Nuclear bomb–produced isotope reveals how vegetation cycles through carbon

It’s hard to figure out exactly how much carbon is taken from the atmosphere by plants. During photosynthesis, plants use carbon dioxide to build new plant matter. But they also release some of that carbon back into the air, and the complexity of the problem increases significantly at the global scale. That’s why global climate models incorporate a wide range of estimates of vegetation’s yearly carbon uptake, from 43 billion to 76 billion metric tons. Those numbers come from site-based studies, ecological models, and satellite data.

In a new study, Heather Graven (Imperial College London) and colleagues take a different approach to measuring carbon uptake by vegetation and estimate that number to be at least 80 billion metric tons annually, above the range currently used in climate models. That might sound like good news, but the researchers say that carbon from plants also returns to the atmosphere faster than expected, and therefore vegetation may be a less stable carbon sink than previously thought.

The new estimates are derived from measurements of the concentration of radioactive carbon-14 created by the detonation of nuclear weapons in the atmosphere. Starting in 1945, more than 500 nuclear weapons were detonated on Earth’s surface. An international treaty moved most testing underground beginning in 1963. By focusing on the small window from 1963 to 1967 when radiocarbon concentrations peaked but little new bomb radiocarbon was added to the atmosphere, Graven and her colleagues were able to construct a well-constrained budget for ¹⁴C and then extend it to the complete carbon budget. “It’s a short period right after the most intense testing activity,” says Graven, “so the radiocarbon has to enter the vegetation through photosynthesis before it goes into the litter or soils.”

Graven and colleagues used data collected in the 1960s and 1970s that documented radiocarbon concentrations in the stratosphere, the troposphere, and the ocean. Those data had been integrated into atmospheric and ocean models in previous studies to produce robust inventories of radiocarbon movement between reservoirs. Over that time, radiocarbon moved from the stratosphere to the troposphere and then into vegetation and the ocean. The deficit between what was lost from the atmosphere and what was gained by the ocean mostly went into new plant growth.

Detailed comparison of the researchers’ results with a suite of the most widely used climate models shows that most simulations underestimate the annual uptake of carbon by plants. Because that finding creates an imbalance in the existing models, it implies that the models also overestimate how long carbon is stored by plants. Given that many climate change mitigation strategies target increased vegetation as a means to remove CO₂ from the atmosphere, it is critical to know the role of plants in the carbon cycle. The new study highlights gaps in that understanding. The researchers emphasize that the capacity to pull carbon from the atmosphere by increasing vegetation, such as through reforestation, may be more limited than we thought. (H. D. Graven et al., Science 384, 1335, 2024 .)