In a rooftop greenhouse near downtown Denver, cash crops are thriving on hydroponic life support. Arugula. Chard. Escarole. Cabbage. “And basil,” said Altius Farms CEO Sally Herbert, plucking a bright leaf. “Which you really should taste. Because it’s magnificent.” The vertical farm is one of many Colorado models for coping with increasing water scarcity in the western United States, as climate change makes droughts more frequent and more severe.