“When the San Francisco Bay Area mandated shelter-in-place March 16, it created a natural experiment for UC Berkeley’s Ron Cohen, who had established an inexpensive pollution sensor network in local neighborhoods. The sensors showed carbon dioxide emissions plummeting 25 percent in the subsequent six weeks, mostly because of a 48 percent drop in traffic. Networks like this — soon to be emplaced in Glasgow — can track greenhouse gases and progress toward lowering them, including the impact of electric vehicles.”