Tectonic plates were, until recently, the Earth’s major climate driver

Prior to the last 150 years, Earth’s climate fluctuated on the scale of millennia, occasionally getting cold enough to become a snowball Earth, or a hothouse.

What stopped the earth from sticking in one of these extreme temperature zones? According to research in Nature, the answer lies with plate tectonics and the carbon cycle.

According to the researchers’ modelling, the movement of tectonic plates assisted a worldwide carbon conveyor belt, which in turn regulated the Earth’s climate.

“We are the first ones to actually explicitly model all the CO2 emissions and storage processes that are embedded in plate tectonics,” says first author Professor Dietmar Müller, coordinator of the University of Sydney’s EarthByte group.

The researchers used thermochemical models to understand how carbon has moved across the globe over the past several hundred million years.

“These thermochemical models have existed for quite a long time,” says Müller. “Their purpose is: they track materials, rocks and the minerals through temperature and pressure space.

“Our work, described in this paper, is to actually connect such a model that tracks the transformation of minerals on their way into the deeper Earth to a plate tectonic model.

“We connected these two types of models for the first time. And we were therefore able to, over a period of 250 million years, track every parcel of plate material, as it gets created at the mid-ocean ridge in the oceans, and then travels across the ocean basin all the way to a subduction zone, ultimately, where it then goes down.”

This devilishly complex modelling allowed the researchers to see how plate tectonics could drive the carbon cycle. When tectonic plates move quickly, increased volcanic activity vents more carbon dioxide into the atmosphere, causing the Earth to warm.

But as the tectonic plates slow down, they have the opposite effect. This happens because of the growth of massive mountain chains, such as the Himalayas, caused by collision between tectonic plates.

“These mountain chains have a lot of continental rocks with a lot of silicate minerals in them, like granite, sandstone or basalt,” says Müller.

“They get uplifted, and they get eroded. And it turns out the chain reaction that is involved in dissolving these rocks – very weak acid rain, because it has [atmospheric] CO2 dissolved in it – the rocks get eroded. And ultimately the dissolved products get carried to the oceans by rivers.”

From there, the carbon gets stored away by phytoplankton and other ocean life, eventually falling to the bottom of the sea floor as a carbon sink.

“You store more and more carbon, you withdraw carbon from the atmosphere, and then it gets cooler again,” says Müller.

Across 250 million years, it’s understandable that this heating-cooling model is a little simplified.

“Of course, there are many complex feedback processes involved in how this actually works,” says Müller.

“We have the oceans, and the atmosphere, and the biosphere and soils. In reality, the whole chain of [processes] is much more complex than my simplified geological story.

“But our point is that the ultimate driver of these long-term climate cycles is actually plate tectonics.” At least up until the past century, anyway.

“Humans have overtaken this process recently, quite clearly,” says Müller.

But this discovery has implications for managing our current climate change. The silicate dissolving pattern on mountains is a carbon-removal technique – and it’s possible to enhance.

“There’s a carbon capture and storage process that has been explored that can perhaps work on human timescales,” says Müller.

Ground rocks such as olivine could enhance this dissolving process, washing more CO2 from the atmosphere into the oceans. Spreading rock dust on agricultural soil could have a similar effect.

Müller says that their modelling will be relevant to researchers “trying to alleviate human-induced global warming”.

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