In 1815, Mount Tambora erupted on Sumbawa, an island of modern-day Indonesia. Historians regard it as the volcano eruption with the deadliest known direct impact: roughly 100,000 people died in the immediate aftermath.

But far more died over the next several years, due to secondary effects that spread all over the globe, says Gillen D’Arcy Wood, author of Tambora: The Eruption That Changed the World.

“What happened after Tambora is that there was three years of climate change,” he says. “The world got colder, and the weather systems changed completely for three years. And so you had widespread crop failure and starvation all from Asia to the United States to Europe.”

Volcanoes near the equator can cause global weather changes if their eruptions are powerful enough to release gases into the stratosphere. This gas gets trapped since it is too high to be washed away by rain, then travels along the equator and spreads out toward the poles. This decreases the amount of heat that passes through the stratosphere from the sun.

This doesn’t just affect whether you should put on a sweater or not; it has profound effects on the ecosystem you live in. With Tambora’s eruption, cooling temperatures led to decreased rainfall, failed crops, and mass starvation in many parts of the world.

It’s difficult to know how many people died because of starvation conditions, but “the death toll is probably about a million people, at least, in the years afterwards,” Wood says. “If you want to include the fact that Tambora unleashed a global pandemic of cholera … then the death toll goes into tens of millions.”

Cholera already existed before the eruption, but the colder temperatures caused by Tambora’s eruption led to the development of a new strain in the Bay of Bengal. Fewer people had immunity to this new strain of cholera, which then spread throughout the world.

Could there have been volcanoes long ago that caused more deaths than Tambora? Perhaps, but because we have no way of knowing, historians generally agree that Tambora caused the most immediate deaths.

For example, the Krakatoa eruption in Indonesia in 1883 is more famous than Tambora because it was a “new media event” that spread around the world through telegrams and photography, Wood says. But this eruption was actually weaker than Tambora’s. And so, even though it had an enormous death toll at 36,000, it was less deadly overall. And while Mount Vesuvius’ destruction of Pompeii in 79 A.D. is one of the most famous volcanic eruptions, its death toll of 2,000 was only a fraction of Tambora’s.

Joseph Manning, a professor of classics and history at Yale University, says that in today’s world, the after-effects of volcanoes are much more dangerous than the direct impact. Because of technological advances, we’re able to more accurately predict when volcanic eruptions will occur in time for evacuations and safety measures, like when flights were canceled in anticipation of a 2017 Mount Agung eruption in Bali; or in January 2018, when the Philippines began evacuating residents near Mount Mayon before a major eruption.

“There’s probably less and less risk of people actually dying from the event,” he says. “But there’s a lot of risk with hydroclimate shocks and drought all over the world, especially in monsoon areas of the world like India, like East Asia, like East Africa.”

Manning believes we don’t worry enough about “those kinds of impacts, which are going to affect not only direct death toll,” but also our ecosystems for years to come.