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Global agricultural policies have averted hunger, but have also enhanced illness, environmental degradation and climate change while wasting natural resources and undermining biodiversity. The need to rethink those policies has been clear for decades, yet little has changed. Just as it would be a shame to waste the Ukraine crisis by failing to rethink energy policies, it would be equally disheartening if we fail to use this war as an opportunity to rethink our agricultural policies.

Famine in countries around the world, most markedly in Africa, is said to be inevitable. It stems from the presumed loss of Russian and Ukrainian wheat output, about 100 million tons a year or a quarter of the world's total, and Russia's exports of nitrogen fertilizer, about 7 million tons annually preinvasion, or 7% of global use.

For April, the food price index published by the United Nations Food and Agriculture Organization was up 30% from a year ago. Global fertilizer prices were up 125% in January from a year before, and rose another 17% from the beginning of the year to March.

Given the seasonality of agricultural production and lingering supply-chain problems, no policy shift can combat this year's shortfalls. But famine is still avoidable. The U.S. and Canada now have about 44 million tons of wheat in their stockpiles. North American lesser grains double these stocks to levels that can fully sustain 45 million people for a year. Wealthy nations' stocks of dry legumes and nuts raise this to 50 million person-years just months away from the next North American wheat harvest.

And then there is livestock feed. Over 250 million tons of wheat, barley, oats and other cereals are globally used for feed, with more than 90 million in the U.S., Canada, western Europe and Australia alone. This investment delivers shockingly little.

One hundred kilograms of feed protein served to livestock yields 10 to 15 kilograms of egg, dairy or poultry protein, or only 3 kilograms of beef protein. The world thus annually sacrifices for livestock production at least 220 million tons of nutrient-rich cereals, more than double the Russian and Ukrainian wheat shortfall.

The wheat famine in Egypt, Tunisia and Yemen expected this year, as well as the political unrest that such food shortages sometimes propel, could therefore be prevented by utilizing existing cereal and legume stocks and redirecting grain from livestock feed to lifesaving human food.

What about the long-term?

Consider the diets of two hypothetical people. The first eats the mean U.S. diet, in size and composition. The other's diet is identical in every way, except the beef Americans normally consume — some 50 to 70 grams daily — is replaced with a mix of plant-based alternatives. Comparing resource needs of only the beef or its plant-based alternative, that portion of the diet requires about 0.3 acres of cropland and more than 12 kilograms of nitrogen fertilizer annually for the beef eater, but less than 0.1 acres and 3 kilograms of nitrogen fertilizer for the plant eater.

We can use the beef-to-plant replacement to counteract future wheat shortfalls like the one created by the Ukraine invasion. With annual wheat yields of about 1,300 to 1,400 kilograms per acre in the U.S. and Canada, and 2,700 to 3,600 kilograms in northwestern Europe, fully making up the shortfalls by producing 50 million additional tons of wheat in each continent requires 36 million to 38 million acres, and 14 million to 19 million acres, respectively.

Each person who replaces beef with plants saves almost a quarter-acre, which would free these acreages. To fully offset such shortages as the one caused by the Ukraine invasion requires 215 million to 250 million North Americans and Europeans adopting the beef-to-plant transition.

Such a transition would be fertilizer-neutral, with the dietary shift saving just enough fertilizer to offset the elevated demands for extra wheat to counteract hunger in Ukraine.

Beyond saving millions from hunger with no added fertilizer needs, the partial dietary shift would reduce cardiovascular disease and cancer death rates by 10% to 18%. Those who go the extra mile by forgoing all meat will enjoy even higher reductions, while also lowering their weight and blood cholesterol and glucose levels.

If, as predicted, millions will soon go hungry, it will not be a "Putin famine" but a readily preventable famine of choice, arising because the people and leaders of wealthy nations have decided that preventing it is too inconvenient.

Gidon Eshel is a geophysicist and research professor at Bard College. He is author of "Spatiotemporal Data Analysis" and the upcoming "Planetary Eating." He wrote this for Bloomberg Opinion.