Earth's 'geological thermostat' is too slow to prevent climate change

Reactions between rocks, rain and carbon dioxide in the atmosphere have helped to stabilise the climate throughout Earth’s history, but they won’t prevent our carbon emissions from causing severe warming, a study of these processes has concluded. However, the findings could help us 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 ocean. Such an increase in CO2, which is a greenhouse gas, should have led to much higher temperatures. Instead, the climate has remained relatively stable in that time, allowing liquid water to persist and life to flourish.

This stability is largely down to removal of CO2 by the weathering process, says Susan Brantley at Pennsylvania State University. In simple terms, this starts when CO2 gas reacts with rainwater to form carbonic acid, which dissolves rock such as limestone. This rock erosion leads to the production of soluble minerals and bicarbonate – a dissolved form of carbon. These products are then washed into the oceans, where they form carbonate minerals that ultimately lock the carbon away in rock.

Previous studies have found that chemical weathering may speed up in higher temperatures, taking more CO2 out of the atmosphere and thus acting to control the climate, a bit like a thermostat. Brantley and her colleagues wanted to determine if this was true in all conditions.

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

The team looked at several lab studies that detailed the chemical weathering process and compared these findings with field experiments measuring weathering rates in 45 soils from across the world. It has been difficult to reconcile lab data with that from the field, says Brantley, because lab studies can’t accurately replicate weathering processes that take thousands of years in the real world.

Combining all this data, the researchers were able to determine that chemical weathering is only particularly temperature-sensitive in areas with high rainfall and high rates of rock erosion due to this rainfall. This means natural rock weathering is too slow to counteract the very large amounts of CO2 being released by human activities.

However, some scientists have proposed efforts to slow down climate change by mining and grinding rock and laying it out on crop fields so that extra weathering occurs. The results suggest this idea, called enhanced rock weathering, may not be so outlandish. “To make it work in a big enough way you would have to mine a lot of rock and spread it over a very large area and make sure it’s in an area with high rainfall,” says Brantley. “But it might be one of the processes we use to slow down climate change.”

Questions over the temperature sensitivity of weathering are important because they will help us understand past climates and what the climate may look like in the future, says Michael Bickle at the University of Cambridge. “This paper makes an important conceptual advance.”

This study is a big deal, says Penny King at the Australian National University in Canberra. “We now have a new idea that can be tested to explain weathering and this can help us with our goal to trap carbon dioxide in stable minerals,” she says.

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