Science

Volcanoes were a safety valve for Earth's long-term climate, research suggests

Scientists explored the combined impact of processes in the solid Earth, oceans and atmosphere over the past 400 million years.

Chains of volcanoes have been responsible for both emitting and then removing atmospheric carbon dioxide (CO2) over geological time, new research suggests.

These processes stabilised temperatures at Earth’s surface.

Scientists explored the combined impact of processes in the solid Earth, oceans and atmosphere over the past 400 million years.

Natural break-down and dissolution of rocks at Earth’s surface is called chemical weathering.

This is important because the products of weathering – elements like calcium and magnesium – are flushed into oceans by rivers.

Once in the ocean they form minerals that lock up CO2. This feedback mechanism regulates atmospheric CO2 levels, and in turn global climate, over geological time.

Dr Tom Gernon, associate professor in Earth Science at the University of Southampton, and a Fellow of the Turing Institute, said: “In this respect, weathering of the Earth’s surface serves as a geological thermostat.

“But the underlying controls have proven difficult to determine due to the complexity of the Earth system.”

Eelco Rohling, Professor in Ocean and Climate Change at the Australian National University (ANU)and co-author of the study, said: “Many Earth processes are interlinked, and there are some major time lags between processes and their effects.

“Understanding the relative influence of specific processes within the Earth system response has therefore been an intractable problem.”

The researchers constructed a novel Earth network, incorporating machine-learning algorithms and plate tectonic reconstructions.

This enabled them to identify the dominant interactions within the Earth system, and how they evolved through time.

The study found that continental volcanic arcs were the most important driver of weathering intensity over the past 400 million years.

Today, continental arcs comprise chains of volcanoes in, for example, the Andes in South America, and the Cascades in the US.

These volcanoes are some of the highest and fastest eroding features on Earth, and because the rocks are fragmented and chemically reactive, they are rapidly weathered and flushed into the oceans.

Martin Palmer, professor of geochemistry at the University of Southampton and co-author of the study, said: “It’s a balancing act.

“On one hand, these volcanoes pumped out large amounts of CO2 that increased atmospheric CO2 levels.

“On the other hand, these same volcanoes helped remove that carbon via rapid weathering reactions.”

Researchers say the findings cast doubt on a long-held concept that Earth’s climate stability over tens to hundreds of millions of years reflects a balance between weathering of the seafloor and continental interiors.

Dr Gernon said: “The idea of such a geological tug of war between the landmasses and the seafloor as a dominant driver of Earth surface weathering is not supported by the data.”

He added: “Unfortunately, the results do not mean that nature will save us from climate change.

“Today, atmospheric CO2 levels are higher than at any time in the past three million years, and human-driven emissions are about 150 times larger than volcanic CO2 emissions.

“The continental arcs that appear to have saved the planet in the deep past are simply not present at the scale needed to help counteract present-day CO2 emissions.”

However, the researchers suggest their findings still provide critical insights into how society might manage the climate crisis.

They suggest that artificially enhanced rock weathering – where rocks are pulverised and spread across land to speed up chemical reaction rates – could play an important role in safely removing CO2 from the atmosphere.

The findings indicate that such schemes may be deployed optimally by using calc-alkaline volcanic materials (those containing calcium, potassium and sodium), like those found in continental arc environments.

Dr Gernon said: “This is by no means a silver bullet solution to the climate crisis – we urgently need to reduce CO2 emissions in line with IPCC mitigation pathways, full stop.

“Our assessment of weathering feedbacks over long timescales may help in designing and evaluating large-scale enhanced weathering schemes, which is just one of the steps needed to counteract global climate change.”

The research is published in the journal Nature Geoscience.

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