During the last 540 million years, the Earth's temperature has experienced three long-term cooling events. New evidence relates all three to the same phenomenon: enormous continental collisions occurring in the tropics. The enhanced rock weathering these produce draws carbon dioxide out of the atmosphere, and cools the planet.
The era commonly referred to as the last Ice Age, ending around 10,000 years ago, was just part of a multimillion-year glacial era, punctuated by short interglacials like the one we are in now. The warmer interludes are well explained by cycles in the Earth's orbit that alter the amount and timing of sunlight received, but the lower average over the last 35 million years has been debated.
A paper in Science attributes the modern baseline to the collision between India and Asia that caused the Himalayas. Moreover, the authors argue, two similar previous events, known as arc-continent collisions, also produced long planetary cool periods. Similar plate collisions outside the tropics, such as the formation of the Ural Mountains, had little effect.
Arc-continent collisions bring large amounts of oceanic basalt to the surface to be exposed to the atmosphere. These rocks are rich in calcium and magnesium, both of which react with atmospheric carbon dioxide and lock it away as carbonate rocks. Remove enough carbon dioxide and the outcome is inevitably a colder planet. The question geologists have pondered is whether enough rocks are exposed in these events to account for major cooling events.
MIT's Dr Oliver Jagoutz argues that in tropical regions, the rock exposure is sufficient, but at higher latitudes, weathering is too slow to cool the climate to the point where glaciers develop at the poles. Tropical basalt provinces can draw down CO2 100 times as fast as non-tropical basalt regions of the same size.
"We think that arc-continent collisions at low latitudes are the trigger for global cooling," Jagoutz said in a statement. "This could occur over 1-5 million square kilometers, which sounds like a lot. But in reality, it's a very thin strip of Earth, sitting in the right location, that can change the global climate."
Skeptics might object the Himalayas, at around 25º north, aren't tropical, but they spring from two separate collisions, which each took place closer to the equator before continental movements pushed everything north.
Jagoutz shows similar low-latitude collisions coincided with the Late Ordovician and Permo-Carboniferous cool periods, respectively 455-440 and 335-280 million years ago.
The movement of the Australian continental plate northwards is creating a similar province in Indonesia, whose rocks account for 9-14 percent of global carbon dioxide drawdown. Sadly, this is still many times slower than we are burning fossil fuels, and the practicality of ideas to speed it up are debated.