AT 50, Americo González Caldua has lived a life that coincided with the retreat of the glaciers of the high Andes. With each passing year, as the cold mountain temperatures rise, the ice moves uphill another 20 yards.

In the dry season, when the storms break and the research season begins, Mr. González heads to the icy heights of the nearby peaks with survey prisms and markers. He brings heavy drilling augers that bore core samples deep below the surface of the glaciers and reveal a picture of the atmosphere from a century ago.

He knows the ice sheet as well as anyone. Yet, Mr. González is neither a mountaineer nor a scientist: He’s the man most often found trailing behind them, carrying their equipment.

Mount Everest has the Sherpas. The Cordillera Blanca, a snowy mountain range in northern Peru, has the ayudantes de campo, or field helpers, in Spanish. They are the mountain men, mainly indigenous, who have watched a huge expanse of ice that was known to them for centuries shrink drastically in the space of one generation.

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