Rock weathering ‘thermostat’ is too slow to prevent climate change

Rock weathering has helped keep Earth’s climate relatively stable for millions of years, but the process isn’t fast enough to keep up with human carbon footprint.

To Jason Arunn Murugesu

Atmospheric rocks, rain, and carbon dioxide reactions have helped stabilize climate throughout Earth’s history, but they cannot prevent carbon dioxide emissions from causing severe warming, they say. But the findings could help devise better ways to trap CO2 and slow climate change.

Over a million years on Earth, gas emissions from volcanoes should have nearly tripled the amount of carbon in the atmosphere and oceans. Such an increase in carbon dioxide, a greenhouse gas, would have led to much higher temperatures. Instead, the climate remained relatively stable during that time, allowing liquid water to persist and life to thrive.

This stability is largely dependent on the removal of CO2 by weathering processes, says Susan Brantley of Pennsylvania State University. Simply put, it starts when CO2 gas reacts with rainwater to produce carbonic acid, which dissolves rocks such as limestone. This rock erosion produces soluble minerals and bicarbonates (dissolved carbon). These products are washed out to sea, eventually forming carbonate minerals that trap carbon in rocks.

Previous studies have found that chemical weathering accelerates at higher temperatures, extracting more CO2 from the atmosphere and potentially acting like a thermostat to regulate climate. Brantley and her colleagues wanted to determine whether this was the case in all conditions.

“If we’re going to disrupt this system by pumping more carbon dioxide into the atmosphere, we need to understand how this system works,” says Brantley.

The team looked at several lab studies detailing chemical weathering processes and compared these findings to field experiments that measured weathering rates in 45 soils around the world. According to Brantley, reconciling lab data with field data was difficult. This is because laboratory studies cannot accurately replicate weathering processes that take thousands of years in the real world.

Combining all this data allowed the researchers to determine that chemical weathering is particularly sensitive to temperature only in areas with high rainfall and high rates of rock erosion due to this rainfall. It means that natural rocks weather too slowly to counteract the very large amounts of CO2 released by human activity.

But some scientists have proposed efforts to slow climate change by mining and crushing the rocks and then laying them down in crop fields to allow extra weathering to occur. It suggests that this idea, called weathering of rocks, may not be so exotic. “To make it work on a large enough scale, you have to mine a lot of rock, spread it out over a very large area, and place it in an area with a lot of rainfall,” he says. “But it could be one of the processes we use to slow climate change.”

The question of temperature sensitivity of weathering is important because it helps us understand past and future climates, says Michael Bickle of the University of Cambridge. “This paper provides an important conceptual advance.”

The study is a big deal, says Penny King of the Australian National University in Canberra. “We now have new ideas that we can test to explain weathering, which will help our goal of locking carbon dioxide in stable minerals,” she says.

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