Alexander Norton: Evaluation of The Global Carbon Cycle in ACCESS-ESM1.6
- The ACCESS Earth System Model (ACCESS‑ESM) simulates carbon exchange between land, ocean, and atmosphere, capturing key feedbacks with climate and atmospheric CO2. In CMIP6, emissions-driven models showed large spread in historical land and ocean carbon sinks, contributing to a mean atmospheric CO2 bias of ~7 ppm and large spread (±14 ppm) across models by 2014. Accurately representing the historical carbon cycle is essential, yet uncertainties in carbon sources and sinks remain a major limitation across CMIP Earth system models.
- ACCESS-ESM1.6, the latest development version for the CMIP7 FastTrack, includes several updates to better represent land and ocean biophysical and biogeochemical processes. Here, we evaluate the global carbon cycle from historical emissions-driven simulations. Model performance is assessed using independent data from the 2025 Global Carbon Budget across the land, ocean, and atmosphere, alongside benchmarking against the ACCESS‑ESM1.5 CMIP6 submission.
- Both ACCESS-ESM1.5 and ACCESS-ESM1.6 reproduce atmospheric CO₂ evolution with a mean absolute error of <5 ppm from 1850-2014, performing well relative to CMIP6 spread, but this agreement masks substantial differences in carbon partitioning. ACCESS‑ESM1.6 shows improvements in carbon partitioning, including small increases in ocean and land uptake and substantially higher land-use change emissions following updates to forest management and wood harvest. These changes bring the model closer to independent estimates, although residual biases remain. This highlights ongoing challenges in representing carbon sinks and underscores their importance for future climate projections.
- Developers and users of ACCESS-ESM1.6. Researchers interested in the global carbon cycle.
- Land, ocean, carbon, climate, ACCESS-ESM