高清福利片

For hundreds of millions of years, Earth鈥檚 climate has warmed and cooled with natural fluctuations in the level of carbon dioxide (CO鈧�) in the atmosphere. Over the past century,聽聽to their highest in 2 million years 鈥撀犅犫�� mostly by burning fossil fuels, causing ongoing global warming that may make parts of the globe uninhabitable.

What can be done? As Earth scientists, we look to how natural processes have recycled carbon from atmosphere to Earth and back in the past to find possible answers to this question.

翱耻谤听聽published in Nature, shows how tectonic plates, volcanoes, eroding mountains and seabed sediment have controlled Earth鈥檚 climate in the geological past. Harnessing these processes may play a part in maintaining the 鈥溾�� climate our planet has enjoyed.

聽have existed in the geological past. The Cretaceous hothouse (which lasted from roughly 145 million to 66 million years ago) had atmospheric CO鈧� levels above 1,000 parts per million, compared with around 420 today, and temperatures up to 10鈩� higher than today.

But Earth鈥檚 climate began to聽聽during the聽, culminating in an聽聽in which temperatures dropped to roughly 7鈩� cooler than today.

What kickstarted this dramatic change in global climate?

Our suspicion was that Earth鈥檚 tectonic plates were the culprit. To better understand how tectonic plates store, move and emit carbon, we built a computer model of the tectonic 鈥渃arbon conveyor belt鈥�.

Tectonic processes release carbon into the atmosphere at mid-ocean ridges - where two plates are moving away from each other - allowing magma to rise to the surface and create new ocean crust.

At the same time, at ocean trenches - where two plates converge - plates are pulled down and recycled back into the deep Earth. On their way down they carry carbon back into the Earth鈥檚 interior, but also release some CO鈧� via volcanic activity.

The Earth鈥檚 tectonic carbon conveyor belt shifts massive amounts of carbon between the deep Earth and the surface, from mid-ocean ridges to subduction zones, where oceanic plates carrying deep-sea sediments are recycled back into the Earth鈥檚 interior. The processes involved play a pivotal role in Earth鈥檚 climate and habitability.聽

Our model shows that the Cretaceous hothouse climate was caused by very fast-moving tectonic plates, which dramatically increased CO鈧� emissions from mid-ocean ridges.

In the transition to the Cenozoic icehouse climate tectonic plate movement slowed down and volcanic CO鈧� emissions began to fall. But to our surprise, we discovered a more complex mechanism hidden in the conveyor belt system involving mountain building, continental erosion and burial of the remains of miscroscopic organisms on the seafloor.

Tectonic plates slow down due to collisions, which in turn leads to mountain building, such as the Himalayas and the Alps formed over the last 50 million years. This should have reduced volcanic CO鈧� emissions but instead our carbon conveyor belt model revealed increased emissions.

We tracked their source to carbon-rich deep-sea sediments being pushed downwards to feed volcanoes, increasing CO鈧� emissions and cancelling out the effect of slowing plates.

So what exactly was the mechanism responsible for the drop in atmospheric CO鈧�?

The answer lies in the mountains that were responsible for slowing down the plates in the first place and in carbon storage in the deep sea.

As soon as mountains form, they start being eroded. Rainwater containing CO鈧� reacts with a range of mountain rocks, breaking them down. Rivers carry the dissolved minerals into the sea. Marine organisms then use the dissolved products to build their shells, which ultimately become a part of carbon-rich marine sediments.

As new mountain chains formed, more rocks were eroded, speeding up this process. Massive amounts of CO鈧� were stored away, and the planet cooled, even though some of these sediments were subducted with their carbon degassing via arc volcanoes.

The Intergovernmental Panel on Climate Change (IPCC)聽聽large-scale deployment of carbon dioxide removal methods is 鈥渦navoidable鈥� if the world is to reach net-zero greenhouse gas emissions.

The weathering of igneous rocks, especially rocks like basalt containing a mineral called olivine, is very efficient in reducing atmospheric CO鈧�. Spreading olivine on beaches could聽, according to聽.

The speed of current聽聽is such that reducing our carbon emissions very quickly is essential to avoid catastrophic global warming. But geological processes, with some human help, may also have their role in maintaining Earth鈥檚 鈥淕oldilocks鈥� climate.

This study was carried out by researchers from the University of Sydney鈥檚聽, The University of Western Australia, the University of Leeds and the Swiss Federal Institute of Technology, Zurich using聽聽open access modelling software. This was enabled by Australia鈥檚 National Collaborative Research Infrastructure Strategy (NCRIS) via聽聽and The Office of the Chief Scientist and Engineer, NSW Department of Industry.

This piece was originally published in . Hero image:聽Ben Turnbull on Unsplash.