4C developed state-of-the-art Earth System models (ESMs) and their individual land and ocean components including the biogeochemical processes that are of importance for climate and carbon feedbacks. The project made use of new observations to better constrain the contemporary carbon cycle and its variability on seasonal to multi- decadal timescales. These include combined CO2, oxygen and carbon isotopes measurements that together enable the identification of underlying processes and drivers of interannual to decadal variability. In parallel, 4C develops new and improved data-based products of land and ocean carbon fluxes to evaluate the ESM carbon cycle models and to improve process representation and reduce the carbon budget imbalance. These new products include water fluxes and storage on the land, neural network-based upscaling of surface ocean pCO2 measurements, ocean interior changes in carbon stocks, new atmospheric data of COS, satellite observations of SIF, and forest net ecosystem productivity. These data provide new information on ocean carbon uptake and its vertical export as well as terrestrial photosynthesis and related carbon sink.
4C developed state-of-the-art ESMs decadal predictions over the coming decade, where models are driven by current and future near-term trajectories of CO2 and other greenhouse gases emissions, also accounting for the natural variability of the global carbon cycle driven by the variability of the climate system.
4C developed novel emergent constraints and weighting methods to reduce uncertainty in future projections of the transient climate response to CO2 emissions, carbon cycle feedbacks and climate.
4C produced original adaptive scenarios and modelling framework to drive Earth System Models ESMs in a configuration where future emissions are refined to keep warming aligned with a predefined target (ex 1.5°C), providing our best estimates of the remaining carbon budgets consistent with the Paris Agreement ambitions, accounting for the major Earth system feedbacks.
In summary, 4C made major advances in our understanding of the key processes regulating the interactions and feedbacks between the carbon cycle and the physical climate system, using observational constraints and improved process understanding to provide, for the first time, near-term predictions and long-term projections of the coupled climate-carbon system under ambitious mitigation scenarios. 4C supported two central elements of the UNFCCC Paris Agreement: the global stocktake to track progress towards the long-term goal and the mitigation effort to achieve a long-term goal of keeping the increase in global average temperature to well below 2°C.
