In ancient fossil aquifers – in the Great Plains of the United States, the North China Plain, or Saudi Arabia – water levels are not recharged by rainfall. Elsewhere, as in northern India, ground water is used faster than it can be replenished. According to the United Nations, ground-water extraction globally has tripled in the past 50 years, during which time India and China's ground-water use has risen 10-fold.
As a result, half of the global population lives in countries where water tables are rapidly falling. These supply problems are compounded by new land use patterns, like deforestation and soil grading, as well as leakage from poorly maintained infrastructure in cities.
To make things worse, climate change is expected to cause water shortages in many parts of the world, making ground water all the more important as a buffer.
The spike in global grain prices caused by the US drought last summer, on the heels of an epic winter drought in Spain and summer heat waves in southern Europe, showed the cascade effect of the sort of droughts that the Intergovernmental Panel on Climate Change expects will multiply in the decades ahead.
Not surprisingly, the greatest impact is on the poor. While American households spend, on average, 13 percent of their budgets on food, that expenditure is often 50 percent or more in the developing world. So a spike in food prices can trigger explosive riots like those that erupted there in the past five years.