In the past 150 years, humanity has released over 2,000 gigatons of carbon dioxide (CO2) into the atmosphere, elevating the CO2 concentration by 50 percent compared to pre-Industrial Revolution figures.

While a portion of that carbon lingers in the atmosphere or is absorbed by oceans, roughly one-third reverts back to the land, “consumed” by vegetation during the process of photosynthesis. This so-called land sink serves as a significant method through which the Earth removes CO2 from the air to maintain its carbon equilibrium. However, substantial inquiries linger regarding the duration of carbon retained by this mechanism.

A recent study from Caltech reveals that the carbon within the land sink is predominantly stored in “nonliving pools”—within soils and sediments rather than in living entities like trees and plants. This is noteworthy, as carbon in these nonliving reservoirs will remain sequestered there for significantly longer—10 to 100 times longer—than it would in plant matter.

The investigation is a collaborative effort between the labs of Woody Fischer, a 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 overseen by Caltech on behalf of NASA. The findings are detailed in a paper published in the journal Science on March 21.

A tree experiencing a surplus of CO2 resembles a person deciding how to allocate extra cash. A tree can use CO2 to produce more leaves, develop deeper roots, or grow thicker bark. Each of these choices has varied consequences for how long that carbon can be kept out of the atmosphere. For instance, leaves decompose rapidly and return carbon to the air, while roots release carbon into the soil where it may last for centuries to millennia.

“There exists a fundamental question: If we’re introducing more CO2 into the atmosphere, does it encourage the growth of more trees and plants?” Fischer mentions. “We recognize that one-third of that CO2 goes back to the land. But where exactly does it go? What are the limits of the land sink in absorbing excess CO2?”

In the innovative study, former postdoctoral researcher at Caltech, Yinon Bar-On, utilized time-series data from 1992 to 2021 to compile an inventory of the Earth’s vegetation over time. This included various methodologies, ranging from analyzing minute details of small forest areas—such as leaf fall, tree growth, and the carbon and water exchanges within the system—to reviewing global biomass surveys collected from satellites. Bar-On is skilled in comprehensive quantitative analyses of intricate biogeochemical systems like the carbon cycle.

Although models have anticipated that heightened atmospheric CO2 would lead to increased trees and vegetation, the research team discovered that this was not the case. Conversely, carbon within the land sink is primarily held in nonliving pools such as soils and sediments.

“The Earth is assisting us by absorbing our surplus carbon emissions,” Frankenberg states. “And since this carbon is seemingly stored in these nonliving reservoirs, we can expect it to remain in the land for an extended period.”

Comprehending where excess carbon is stored on land is crucial for shaping policies and decisions regarding land use to combat climate change, as human actions can directly influence the land sink. For instance, the findings indicated that disposing of organic waste like wood and paper in landfills appears to be a significant sink for global CO2, but concurrently, it may produce methane, which has a stronger warming impact than CO2.

This research paper is titled “Recent increases in global terrestrial carbon stocks are predominantly stored in nonliving pools.” Bar-On is the principal author of the study. Alongside Fischer and Frankenberg, co-authors include Xiaojun Li and Jean-Pierre Wigneron from Université de Bordeaux in France, Michael O’Sullivan and Stephen Sitch from the University of Exeter in the UK, and Philippe Ciais from Université Paris-Saclay in France. Funding was supplied by the Rothschild Postdoctoral Fellowship, the Resnick Sustainability Institute at Caltech, the David and Lucile Packard Foundation, and the Schmidt Science Fellows.