For billions of years, our planet has remained within a relatively narrow temperature band, allowing life as we know it to evolve.
Now, a new study on a key player in Earth’s global thermostat is helping to better explain how and why that is. The study begins to answer some long-standing questions about silicate weathering, a chemical process involving rocks and rainwater that has slowly locked away carbon dioxide from the atmosphere.
Giving and Taking Carbon
Volcanic eruptions release large amounts of carbon dioxide into the atmosphere over geologic timescales. Because it is a greenhouse gas, and therefore traps heat, unchecked volcanic carbon dioxide would cause average temperatures to keep rising; something must also be taking the gas back out of the atmosphere for Earth’s climate to remain stable.
That’s where the silicate weathering process comes in. Carbon dioxide dissolves in rainwater, forming a weak acid that reacts with the silica in rocks, breaking them down and forming compounds like bicarbonate. These minerals flow to the ocean, where marine organisms use them to make shells; when the organisms die, their shells become limestone on the ocean floor, locking away carbon for millions of years.
These reactions happen faster when temperatures are higher, meaning that the hotter Earth gets, the faster carbon dioxide gets locked away. Over time, as more of the gas is pulled out of the atmosphere, the planet cools, and the process slows down again. Scientists have long theorized that this feedback loop could play a big role in keeping Earth’s climate mild, even as such factors as changes to the Sun’s intensity could have pushed the planet’s climate to extremes.
“Everything tells us that these fluxes of carbon into the system and out of the system have to be incredibly balanced on these long timescales,” said Matthew Winnick, an aqueous geochemist at the University of Massachusetts Amherst who wasn’t involved with the study.
“This basic problem of going from the lab to the field has puzzled me my whole life.”
Scientists have conducted laboratory experiments on the process and gathered data on silicate weathering in the field for decades now. But the lab and field measurements of how quickly silicate rocks break down have never agreed with each other.
“They’re often off by orders of magnitude,” said Susan Brantley, a geochemist at Pennsylvania State University and a coauthor of the study. “This basic problem of going from the lab to the field has puzzled me my whole life.”
Simply put, there’s too much complexity in the real world to effectively model it in the lab. Though it’s clear that hotter temperatures speed up weathering reactions, there are other factors involved as well, such as erosion and rainfall, each with its own complexities. And weathering happens over hundreds of thousands of years, whereas typical lab experiments run for a few months to a year.
To get a better grasp of the problem, Brantley and her colleagues evaluated decades of existing lab and real-world studies to link together many of these processes and better understand how they might change weathering rates.
The researchers found that rainfall is the most important factor governing rates of silicate weathering around the world. In many places, it’s simply too dry for weathering to happen at a meaningful rate. Erosion is also key; it was the limiting factor in 72% of the environments where weathering actually happened. Temperature was the most important limiting factor in just 28% of places where silicate weathering occurred.
“Seeing the actual quantification of this to be able to put these into carbon cycle models is really interesting,” Winnick said. “This goes a long way towards us being able to understand how sensitive weathering is to temperature right now.”
The researchers used their findings to estimate how much global silicate weathering rates should increase for every 1°C rise in temperature. Overall, the temperature sensitivity of weathering was toward the lower end of what previous studies had predicted; reaction rates increased by about 3% for every 1°C increase in temperature.
That temperature sensitivity varies from place to place, depending on other factors, Brantley said. It’s very high in places that have both a lot of rainfall and high erosion rates, she added. Erosion tumbles rocks through a landscape, breaking them apart and exposing more of their surface to rainfall, which provides new fuel for weathering reactions.
In dry environments, weathering is less sensitive to temperature because there is less water available to react with rocks. Even in some wet regions, thick soils held in place by plants limit erosion rates, meaning weathering will also occur slowly.
Ultimately, most places on Earth aren’t ideal for silicate weathering to happen quickly, and the process won’t speed up by a great amount, even as temperatures warm in the future.
Improving on Nature
Though natural silicate weathering may not save us from climate change, some scientists have proposed that supercharging the process might make a dent. So-called “enhanced weathering” involves spreading ground-up silicate minerals such as feldspar on agricultural fields, making them available to chemically react with carbon dioxide–laden rainwater.
“The more you can understand about silicate weathering, the natural process, the more you will be able to tailor any studies on enhanced weathering.”
The new study has valuable insights for this work as well, pointing out where enhanced weathering is most likely to be effective and showing how much of an impact it could have on global carbon dioxide levels. Predicting how much ground-up rock would be necessary is likely to be difficult, however, she explained.
“The more you can understand about silicate weathering, the natural process, the more you will be able to tailor any studies on enhanced weathering,” said Edward Tipper, an Earth and environmental scientist at the University of Cambridge who was not involved in the study.
Enhanced weathering is likely to offer only limited utility in fighting climate change, though it could play a role, Brantley said.
But there’s still much to be learned, such as how biological organisms affect weathering, Brantley said. Trees hinder erosion, whereas microbes can speed it up. Future work could take them into account. For now, one of the most basic processes behind life on Earth retains some of its ancient mystery.
—Nathaniel Scharping (@NathanielScharp), Science Writer