Most experts agree it would take a superpower’s reach to ensure the safe passage of planes needed to deliver payloads of dust to the stratosphere in a geoengineering exercise. But neither the US nor China would lightly cede control over the global thermostat, says Nielsen, and distrust and poor communication are a significant risk: “Any action related to the issue by one actor can be perceived as malintent by the other.” Overall result not surprising, but I didn’t anticipate the details.