Past and Future Climate Variability Uncertainties in the Global Carbon
Budget using the MPI Grand Ensemble
Abstract
Quantifying the anthropogenic fluxes of CO2 is important to understand
the evolution of carbon sink capacities, on which the required strength
of our mitigation efforts directly depends. For the historical period,
the global carbon budget (GCB) can be compiled from observations and
model simulations as is done annually in the Global Carbon Project’s
(GCP) carbon budgets. However, the historical budget only considers a
single realization of the Earth system and cannot account for internal
climate variability. Understanding the distribution of internal climate
variability is critical for predicting the future carbon budget terms
and uncertainties. We present here a decomposition of the GCB for the
historical period and the RCP4.5 scenario using single model large
ensemble simulations from the Max Planck Institute Grand Ensemble
(MPI-GE) to capture internal variability. We calculate uncertainty
ranges for the natural sinks and anthropogenic emissions that arise from
internal climate variability, and by using this distribution, we
investigate the likelihood of historical fluxes with respect to
plausible climate states. Our results show these likelihoods have
substantial fluctuations due to internal variability, which are
partially related to ENSO. We find that the largest internal variability
in the MPI-GE stems from the natural land sink and its increasing carbon
stocks over time. The allowable fossil fuel emissions consistent with
3°C warming may be between 9–18 PgCyr-1. The MPI-GE is generally
consistent with GCP’s global budgets with the notable exception of
land-use change emissions in recent decades, highlighting that human
action is inconsistent with climate mitigation goals.