The amount of carbon stored by plants is more vulnerable to climate change than previously thought, according to a study published in Science. This work changes how we understand nature’s role in mitigating climate change, including the potential for nature-based carbon removal projects such as mass tree-planting.
Plant productivity has increased since the early 1900s, and more CO2 is currently used up by plants than is released back into the air. Researchers know that approximately 30% of CO2 emissions from human activities are stored in plants and soils each year, reducing climate change and its impacts. However, the exact mechanisms to explain how this storage happens and its future stability are not yet well understood.
In this study, radiocarbon (14C) – a radioactive isotope of carbon – was combined with model simulations to understand how plants use CO2 at a global scale. Radiocarbon exists naturally, but nuclear bomb testing in the 1950s and 1960s increased the level of 14C in the atmosphere. This extra 14C was available to plants globally, giving researchers a good tool to measure how fast they could take it up. The team analysed the levels of 14C in plants between 1963 and 1967 to assess how quickly carbon moves from the atmosphere to vegetation and what happens to it once it’s there. This period was selected because it had no nuclear detonations, and there was a relatively constant level of 14C in the atmosphere.
The results, conducted by a team led by Dr Heather Graven from Imperial College London, show that current climate models significantly underestimate the amount of CO2 that is used by vegetation globally each year but overestimate how long it remains trapped there. “Plants across the world are actually more productive than we thought they were,” said Dr Graven. In other words, plants take up carbon dioxide quicker but also release it sooner than previously thought. This means carbon from human activities will likely be released back into the atmosphere sooner than previously predicted.
“Many of the strategies being developed by governments and corporations to address climate change rely on plants and forests to draw down planet-warming CO2 and lock it away in the ecosystem,” said Dr Graven. “But our study suggests that carbon stored in living plants does not stay there as long as we thought. It emphasises that the potential for such nature-based carbon removal projects is limited, and fossil fuel emissions need to be ramped down quickly to minimise the impact of climate change.”
The authors emphasise that we need to improve theories about how plants grow and interact with their ecosystems to understand better how the biosphere is mitigating climate change. “Scientists and policymakers need improved estimates of historical land carbon uptake to inform projections of this critical ecosystem service in future decades. Our study provides critical insights into terrestrial carbon cycle dynamics, which can inform models that are used for climate change projections,” said Dr Will Wieder.
Graven HD, Warren H, Gibbs HK, Khatiwala S, Koven C, Lester J, Levin I, Spawn-Lee SA, Wieder W. Bomb radiocarbon evidence for strong global carbon uptake and turnover in terrestrial vegetation. Science. 2024 Jun 21;384(6702):1335-1339