Mount Pinatubo
The Philippine volcano whose 1991 eruption cooled the entire planet
Before 1991, few outside the Philippines had heard of Mount Pinatubo, a forested peak on Luzon that had slept for some five centuries. Then it produced the second-largest volcanic eruption of the twentieth century, hurling a column of ash 35 kilometres into the sky and injecting so much sulphur into the stratosphere that it measurably cooled the entire planet for two years. That a single mountain could nudge the global thermostat made Pinatubo a landmark in earth science, and the months-long evacuation that preceded the climax stands as one of volcanology's greatest life-saving successes.
Pinatubo lies on the Luzon volcanic arc, where the South China Sea floor subducts beneath the Philippines. Its pre-eruption summit reached 1,745 metres, and the 1991 climax blew away the top and left a caldera 2.5 kilometres wide, now filled by a striking crater lake. The eruption coincided, disastrously, with Typhoon Yunya, whose rains turned falling ash into heavy wet cement that collapsed roofs across the region. For years afterward, monsoon rains remobilised the loose ash into devastating mudflows called lahars that buried towns far from the volcano itself.
The eruption's greatest scientific legacy floated invisibly overhead. Some 20 million tonnes of sulphur dioxide formed a stratospheric haze that reflected sunlight and lowered average global temperatures by about half a degree Celsius, an event so well documented it became a benchmark for climate models and even for debates about deliberate climate engineering. On the ground, the accurate forecasting by Filipino and U.S. scientists evacuated tens of thousands and demonstrated that volcanic disasters need not be unforeseeable. The eruption also hastened the closure of the nearby American Clark Air Base, reshaping the region's geopolitics.