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Rõ ràng là lượng nước bị mất từ mặt đất bốc hơi không thể vượt quá số tiền mà rơi như mưa, nhưng sự bay hơi tiềm năng có thể vượt quá lượng mưa. | 214 GRASSLANDS evaporation pan that is left standing in the open. Obviously the amount of water lost from the ground by evaporation cannot exceed the amount that falls as precipitation but the potential evaporation can exceed the precipitation. It is possible therefore for precipitation to increase but for the ground to become drier. If the average temperature rises by less than about 3.6 F 2 C precipitation will increase more than the potential evaporation and so the ground will become more moist. This might allow some plant species to grow more vigorously at the expense of others altering the balance of species in the natural grassland community and it might encourage the growth of more trees. Where grassland has been converted to arable farmland soil that is moister might increase crop yields although crops could be damaged if some of the increased precipitation arrives in the form of violent hailstorms. If the average temperature rises by more than about 3.6 F 2 C potential evaporation will increase faster than precipitation and the ground will become drier. This would be a more serious situation. Crops would need more irrigation to make good the shortfall and in time natural grassland would become dominated by plants that tolerate drought while less tolerant plants would disappear. At present the evidence suggests that the rise in temperature will be modest and grasslands will not suffer although their composition may change. It is possible that forest might expand into moister areas of what is now grassland. If the rate of warming were to increase substantially however grasslands would be confined to the moister regions while other areas turned to semidesert or desert. Expansion of towns and roads Nowadays most Americans and Europeans live in cities. In the United States 79 percent of the population lives in urban areas. In the United Kingdom 89.5 percent of people are city dwellers in France the proportion is 75.5 percent and in Germany it is 87.7 percent.
