Past and Future Climate Variability Uncertainties in the Global Carbon
Budget using the MPI Grand Ensemble
Abstract
Quantifying the anthropogenic sources and sinks 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 in the Global Carbon Project’s (GCP) annual
carbon budgets. However, the historical budget only considers a single
realization of the Earth system and cannot account for internal climate
variability uncertainties. 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 are
compatible with the RCP4.5 scenario, requiring at least 40 ensemble
members. The MPI-GE allows us to investigate the likelihood of
historical fluxes relative to the distribution from internal
climate variability. Our results show that these likelihoods have
substantial fluctuations due to internal variability, which are at
least 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 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.