Did you know that the Earth’s climate when it’s in a normal state is warm and comfortable? You wouldn’t be criticized for not knowing that, considering that over the past billion years, this general state has been interrupted by severe cold weather that covers the poles—and sometimes the rest of the planet—with ice. But periods with no glaciers dominated for three quarters of the past billion years.

So, what caused the ice ages?

According to research from the University of California, Berkeley, the ice ages were triggered by mountain formation in the tropics, which happens when continental land masses collide with volcanic island arcs such as those in Indonesia or Hawaii.

The study, which appeared in the most recent issue of the journal Science, concludes that when volcanic arcs and continents collide due to plate tectonics, they trigger global cooling. And there’s just such a collision happening right now. Parts of the Indonesian Archipelago are being pushed upward into mountains on the northern margin of Australia.

The earth’s climate is largely controlled by the amount of carbon dioxide in the atmosphere. When islands and continents collide, mountains containing rocks known as ophiolites, which have a high capacity to remove carbon from the atmosphere, emerge. Thus, when ophiolites emerge and store much of the carbon in the atmosphere, the earth cools.

“Earth has a long-running carbon sequestration program,” said researcher Nicholas Swanson-Hysell of UC Berkeley. “We know that these processes keep Earth’s climate in balance, but determining what causes shifts between non-glacial and glacial climates on million-year time scales is a long-standing puzzle.”

The researchers used state-of-the-art models of Earth’s paleogeography to see the position of mountain-building events like the one happening in the Indonesian archipelago over the last half-billion years. They found that all three of the major ice ages during that time period had begun with volcanic archipelago-continent collisions in the tropics. No collisions that happened in more temperate or cooler regions triggered ice ages.

“While we thought this process was important, the relationship between such environments in the tropics and glacial climate was clearer than we expected,” said Swanson-Hysell.

This theory also explains why ice ages end: basically, once the collisions stop and less rock is exposed—or as the rocks drift out of the tropics—carbon sequestration becomes less efficient, and CO2 levels rise as volcanic gas emissions continue.

You might think that all these continental collisions and carbon storage would be good news because they may be able to reverse global climate change. Alas, no. The geologic processes that consume carbon dioxide are slow and can’t keep up with the massive CO2 emissions that are coming from burning fossil fuels. Swanson-Hysell said that over the millennia, Earth’s natural carbon sequestration program will restore balance, but that will be a long wait for modern civilization—which, of course, became so successful in the planet’s current cooler climate.

Photo: Indonesia’s Mount Bromo volcano with smoke emanating from the crater. Credit: Shutterstock