The discovery would be "crucial for shaping future climate" policies for reducing greenhouse gas emissions and improving the capture and storage of carbon dioxide from the atmosphere, the study's authors said.
About one-third of CO2 released by human activities is stored in land-based carbon sinks like forests, which along with oceans help slow global warming by absorbing excess heat-trapping emissions.
But forests are under threat, and their capacity to soak up CO2 has been diminished due to global warming, disease, wildfires and large-scaled land clearing.
Recent studies have shown that Earth's carbon stocks are increasing, but how this is spread across land-based ecosystems has been less clear.
A major uncertainty has been the distribution between living vegetation like trees and other plants, and non-living matter like decaying wood and soil.
The authors said that understanding this in greater detail was vital because ecosystems face different environmental threats, and boast differing capacities to lock away carbon.
To address this question, an international team of scientists conducted a comprehensive assessment of global changes in carbon stored in woody vegetation between 1992 and 2019.
This study, published in Science, revealed that most of the CO2 accumulated over that period was locked away as non-living organic matter in soil, deadwood, and reservoirs like dams and landfills.
"Most terrestrial carbon gains are sequestered as nonliving matter and thus are more persistent than previously appreciated," the study said.
"These pools persist far longer than living biomass, suggesting that terrestrial carbon storage may be more stable over time than previously assumed," said a statement accompanying the study's release.
These findings contrast sharply with earlier studies that estimated living matter accounted for roughly 70 percent of the carbon stored on land.
Some parts of the Amazon, due to climate change and deforestation, have shifted from being a sink to source of CO2, while other landscapes under pressure are also transforming.
After storing carbon dioxide in frozen soil for thousands of years, the Arctic tundra has changed to being an overall source of CO2 emissions as the region warms up and is torched by wildfire.
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