Introduction
Exciting new research by LEMONTREE PI Dr. Heather Graven and her team has been published in the journal Science, shedding new light on global carbon turnover in terrestrial vegetation. By analysing the radiocarbon signal from nuclear bomb tests in the 1950s, these scientists are gaining a better understanding of carbon cycling. This groundbreaking work is crucial for refining our global carbon cycle models and improving our predictions of net primary productivity (NPP).
As the LEMONTREE project aims to develop a land ecosystem model based on new theory, observations and experiments, work that assesses current models and identifies ways to improve them is core to our project goals. This work by Dr. Graven and colleagues has assessed some current models and brings to light aspects of carbon cycling for the LEMONTREE team to consider in their own models.
Context
During the 1950s and 1960s, nuclear bomb testing released a wave of carbon-14 (¹⁴C) into the atmosphere. This ¹⁴C was incorporated into vegetation during photosynthesis.
The researchers focused on the period from 1963 to 1967 because it followed the intense period of nuclear testing but did not include any large nuclear detonations. Since it is difficult to estimate the amount of 14C produced by the bombs, they could account for the movement of 14C between the atmosphere, ocean and biosphere over 1963 to 1967 without having to consider new production of 14C by bombs. Using a budgeting approach with measurements of 14C in the atmosphere and ocean allowed them to quantify how much ¹⁴C went into vegetation over 1963 to 1967 and compare it to models.
Model Performance
The only model that explicitly simulates 14C is the Community Land Model version 5.0, a widely-used model that is part of the Community Earth System Model version 2 (CESM2). Interestingly, estimates for the 1960s from CLM5.0 are significantly lower than the observation-based estimates from the research team. This discrepancy suggests that the NPP in the model is too low and/or the carbon allocation between short-lived and long-lived pools is incorrect.
In order to compare the estimated 14C accumulation with other models that do not explicitly simulate 14C, the authors created model emulators for vegetation for a further six models. Models with high NPP perform best in estimating ¹⁴C accumulation. The authors found that increasing NPP to woody vegetation pools or increasing the amount of carbon in nonwoody pools like leaves and fine roots are needed to increase ¹⁴C accumulation. Overall, it seems NPP needs to be higher than in current model formulations. This research highlights several ways current models need adjusting and is something that the LEMONTREE team will need to consider in future modelling efforts.
Crucially, this work reveals that global carbon sinks are more vulnerable and likely have a higher carbon turnover rate than we previously thought. This is particularly important considering the reliance of climate change mitigation strategies on nature-based carbon removal solutions. If predictions of biospheric carbon storage are wrong, this could significantly impact not only our ability to use these mitigation strategies, but our predictions of our future climate risk.
The research team who published this paper included scientist German physicist Dr. Ingeborg Levin, a pioneer of greenhouse gas research, who sadly passed away in February this year. The authors have dedicated the paper to her memory.
You can find the full publication by Dr. Graven and her colleagues here.
Heather D. Graven et al. Bomb radiocarbon evidence for strong global carbon uptake and turnover in terrestrial vegetation. Science 384,1335-1339 (2024) DOI:10.1126/science.adl4443
By Sophia Cain and Dr. Heather Graven