et al., 2021). The inter-model uncertainty associated with CMIP6 NPP projections is larger than in CMIP5, and it is consistently larger than the scenario uncertainty. For each SSP across the CMIP6 ensemble, individual models project both increases and decreases in global primary production, reflecting a diverse suite of bottom-up and top- down ecological processes, which are variously parameterised across models . Furthermore, accurate simulation of many of the biogeochemical tracers upon which NPP depends  remains a significant and ongoing challenge to ESMs  .
IPCC
Climate Change 2022: Impacts, Adaptation and Vulnerability
IPCC Report