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WASHINGTON — If the world hopes to make meaningful progress on climate change, it won’t be enough for cars and factories to get cleaner. Our cows and wheat fields will have to become radically more efficient, too.

That’s the basic conclusion of a sweeping new study issued Wednesday by the World Resources Institute, an environmental group. The report warns that the world’s agricultural system will need drastic changes in the next few decades in order to feed billions more people without triggering a climate catastrophe.

The challenge is daunting: Agriculture already occupies roughly 40 percent of the world’s land and is responsible for about a quarter of humanity’s greenhouse gas emissions. But with the global population expected to grow from 7.2 billion people today to nearly 10 billion by 2050, and with many millions of people eating more meat as incomes rise, that environmental impact is on pace to expand dramatically.

Based on current trends, the authors calculated, the world would need to produce 56 percent more calories in 2050 than it did in 2010. If farmers and ranchers met that demand by clearing away more forests and other ecosystems for cropland and pasture, as they have often done in the past, they would end up transforming an area twice the size of India.