After a smaller eruption in the 1960s a high-altitude plane had flown through the plume and come back with a residue on its windshield that an Australian scientist had evaluated by licking it. "Painfully acid" was his verdict. He’d experienced exactly the same sensation as Texas oilmen sampling sour crude on their fingers. So there was already evidence, prior to Pinatubo, that volcanoes hurled sulfur compounds into the stratosphere. The 1991 blast found scientists ready to make more sophisticated measurements than windshield licking, and provided the basis for decades of research and modeling of so-called solar geoengineering—the term for any climate mitigation scheme that was based on bouncing part of the sun’s rays back into space.