Parra, like many farmworkers here, was not fully aware of the invisible consequences of his work. But scientists who have studied this valley for decades know that in these precise moments and conditions — when water mixes with nitrogen fertilizer, and when no crop is in the ground to absorb it — huge surges of nitrous oxide gas are released into the atmosphere.

Down on the valley floor, Rafael Parra bent to the work of feeding the world — and unintentionally warming it.

A layer of chalk-white fertilizer had been scattered on the barren ground. Tractors had cut long furrows in the dry and crumbling soil. The wheat seeds would not be planted for days, but it was time to release the laughing gas.

Parra plunged one end of an old, plastic tube into an irrigation canal, generating the suction that sent water gurgling into the drought-parched earth. It was a low-tech, gravity-fed form of irrigation used for generations here in the Yaqui Valley, a storied breadbasket of Mexico.

“That’s all there is to it,” he said.

Parra, like many farmworkers here, was not fully aware of the invisible consequences of his work. But scientists who have studied this valley for decades know that in these precise moments and conditions — when water mixes with nitrogen fertilizer, and when no crop is in the ground to absorb it — huge surges of nitrous oxide gas are released into the atmosphere.

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