After Tambora

IF ALIENS had been watching the Earth during 1815 the chances are they would not have noticed the cannon fire of Waterloo, let alone the final decisions of the Congress of Vienna or the birth of Otto von Bismarck. Such things loom larger in history books than they do in astronomical observations. What they might have noticed instead was that, as the year went on, the planet in their telescopes began to reflect a little more sunlight. And if their eyes or instruments had been sensitive to the infrared, as well as to visible light, the curious aliens would have noticed that as the planet brightened, its surface cooled.

Mount Tambora (pictured), a volcano on the Indonesian island of Sumbawa, was once similar in stature to Mont Blanc or Mount Rainier. But in April 1815 it blew its top off in spectacular fashion. On the 10th and 11th it sent molten rock more than 40 kilometres into the sky in the most powerful eruption of the past 500 years. The umbrella of ash spread out over a million square kilometres; in its shadow day was as night. Billions of tonnes of dust, gas, rock and ash scoured the mountain’s flanks in pyroclastic flows, hitting the surrounding sea hard enough to set off deadly tsunamis; the wave that hit eastern Java, 500km away, two hours later was still two metres high when it did so. The dying mountain’s roar was heard 2,000km away. Ships saw floating islands of pumice in the surrounding seas for years.