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Climate-Carbon Interactions in the Current Century

Periodic Reporting for period 1 - 4C (Climate-Carbon Interactions in the Current Century)

Reporting period: 2019-06-01 to 2020-11-30

Climate carbon cycle feedbacks can potentially amplify climate change over the 21st century. Hence, these processes play an important role in determining the climate response to anthropogenic emissions of carbon dioxide (CO2). The EU-funded 4C project 4C addresses the crucial knowledge gap in the sensitivity of the climate system to CO2 emissions, and aims to improve our quantitative understanding of carbon-climate interactions and feedbacks. This will be achieved through innovative integration of models and observations, providing new constraints on carbon-climate interactions and climate projections, and supporting IPCC assessments and policy objectives. To meet these goals, 4C has four overall objectives:
1) to make a major improvement in our understanding of the global carbon cycle over the historical period, by producing a comprehensive set of novel constraints using innovative methods and new observations, and applying these new constraints to critically assess and improve current carbon cycle models;
2) to develop new tools and methods to predict, for the first time, the evolution of global carbon cycle variability over the coming decade, including atmospheric CO2, land and ocean carbon sinks, and climate response to track the overall progress towards the goal of the Paris Agreement;
3) to improve our understanding of climate-carbon feedbacks and provide a robust quantification of their evolution over the 21st century, using new constraints from historical observations to inform the analysis of ESM projections;
4) to ensure the usability of the knowledge generated by scientific research and engage in bilateral interactions among scientists and policymakers, while also fostering the understanding of the findings for the broad society.
We develop new observational constraints on the combined global and regional land and ocean CO2 fluxes to support quantitative understanding of the global carbon budget. We are developing global budgets of oxygen and 13C in addition to the global carbon budget, also producing an extended dataset of satellite-based atmospheric CO2 (xCO2). In parallel we also develop new observation-based data products on ocean surface pCO2, ocean interior inorganic carbon, land soil water content and forest net ecosystem productivity. These new observations are being used to constrain historical simulations of the global carbon cycle in order to reduce un certainties in the quantification of the land and ocean carbon sinks.
We develop new modelling methods to predict the near term evolution of the carbon cycle. We first use Earth system models to understand and quantify the potential predictability of the land and ocean carbon sinks. Next, we validate our modelling systems by quantifying their ability to predict the recent past once the observed state of the climate is assimilated into initial conditions. And, finally we use these models to perform actual predictions of the near future.
We also we develop novel emergent constraints for the land carbon cycle, based on soil moisture observation or on observed soil turnover time to constrain future changes in the land carbon cycle.
In parallel, we also develop novel emergent constraints for the ocean carbon cycle, using observed sea surface salinity or density of Arctic Ocean surface waters, to constrain projections of the ocean carbon cycle.
We analysed the transient climate response to cumulative CO2 emissions, its main sources of uncertainties, including a quantification of the role of non-CO2 emissions.
We disseminate our results via direct interaction with policy makers and IPCC, the development of a new science-user platform (ScienceBrief), targeted policy briefs and carbon outlooks, short videos, press releases, and continuous activity on social media.
4C develops beyond state-of-the-art Earth System models (ESMs) and their individual components including physical and biogeochemical processes that are of importance for climate and carbon feedbacks. The project will make use of new observations to better constrain the contemporary carbon cycle and its variability on seasonal to multi- decadal timescales. These include combined CO2, O2 and 13C measurements, and satellite atmospheric CO2 that together will enable the identification of underlying processes and drivers of variability. In parallel, 4C develops new and improved data-based products to evaluate the ESM carbon cycle models and to guide and improve process representation so as to 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 will provide new information on ocean carbon uptake and vertical export as well as land photosynthesis and related carbon sink.
4C makes use of 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 develops 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.
4C will produce original adaptive scenarios to drive the 4C ESMs in their new configuration to provide best estimates of carbon budgets consistent with the Paris Agreement ambitions, accounting for the major Earth system feedbacks.

In summary, 4C will make a major advance 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. This will allow the 4C team to deliver policy-relevant and observationally-constrained carbon dioxide emission pathways consistent with the Paris Agreement ambitions. Via its objectives, 4C will support two central elements of the UNFCCC Paris Agreement: the 2023 and 5-year 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.
The Global Carbon Cycle