As humans turn up the heat on Earth, geoscientist Aaron Bufe and other researchers are figuring out how our planet has stayed so cool in the past. Throughout Earth’s history, volcanoes have belched out varying amounts of atmosphere-warming carbon dioxide, but the runaway greenhouse effect that should have boiled our oceans away eons ago never came to fruition, and temperatures have been relatively stable.
“Over 500 million years and probably longer, Earth’s temperature has mostly varied between about 10°C and 30°C. Somehow, Earth has always sort of managed to get back to a temperature that makes life possible,” Bufe said.
Bufe, of Ludwig Maximilian University of Munich, and his colleagues found that mountains with moderate uplift rates have been sucking carbon out of the atmosphere, outpacing young, fast-eroding ranges once thought to be carbon drawdown champs. Their recent work, published in Science, weighs in on an ongoing debate over whether mountains act as sinks or sources of atmospheric carbon.
Mountains Rise, Carbon Falls
Since the late 1970s, many scientists have thought that the rapid rise of mountains such as the Himalayas, Rockies, and parts of the Andes removes naturally emitted carbon, thus keeping the greenhouse effect in check.
Warmer climates produce more rain, and carbonic acid in raindrops dissolves silicate minerals brought to the surface within the uplifting mountains. Carbon, calcium, and other molecules then flow into the ocean, where they form compounds used by marine organisms to build shells and skeletons. Earth has sequestered carbon through this silicate weathering process for hundreds of thousands of years.
“Then how do you explain the long-term decline in atmospheric CO2, growth of polar ice sheets, and cooling over millions of years?”
But about a decade ago, scientists discovered that sulfide minerals such as pyrite add carbon dioxide (CO2) to the atmosphere in amounts that could negate silicate weathering storage. That’s because sulfides (and carbonates) break down very fast in places where there’s lots of erosion, such as in the Himalayas.
The finding puzzled researchers. “Then how do you explain the long-term decline in atmospheric CO2, growth of polar ice sheets, and cooling over millions of years?” said Jeremy Rugenstein, a paleoclimatologist at Colorado State University and a coauthor of the new study.
The Link Between Erosion and Weathering
A few years ago, Bufe and Rugenstein and their colleagues set out to solve that mystery and began by asking, “Can we actually measure the sensitivity of CO2 drawdown and release to erosion?”
Erosion (the removal of rocks and minerals from an outcrop) influences weathering (the breakdown of rocks and minerals) because it exposes fresh minerals to the elements. But existing studies disagree about the relationship between these processes. Some say silicate weathering tracks with erosion, whereas others show that the amount of silicate in rivers doesn’t change regardless of erosion rate.
“This optimum emerged that you couldn’t really see from the data alone.”
In 2021, the researchers got a hint that the answer may lie somewhere in the middle, when data Bufe collected in southern Taiwan, where erosion varies wildly, revealed that as erosion rates increased, silicate weathering eventually leveled off. Quickly dissolving sulfides and carbonates, meanwhile, kept up with the flow of sediments from mountain to river, turning the topography from a sink to a source of atmospheric CO2.
In expanding the scope of the study to include mountain ranges in New Zealand and Sichuan, China—places where erosion rates also vary—the researchers saw a pattern. “This optimum emerged that you couldn’t really see from the data alone,” Bufe said.
Maximum CO2 consumption in all three locations occurred where erosion was about 0.07 millimeter per year. Slower than that, there aren’t enough silicate rocks exposed to weather; faster, and silicate doesn’t have time to completely dissolve.
Bufe pointed to medium-sized mountains such as the Juras in Europe and those in the Black Forest in Germany and along the Oregon coast as prime examples in this Goldilocks range, where the erosion rate is just right. There, most carbon-sourcing sulfides and carbonates weathered away long ago.
The fact that all three data sets end up supporting each other is “incredibly powerful.”
“They’ve taken a bit of a risk by looking at these three quite different mountain locations. But the results are really cool and worth it,” said Bob Hilton, a sedimentologist at the University of Oxford who was not involved in the study. The fact that all three data sets end up supporting each other is “incredibly powerful,” he said.
More research is needed to get a complete picture of how landscapes process carbon, Bufe said. In floodplains downriver from where he took samples, silicate rock may continue to weather. And organic carbon isn’t accounted for in these results, nor is a common contributor to the sediment: basalt.
This “more nuanced way of thinking about the Earth system over long timescales” is what’s needed to puzzle out how minerals, erosion, and rain control climate, said Pennsylvania State University aqueous geochemist Susan Brantley, who wasn’t involved in the research.
—Martin J. Kernan, Science Writer
2 May 2024: This article has been updated to clarify that basalt contributes to sediment.