We are witnessing the beginning of one of the great tragedies of history. The United States, in a misguided effort to reduce its oil insecurity by converting grain into fuel for cars, is generating global food insecurity on a scale never seen before.

Humanity's greatest challenge may soon be just making it to the next harvest.

Our early 21st century civilization is in trouble. We need not go beyond the world food economy to see this. Over the last few decades we have created a food production bubble-one based on environmental trends that cannot be sustained, including overpumping aquifers, overplowing land, and overloading the atmosphere with carbon dioxide.

It takes 1,000 tons of water to produce 1 ton of grain. As water becomes scarce and countries are forced to divert irrigation water to cities and industry, they will import more grain. As they do so, water scarcity will be transmitted across national borders via the grain trade. Aquifer depletion is a largely invisible threat, but that does not make it any less real.

Rising oil prices have focused the world's attention on the depletion of oil reserves. But the depletion of underground water resources from overpumping is a far more serious issue. Excessive pumping for irrigation to satisfy food needs today almost guarantees a decline in food production tomorrow.

The foundation is being laid for the emergence of both wind and solar cells as cornerstones of the new energy economy.

The challenge is either to build an economy that is sustainable or to stay with our unsustainable economy until it declines. It is not a goal that can be compromised. One way or another, the choice will be made by our generation, but it will affect life on earth for all generations to come.