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Sowing and reaping: It’s at the core of human society. In ancient times, agriculture helped make the first great civilizations possible. Now, with the help of modern machinery and fertilizers, farm yields are so high that we produce more than enough food to feed everyone on the planet.

Yet, hunger remains stubborn. Malnutrition is growing in some parts of the world. And, the planet’s land and water resources are so poorly used, according to a new United Nations report, that, as climate change puts ever-greater pressure on agriculture, the ability of humanity to feed itself is in peril.

We are reaping what we sow in another sense.

The report , published in summary form Thursday by the Intergovernmental Panel on Climate Change, magnifies a dual challenge: how to nourish a growing global population, but do so in a way that minimizes agriculture’s carbon footprint.

Answering that challenge requires a huge overhaul of how we use land and water for food production, experts say. And it also requires a hard look at who gets to eat what.