AS CLIMATE-CHANGE negotiators from nearly 200 countries meet this week in South Africa, Carter Roberts, the World Wildlife Fund’s president, argues that the leading environmental challenge of this century won’t be global warming. It will be feeding people.

The WWF reckons that about 70 percent of the world’s land either is used to produce food or is unsuitable for that. Global population is heading from 7 billion toward a possible 10 billion by 2100. Per capita consumption rises as countries develop. Some vacant land may not be all that fertile.

The smart response is to improve how humans produce food by applying ever-more-efficient agricultural techniques more widely. Tim Searchinger, a land-use expert at Princeton, points to South America, where cattle ranchers could raise more livestock on less land by seeding their fields with clover or growing legumes, for example. The WWF is pressing large companies to demand that suppliers intensify their agricultural production on existing farmland.

What you won’t hear the WWF and other environmental groups cheering for, though, is another important tool for efficiency: genetic modification, which enhances the ancient practice of artificial genetic selection and could make crops more productive and more resistant to drought and bad weather. Unfounded opposition is particularly extreme in Europe, blocking just the sort of breakthroughs environmentalists and world poverty advocates should encourage.

Ending government subsidies for supposedly green biofuels, which backers promote as renewable, would also reduce strain on land resources.

Even with the full application of scientists’ know-how and a reduction in irrational subsidies, though, Mr. Searchinger argues in a new report from the National Wildlife Federation that simply using land better won’t guarantee conservation in some parts of the tropics. More efficient agriculture could enhance incentives to clear more land, some of it extremely ecologically valuable, since local producers might then be able to grow food cheaply enough to replace imports.

Governments will have to adjust those incentives to reflect the large amounts of carbon emitted when clearing rain forest, making sensible conservation profitable, just as environmentalists will have to embrace the science that makes more efficient agriculture possible.