Uncertainty in response of net land sink to rising atmospheric CO2 undermines climate projections

Background/Question/Methods

The terrestrial carbon cycle plays a critical role in regulating the accumulation of carbon in the atmosphere and recent work suggests that the strength of the terrestrial sink is growing. Understanding the factors controlling land carbon uptake is critical for reducing uncertainties in projections of future climate. The relative importance of changing climate, rising atmospheric CO₂, and other factors, however, remains unclear despite decades of research. For example, results from some recent studies indicate that CO₂ fertilization is the predominant driver of the growth in the terrestrial carbon sink, particularly in the tropics. However, other studies argue that models may be overly sensitive to changes in atmospheric CO₂ concentration, calling into question our understanding of carbon cycle-climate feedbacks. And, while CO₂ fertilization may be a dominant driver for the influx of carbon to the biosphere, its importance for the net land carbon sink, relative to the full suite of environmental factors, is unclear.

Results/Conclusions

We use an ensemble of twelve models from the Multi-scale Synthesis and Terrestrial Model Intercomparison Project (MsTMIP) and a series of sensitivity simulations to attribute changes in historical global land carbon uptake (and loss) to key biophysical and biogeochemical drivers. Because models vary widely in their representation of land-atmosphere carbon dynamics, each model can be viewed as one possible realization of terrestrial carbon cycling and its key drivers. We show that models disagree on the primary driver of cumulative carbon uptake for 85% of vegetated land area. Disagreement is largest in model sensitivity to rising atmospheric CO₂ which shows almost twice the variability in cumulative land uptake since 1901 (1 s.d. of 212.8 PgC vs. 138.5 PgC, respectively). We find that variability in CO₂ and temperature sensitivity is attributable, in part, to their compensatory effects on carbon uptake, whereby comparable estimates of carbon uptake can arise by invoking different sensitivities to key environmental conditions. Conversely, divergent estimates of carbon uptake can occur despite being based on the same environmental sensitivities. Together, these findings imply an important limitation to the predictability of carbon cycling and climate under unprecedented environmental conditions.