Volunteers who leave water in the desert describe rising fears of vigilantes and climate peril

It was a blustery day in the Sonoran desert as a group of humanitarian aid volunteers hiked through a vast dusty canyon to leave gallons of bottled water and canned beans in locations where exhausted migrants could find them.

Empty plastic bottles, rusty cans and footprints heading north were among the signs of human activity strewn between the towering saguaro and senita cacti, in an isolated section of the Organ Pipe Cactus national monument – about 20 miles (32km) north of the US-Mexico border.