. At least one climate model predicts that, under current levels of greenhouse gas emissions, the chances of such a drought would rise from 5% now (one every 20 years) to 50% by 2030 and to 90% by 2100. The multimodel prediction in Figure 5.3 indicates substantial decreasing soil moisture despite increasing precipitation.
Downstream of the Amazon basin drought area, in the city of Manaus, the level of the Amazon River dropped by 3 m during the current drought. Many communities dependent on the river for transport were left stranded as tributaries dried up.
In China, the devastating drought that has plagued the drainage area of the Yellow River, which is supposed to supply irrigation water to more than a million hectares (a hectare is 2.7 acres) of farmland, shows no signs of abating. More than 300 000 farmers are now in urgent need of irrigation water. Farmers in the region are being encouraged to plant crops that need less water. Even if these measures are taken, a shortage of more than half a billion cubic meters will remain. More than 48 million people and 5 million hectares of arable land have been affected in some way. More than 3 million hectares yielded no harvest last year. The central and western parts of the Shandong Province, including Heze, Jining, Liaocheng, Jinan, Zibo, Binzhou, and Dongying, are the most affected. Figure 5.3 indicates increasing precipitation and decreasing soil moisture throughout most of China.
Water reservoirs in Australia are dropping fast, crop forecasts have been slashed, and great swaths of the continent are entering what scientists have called a "one-in-a-thousand-years drought." Many regions are in their fifth year of drought, and more than half of Australia's farmland is experiencing drought. The Murray-Darling river system, which receives 4% of Australia's water but provides three quarters of the water consumed nationally, is already 54% below the previous record minimum. The drought will affect drinking water supplies to many areas. Sydney's largest reservoir is now only 40% full, and many small rural towns in east Australia face shortages. Thus, a serious impact on crops is expected: its lowest wheat crop for 12 years, a decrease of more than 50% below last year's, which was already well below average.
The Commonwealth Scientific and Industrial Research Organization, the leading scientific body in Australia, has predicted that rainfall in parts of eastern Australia could drop by 40% by 2070, along with a 7°C rise in temperature. Figure 5.3 shows decreasing precipitation except in the north and decreasing soil moisture, especially on both coasts.
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