In the foothills of the Sierra Madre mountains, the carcasses of starving cattle rotted in a bone-dry reservoir. Useless fishing nets hung on dusty fences. Rowboats were stranded in the sand. 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, 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.