H2020 4C Project: Climate-Carbon Interactions in the Current Century

4C developed state-of-the-art Earth System Models (MES) and their individual land and ocean components, including biogeochemical processes important for climate and carbon feedbacks. The project leveraged new observations to better constrain the contemporary carbon cycle and its variability on timescales ranging from seasonal to multidecadal. These include combined measurements of CO2, oxygen, and carbon isotopes that together identify the underlying processes and drivers of interannual to decadal variability. In parallel, 4C is developing new and improved products based on land and ocean carbon flux data to evaluate MES carbon cycle models, improve process representation, and reduce carbon budget imbalances. These new products include water fluxes and land storage, neural network-based scaling of pCO2 measurements, and a wide range of other products. at the ocean surface, changes in carbon stocks in the ocean interior, new atmospheric SOC data, satellite observations of SIF, and net productivity of forest ecosystems. These data provide new insights into ocean carbon uptake and vertical export, as well as terrestrial photosynthesis and related carbon sinks. 4C developed state-of-the-art Earth System Model (ESM) decadal predictions for the next decade, where models are driven by current and near-term future trajectories of CO2 and other greenhouse gas emissions, and also account for natural variability in the global carbon cycle driven by climate system variability. 4C developed new emerging 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 a modeling framework to drive Earth System Models (ESMs) in a setting where future emissions are refined to keep warming aligned with a predefined target (e.g., 1.5°C), providing our best estimates of remaining carbon budgets consistent with the ambitions of the Paris Agreement, accounting for major Earth system feedbacks. In summary, 4C made important 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 core elements of the UNFCCC Paris Agreement: the global inventory to track progress toward the long-term goal, and the mitigation effort to achieve a long-term objective of keeping the global average temperature increase to well below 2°C.