How the Planet Stores Our Excess Carbon Emissions

Over the last 150 years, humans have emitted over 2,000 gigatons of carbon dioxide (CO2) into the atmosphere, increasing the CO2 concentration by 50 percent from pre-Industrial Revolution levels.

While some of that carbon remains in the atmosphere or is absorbed by the oceans, about one-third goes back into the land, "eaten" by plants that consume CO2 during photosynthesis. This so-called land sink is a major mechanism by which the planet draws CO2 down from the air to balance its own carbon budget. But major questions remain about the longevity of carbon stored by this process.

A new study from Caltech finds that carbon in the land sink is stored primarily in "nonliving pools"—in soils and sediments, rather than in living matter like trees and plants. This is remarkable, as carbon in nonliving reservoirs will stay sequestered there for much longer—10 to 100 times as long—than it would in plants.

The research is a collaboration between the laboratories of Woody Fischer, professor of geobiology, and Christian Frankenberg, the Chandler Family Professor of Environmental Science and Engineering and a research scientist at NASA's Jet Propulsion Laboratory, which is managed by Caltech for NASA. The study is described in a paper appearing in the journal Science on March 21.

A tree with a surplus of CO2 is like a person deciding how to spend a surplus of cash. A tree can utilize CO2 to grow more leaves, or deeper roots, or thicker bark. Each of these options has different ramifications for how long that carbon can stay out of the atmosphere. Leaves, for example, decompose quickly and release carbon back into the air, whereas roots secrete carbon into soil where it will remain for hundreds of years to millennia.

"There's this fundamental question: If we're putting more CO2 into the air, is it promoting the growth of more trees and plants?" says Fischer. "We know one-third of that CO2 goes back into the land. Where specifically is it going? What are the limits of the land sink to take up excess CO2?"

In the new study, former Caltech postdoctoral scholar Yinon Bar-On used time-series data from 1992 to 2021 to take an inventory of the Earth's vegetation over time. This involved multiple approaches, from studying the minutiae of small patches of forest—how many leaves fell, how much did the trees grow, how much carbon and water went in and out of the system—to examining global surveys of biomass taken from satellites. Bar-On specializes in holistic quantitative analyses of complex biogeochemical systems like the carbon cycle.

Though models have predicted that more atmospheric CO2 would result in more trees and vegetation, the team found that this was not the case. Instead, carbon in the land sink is stored in nonliving pools such as soils and sediments.

"The planet is doing us a favor by taking up our excess carbon emissions," Frankenberg says. "And because this carbon is apparently stored in these nonliving pools, we can expect the carbon to remain in the land for a longer time."

Understanding where excess carbon is being stored on land is important for policies and decision-making around land use to mitigate climate change because human activities can directly impact the land sink. For example, the study found that burial of organic waste such as wood and paper in landfills seems to be an important sink for CO2 globally, but, at the same time, may generate methane, which has an even stronger warming effect than CO2.

The paper is titled "Recent gains in global terrestrial carbon stocks are mostly stored in nonliving pools." Bar-On is the study's first author. In addition to Fischer and Frankenberg, co-authors are Xiaojun Li and Jean-Pierre Wigneron of Université de Bordeaux in France, Michael O'Sullivan and Stephen Sitch of the University of Exeter in the United Kingdom, and Philippe Ciais of Université Paris-Saclay in France. Funding was provided by the Rothschild Postdoctoral Fellowship, the Resnick Sustainability Institute at Caltech, the David and Lucile Packard Foundation, and the Schmidt Science Fellows.