Old bombs reveal new insights: Plants store more carbon, but for a shorter time frame, than we thought

Analysis of radiocarbon produced during nuclear bomb testing in the 1960s suggests that current Earth system models underestimate carbon uptake into terrestrial vegetation and soils. But, say the study’s authors, this storage is more short-lived than previously thought. The findings suggest that anthropogenic carbon dioxide will not reside as long in the terrestrial biosphere as models currently predict. Accurate climate predictions, crucial for developing effective climate policies, require a robust representation of the global carbon cycle. It’s thought that vegetation and soils account for taking up approximately 30% of anthropogenic carbon dioxide (CO₂) emissions, yet the processes underlying the net CO₂ sink into the terrestrial biosphere are poorly constrained. This is particularly true for processes involved in global net primary productivity (NPP) – the rate at which plants produce new tissues from atmospheric CO₂ – and for carbon turnover rates. Current NPP estimates vary widely due to reliance on statistical or model-based predictions and limited site-scale measurements. Nuclear bomb testing in the 1950s and 1960s increased atmospheric radiocarbon (¹⁴C), which was absorbed by the terrestrial biosphere. Tracking the accumulation of bomb radiocarbon in the biosphere post-nuclear testing could help evaluate NPP and carbon turnover rates. However, direct observation of global ¹⁴C accumulation has been a challenge. Heather Graven and colleagues address this by combining a new budget of radiocarbon produced by nuclear bombs with model simulations to provide a top-down constraint on global NPP for the 1960s (1963 to 1967). Graven et al. found that current models underestimate the magnitude of NPP in the 1960s. According to the findings, NP in the 1960s was at least 63 petagrams of carbon per year (PgC/yr), which implies a current rate of 80PgC/yr given increases in productivity over time. This is more than the 43 to 76 PgC/yr that current models predict. Graven et al. argue that this discrepancy is due to underestimating the carbon stored in short-lived or non-woody plant tissues. “Because the uptake and turnover of carbon are the main controls on the anthropogenic CO₂ sink in the terrestrial biosphere, the results of our study suggest that the storage of anthropogenic carbon in the terrestrial biosphere is likely more short-lived and more vulnerable to future changes than previously thought,” Graven et al. write.