In the desert near Agua Dulce, north of Los Angeles, hikers along the Pacific Crest Trail who reached mile marker 502 encountered a cistern of water that smelled bad and tasted worse, with a dead rat floating inside. They got out their filters and refilled their bottles anyway. “Will update if I get sick,” one wrote on a message board to those coming up behind. The message was just one sign of how global warming is affecting life along the trail, where, during a hot season nearly devoid of rain, water tanks and caches were more important than ever, the last line of defense against dehydration. At least some hikers willing to take their chances.