ecology

  The world's animal protein are also under excessive pressure. As human populations grow, so do livestock numbers. With 180 million people worldwide now trying to make a living raising 3.3 billion cattle, sheep, and goats, grasslands are simply collapsing under the demand. As a result of overstocking, grasslands are now deteriorating in much of Africa, the Middle East, Central Asia, the northern part of the Indian sub-continent, and much of northwestern China.

In China, the combination of overplowing and overgrazing to satisfy rapidly expanding food needs is creating a dust bowl reminiscent of the US Dust Bowl of the 1930s - but much larger. In a desperate effort to maintain grain self-sufficiency, China has ploughed large areas of the northwest, much of it land that is highly erodible and should never have been ploughed.

As the country's demand for livestock products - meat, leather, and wool - has climbed, so have the numbers of livestock, far exceeding those of the US, a country with comparable grazing capacity. In addition to the direct damage from overplowing and overgrazing, the northern half of China is literally drying out as aquifers are depleted by overpumping. These trends are converging to form some of the largest dust storms ever recorded. The huge dust plumes, traveling eastward, affect the cities of northeast China - blotting out the sun and reducing visibility. Eastward-moving winds also carry soil from China's northwest to the Korean Peninsula and Japan, where people regularly complain about the dust clouds that filter out the sunlight and blanket everything with dust.

Unless China can reverse the overplowing and overgrazing trends that are creating the dust bowl, these trends could spur massive migration into the already crowded cities of the northeast and undermine the country's economic future.

Water tables are falling under large expanses of the three leading food- producing countries - China, India, and the US. Under the North China Plain, which accounts for 25 per cent of China's grain harvest, the water table is falling by roughly 1.5 metres (5 feet) per year. China's Yellow River, the cradle of Chinese civilization, runs dry for part of each year, depriving farmers in its lower reaches of irrigation water.

Deforestation also diminishes the recycling of water inland, thus reducing rainfall in the interior of continents. When rain falls on a healthy stand of dense forest, roughly one quarter runs off, returning to the sea, while three quarters evaporates, either directly or through transpiration. When land is cleared for farming or grazing or is clear cut by loggers, this ratio is reversed - three quarters of the water returns to the sea and one quarter evaporates to be carried further inland.