The last three ice ages might have been triggered by tectonic collisions between Earth's continents, according to new research, and it could give us clues as to how the planet's climate might suddenly shift again in the future.
What's likely to have happened, the new study says, is these massive collisions pushed oceanic rocks up above the surface, into hot and humid tropical environments where they began to pull carbon dioxide out of the air. Given enough time, these chemical reactions could cool global temperatures and start an ice age.
The findings are based initially on an analysis of two sutures in the Himalayas – major fault zones where oceanic and continental plates have collided. Both sutures were originally formed near the equator, 50 million and 80 million years ago respectively, and happened just before major global cooling events.
"We think that arc-continent collisions at low latitudes are the trigger for global cooling," says one of the team, Oliver Jagoutz from the Massachusetts Institute of Technology (MIT).
"This could occur over 1-5 million square kilometres [0.4-1.9 square miles], 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."
You could think of it as the opposite of the process that's often associated with major extinction events – where huge amounts of carbon dioxide get released into the atmosphere from underground stores buried in rock.
Specifically in this case, it's the calcium and magnesium in the newly exposed rock that reacts with the carbon dioxide in the atmosphere and pulls it out of the air.
Adding the new research to previous work, the researchers were able to model the eruption and movement of more sutures even further back through time.
They found three major periods of suture formation that coincided with ice ages – in the Late Ordovician period (455-440 million years ago), the Permo-Carboniferous period (335-280 million years ago), and the Cenozoic period (35 million years ago to the present day). No sutures meant no ice ages.
Models of major tectonic activity (orange lines) over the last 540 million years. (MIT)
"We found that every time there was a peak in the suture zone in the tropics, there was a glaciation event," says Jagoutz. "So every time you get, say, 10,000 kilometres [6,214 miles] of sutures in the tropics, you get an ice age."
The current cooler period we're technically in from a 'big picture' perspective – despite the rapid warming we've managed to bring about over the last hundred years or so – may have been caused by a major suture zone active today in Indonesia, according to the team.
Carbon sequestration – where CO2 is artificially pulled out of the air – is something scientists are looking at to combat rising temperatures, and it's possible that we could learn a few tricks from the way that these tectonic movements suck up carbon dioxide.
However, the team behind the new study is skeptical that this process, which happens naturally over millions of years, is going to be any help to us as we struggle to keep temperatures down to habitable levels.
The same rocks are likely to be responsible for ending the ice ages they began as well, with oceanic rock gradually eroding to be replaced by rock that was less hungry for CO2.
"We showed that this process can start and end glaciation," says Jagoutz. "Then we wondered, how often does that work? If our hypothesis is correct, we should find that for every time there's a cooling event, there are a lot of sutures in the tropics."
The research has been published in Science.