How Earth’s thermostat controls the weather

Rocks, rain, and carbon dioxide help control Earth’s climate for thousands of years—like a thermostat—through a process called weathering. A new study led by scientists at Penn State University may improve our understanding of how this thermostat responds when temperatures change.

“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 exist and support life,” said Susan Brantley, Geosciences professor at Penn State ⁽¹⁾. “The idea is that the weathering of silicate rocks is this thermostat, but no one has ever really agreed on its sensitivity to temperature.”

Since many factors are involved in weathering, it has been difficult to use only the results of laboratory experiments to create global estimates of how weathering responds to changes in temperature, the scientists explain.

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

“When you do experiments in the lab and you don’t take samples from the soil or a river, you get different values,” explains Brantley. “So what we’re trying to do in this research is look at these different spatial scales and figure out how we can make sense of all this data that geochemists around the world have been accumulating about weathering on the planet. And this study is a model of How can we do it”.

A weak acid falling to Earth

Weathering is part of a balancing act of carbon dioxide in Earth’s atmosphere. Volcanoes have emitted vast amounts of carbon dioxide throughout Earth’s history, but instead of turning the planet into a hot house, the greenhouse gas is slowly removed through weathering. The rain takes carbon dioxide from the atmosphere and creates a weak acid that falls to Earth and wears away silicate rocks on the surface. The by-products are carried by streams and rivers to the ocean, where the carbon ends up locked up in sedimentary rocks, the scientists explain.

“It has long been hypothesized that the balance between carbon dioxide entering the atmosphere from volcanoes and carbon dioxide carried away by erosion over millions of years keeps the planet’s temperature relatively constant. Brantley explains. “The key is that when there is more carbon dioxide in the atmosphere and the planet warms up, weathering speeds up and pulls out more carbon dioxide. And when the planet is colder, weathering slows down.”

But much is still unknown about the sensitivity of weathering to changes in temperature, partly because of the long spatial and temporal scales involved. “In a soil profile, you see an image of a soil where the camera shutter has been open for a million years: there are built-in processes that have been going on for a million years and you are trying to compare it to an experiment in a two-year-old flask,” Brantley explained.

According to Brantley, the field of hotspot science—which examines landscapes from the highest vegetation to the deepest groundwater—has helped scientists better understand the complex interactions that influence weathering.

For example, rocks must fracture for water to get into the cracks and begin to break down the materials. For that to happen, the rock must have large exposed surfaces, and that is less likely to happen in regions where the soil is deeper.

weathering to climate change

“Only when you start to cross spatial and temporal scales do you start to see what’s really important,” Brantley said. “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 area forms out there in the natural system, you’re never going to be able to predict the system.” real.”

The scientists reported in the journal Science ⁽²⁾ that the temperature sensitivity measurements in the laboratory were lower than the estimates for soils and rivers in their study. Using laboratory and field observations, they extended their conclusions to estimate the global temperature dependence on weathering.

Their model can be useful for understanding how weathering will respond to climate change in the future and for evaluating human attempts to increase weathering to extract more carbon dioxide from the atmosphere, such as carbon sequestration.

“One idea has been to improve weathering by digging up a lot of rock, crushing it, transporting it and putting it out in the field to weather,” Brantley explains. “And that will work, it’s already working. The problem is that it’s a very slow process.” Although warming can speed up weathering, scientists say it could take thousands or hundreds of thousands of years to extract all the carbon dioxide that humans have added from the atmosphere.