For hundreds of millions of years, the Earth’s climate has warmed and cooled with natural fluctuations in the level of carbon dioxide (CO₂) in the atmosphere. Over the last century, humans have increased their CO₂ levels to a maximum of two million years, exceeding natural emissions, especially by burning fossil fuels, causing global warming that can make uninhabitable parts of the world uninhabitable.
What can be done? As Earth scientists, we look at how natural processes have recycled carbon from the atmosphere on Earth and in the past to find possible answers to this question.
Our new research published in Nature shows how tectonic plates, volcanoes, eroded mountains, and seabed sediments have controlled the Earth’s climate in the geological past. Taking advantage of these processes can play an important role in maintaining the “golden age” climate that our planet has enjoyed.
From the greenhouse to the ice age
Greenhouse and ice climates have existed in the geological past. The Cretaceous greenhouse (which lasted approximately 145 million to 66 million years ago) had atmospheric CO₂ levels above 1,000 parts per million, compared to the current 420, and temperatures up to 10 ℃ higher than current.
But the Earth’s climate began to cool about 50 million years ago during the Cenozoic era, culminating in an icy climate in which temperatures dropped to about 7 ℃ colder than today.
What drove this dramatic change in the global climate?
The Earth evolved from a greenhouse climate in the Cretaceous period (left) to an ice climate in the following Cenozoic era (right), giving rise to layers of ice inside. Image credit: F. Guillén and M. Antón / Wikimedia Commons
Our suspicion was that the Earth’s tectonic plates were to blame. To better understand how tectonic plates store, move, and emit carbon, we built a computer model of the tectonic “carbon conveyor belt.”
The carbon conveyor belt
Tectonic processes release carbon into the atmosphere in the mid-ocean ridges, where two plates move away from each other, allowing magma to rise to the surface and create a new oceanic crust.
At the same time, in the ocean trenches, where two plates converge, the plates are thrown down and recycled back to the deep Earth. On their way down they transport carbon back into the Earth’s interior, but they also release some CO₂ through volcanic activity.
The Earth’s tectonic carbon conveyor belt transports massive amounts of carbon between the Earth’s depths and the surface, from oceanic ridges to subduction zones, where oceanic plates carrying sediments from the deep sea are recycled from new inside the Earth. The processes involved play a key role in the Earth’s climate and habitability. Image Credit: Author provided
Our model shows that the climate of the Cretaceous greenhouse was caused by very fast tectonic plates, which drastically increased CO₂ emissions from ocean ridges.
In the transition to the Cenozoic ice climate, the movement of tectonic plates slowed and volcanic CO₂ emissions began to decline. But to our surprise, we discovered a more complex mechanism hidden in the conveyor belt system that involved building mountains, continental erosion, and burying the remains of microscopic organisms on the seabed.
The hidden cooling effect of Cenozoic slow-moving tectonic plates
Tectonic plates are slowing down due to collisions, which in turn lead to the construction of mountains, such as the Himalayas and the Alps formed over the past 50 million years. This should have reduced volcanic CO₂ emissions, but instead our carbon conveyor belt model revealed an increase in emissions.
We tracked its source to the deep carbon-rich deep marine sediments that were pushed down to feed the volcanoes, increasing CO₂ emissions and canceling out the slowing effect of the plates.
So what exactly was the mechanism responsible for the fall in atmospheric CO₂?
The answer lies in the mountains that were responsible for braking the plates in the first place and in the storage of carbon in the depths of the sea.
As soon as the mountains form, they begin to erode. CO₂-containing rainwater reacts with a series of mountain rocks, breaking them. Rivers carry dissolved minerals to the sea. Marine organisms then use the dissolved products to build their shells, which eventually form part of the carbon-rich marine sediments.
As new mountain ranges formed, more rocks eroded, accelerating this process. Massive amounts of CO₂ were stored and the planet cooled, although some of these sediments were subdued by their degassing of carbon through arc volcanoes.
The limestone on the white cliffs of Dover is an example of carbon-rich marine sediment, made up of the remains of small skeletons of marine plankton calcium carbonate. Image credit: I Giel / Wikimedia, CC BY
Weathering of rocks as a possible carbon dioxide removal technology
The Intergovernmental Panel on Climate Change (IPCC) says large-scale deployment of carbon sequestration methods is “inevitable” if the world is to achieve zero net greenhouse gas emissions.
Weathering of igneous rocks, especially rocks such as basalt that contain a mineral called olivine, is very efficient in reducing atmospheric CO₂. Spreading olivine on beaches could absorb up to a trillion tons of CO₂ from the atmosphere according to some estimates.
The current rate of human-induced warming is such that reducing our carbon emissions very quickly is essential to prevent catastrophic global warming. But geological processes, with a little human help, can also play a role in maintaining the Earth’s “Golden Rites” climate.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Image credit: David Mark of Pixabay