Study unveils new clues as to how the ‘Earth’s thermostat’ controls climate

Rocks, rain, and carbon dioxide help control Earth’s climate — like a thermostat — for thousands of years through a process called weathering. A new study led by Penn State scientists could improve our understanding of how this thermostat responds to changes in temperature.

“Life has existed on this planet for billions of years, so we know that Earth’s temperature has remained constant enough for liquid water to support life,” said Susan Brantley, professor at Evan Pugh University and Barnes Professor of Geosciences at Penn State. “The idea is that the weathering of silicate rock is that thermostat, but no one has ever really settled on its sensitivity to temperature.”

Because many factors go into weathering, using results from laboratory experiments alone to produce global estimates of how weathering responds to temperature changes has been a challenge, the scientists said.

The team combined laboratory measurements and soil analysis from 45 soil locations around the world and many watersheds to better understand the weathering of major rock types on Earth, and used those results to create a global estimate of how weathering responds to temperature .

“If you do experiments in the lab or you take samples from the ground or from a river, you get different readings,” Brantley said. “So in this research we tried to look at these different spatial scales and figure out how to make sense of all this data that geochemists around the world have been collecting about weathering on the planet. And this study is a model for how we can do that.”

Weathering is part of a balancing act of carbon dioxide in Earth’s atmosphere. Volcanoes have released large amounts of carbon dioxide throughout Earth’s history, but instead of turning the planet into a greenhouse, the greenhouse gas is slowly being removed through weathering.

Rain removes carbon dioxide from the atmosphere and creates a weak acid that falls to the ground, eroding silicate rock from the surface. The byproducts are carried by streams and rivers to the ocean, where the carbon is eventually trapped in sedimentary rock, the scientists said.

“It has long been thought that the balance between carbon dioxide entering the atmosphere from volcanoes and being pulled out by weathering over millions of years keeps the planet’s temperature relatively constant,” Brantley said. “The key is, when there’s more carbon dioxide in the atmosphere and the planet gets hotter, weathering goes faster and pulls out more carbon dioxide. And when the planet is cooler, weathering slows down.”

However, much is still unknown about how sensitive weathering is to temperature changes, due in part to the long spatial and temporal scales.

“In a soil profile, you see an image of the soil that has the camera shutter open sometimes for a million years — there are built-in processes that go on for a million years and you’re trying to compare that to a two-year flask experiment. ‘ Brantley said.

Brantley said the field of critical zone science — which studies landscapes from the highest vegetation to the deepest groundwater — has helped scientists better understand the complex interactions that affect weathering.

For example, rocks must fracture to allow water to enter cracks and degrade the materials. This requires the rock to have large, exposed surfaces, and this is less likely in regions with deeper soils.

“It’s only when you start transcending space and time scales that you see what’s really important,” Brantley said. “The surface is really important. You can measure all the rate constants you want for that solution in the lab, but until you can tell me how the surface forms out there in the natural system, you’ll never be able to predict the real system.”

The scientists report this in the journal Science that temperature sensitivity measurements in the lab were lower than estimates from soils and fluxes in their study. Using observations from the laboratory and from field sites, they upscaled their results to estimate the global temperature dependence of weathering.

Their model can be helpful in understanding how weathering will respond to future climate change and in assessing human-caused attempts to increase weathering to pull more carbon dioxide from the atmosphere — such as carbon sequestration.

“One idea was to enhance weathering by digging up a lot of rock, grinding it up, transporting it, and putting it in the fields for weathering to take place,” Brantley said. “And that will work – it’s already working. The problem is that it is a very slow process.”

Although warming could speed up weathering, it could take thousands or hundreds of thousands of years to remove from the atmosphere all the carbon dioxide that humans have added, the scientists said.

Other Penn State researchers who participated in the study were Andrew Shaughnessy, a graduate student in the Department of Geosciences, and Marina Lebedeva and Victor Balashov, senior scientists at the Earth and Environmental Systems Institute.