San Francisco Chronicle: For the first half of his life, potato farmer Gregorio Huanuco used the same formula that had dictated the survival of his ancestors for generations. Huanuco, 48, waited for rains to fall on his small parcel of land to sustain his crops of potatoes as well as various tubers and quinoa. When ripened, his family ate what they needed and sold the surplus in the nearby central city of Huaraz. But by 1990, Huanuco began noting strange climatic patterns in this village of 500 …