Wake up, World Bank and FAO.

Of all the problems confronting the world's beleaguered governments, few have as much long-term importance as the challenge of guaranteeing their peoples an adequate food supply. In the face of growing populations which may not stabilize for decades, many countries are likely to find that challenge increasingly difficult to meet. But no country, rich or poor, can chart an effective food policy without first understanding what levels of crop production the world can reasonably count on - not just 10 or 20 years from now, in the wake of some hoped-for technological breakthrough, but today and next year, under today's and next year's real-life constraints.

Those constraints are well documented: cropland is shrinking in the face of urban sprawl; irrigation water is being diverted in ever greater quantities for industrial and municipal needs; fertilizer use in much of the world has reached a saturation point, beyond which extra doses do not increase the harvest; and the overall protein-producing efficiency of the world agricultural system is declining, as newly prosperous Asian populations shift increasingly from direct grain consumption to meat. Meanwhile, demand continues to rise by nearly 90 million mouths per year.

Yet these and other constraints, such as the effects of soil erosion and air pollution on crop yields, are given little weight in the two most important agricultural forecasts, the global grain production projections by the World Bank and the UN Food and Agriculture Organization. Governments all over the world depend on these projections to make tough, real-world decisions - on food production, food imports, and a host of other issues. But the economists who do the forecasting rely mostly on an extrapolation of past production trends, and mechanically infer that because the world's farmers were able to sustain rapid growth in the world grain harvest from 1960 to 1990, they will continue to do so ad infinitum.

No industry has a hope of understanding its future without a solid grasp of the various external pressures - economic, social, or biological - that affect its production. Simple extrapolation from past trends never works over the long term. Important change - whether it's the stagnation in nuclear powerplant construction, or the replacement of the LP record by the compact disk - cannot be predicted in a theoretical vacuum. Yet over the past five years, the best-known projections for the world's most important product - food - have continued to march upward, even while actual production has fallen. Failure to notice what is really happening could lead to serious underinvestment in such key endeavors as family planning and agricultural research.

Of course, planners are not required to accept World Bank and FAO projections uncritically; those sanguine forecasts could simply be ignored - eventually to fade away into bureaucratic obscurity. But rather than abdicate leadership, it would be better for these important institutions to update their forecasting methods. The global food supply can't be reliably estimated without the help of water resource experts, plant physiologists, agronomists, and weather scientists. It's time for the World Bank and FAO econometricians to go beyond mere extrapolations and adopt a more interdisciplinary approach.