Testing the impacts of natural climate variability on land carbon uptake and overshoot emission pathways in an Earth System Model

The eruption of Mount Tambora in 1815 cooled the global climate and caused famines due to crop failures in the "year without summer". Such changes to Earth system conditions, triggered by volcanic activity, increase the uncertainty surrounding future climate change. However, the subsequent response of land carbon uptake and its potential implications for future emission pathways are not well-explored. While multi-annual mean responses are predictable and aid emission budget estimations, higher-order variability in land carbon flux and the effects of non-CO₂ forcings, like volcanic aerosols, remain uncertain. Aerosols cause variability by altering the Earth’s radiation balance, reducing surface temperature, and modulating precipitation patterns, driving substantial regional climate change. Additionally, state-dependent non-linearities, such as regional sensitivity differences to aerosol forcing, complicate the climate’s response.

This study utilizes advanced Earth system model simulation to compare ensembles with both semi-stochastic and constantly recurring explicit volcanic forcing to examine their effect on land carbon flux variability in an overshoot scenario. Analyzing temperature and carbon flux spectra, along with mean standardized anomalies from global to regional scales, reveals the temporal and spatial structures of variability driven by intermittent volcanic forcing. On the event scale, we detect the system response via autoregressive processes, which allows us to quantify the impacts of individual events on the terrestrial carbon stock. We put these findings into the context of emission scales in future pathways.

We find a connection between increased volcanic forcing and more considerable variability in land carbon uptake, which seems to be exacerbated at lower CO₂ forcing.  Because of this state-dependency, the effect varies along the overshoot pathway. Additionally, intermittent volcanic forcing affects carbon flux variability, most prominent on decadal timescales and regional proximity of the eruption. Our study indicates that both positive and negative carbon stock impacts are more variable with increasing event magnitude. However, attribution is challenging due to low signal-to-noise ratios and internal climate variability. The results identify vegetation carbon from the equatorial regions as the primary driver of these polar impacts, with minor positive contributions from soil and litter carbon in northern latitudes. While the average impact across the ensemble approaches zero, the cumulative effects of individual simulations can vary up to the order of the annual terrestrial carbon sink. These results hint that future emission pathways should consider a more realistic volcanic forcing when assessing the land carbon stock's transient behaviour. However, they also require validations through model comparisons. Intermittent volcanic forcing could also represent a natural analog to assess the impacts of stratospheric aerosol injection, a geoengineering method to counteract global warming with sulfur aerosols.