Climate-Carbon Interactions in the Current Century

Project Information

4C

Grant agreement ID: 821003

DOI

10.3030/821003

Project closed

EC signature date 3 May 2019

Start date 1 June 2019

End date 30 November 2023

Funded under

• SOCIETAL CHALLENGES - Climate action, Environment, Resource Efficiency and Raw Materials

Total cost

€ 7 784 750,00

EU contribution

€ 7 784 750,00

Coordinated by

THE UNIVERSITY OF EXETER
United Kingdom

Periodic Reporting for period 3 - 4C (Climate-Carbon Interactions in the Current Century)

Reporting period: 2022-06-01 to 2023-11-30

Summary of the context and overall objectives of the project

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 addressed the crucial knowledge gap in the sensitivity of the climate system to CO2 emissions, and aimed to improve our quantitative understanding of carbon-climate interactions and feedbacks. This was 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. 4C achieved four overall objectives.

1) 4C made a major improvement in our understanding of the global carbon cycle over the recent past, producing novel atmospheric, oceanic and terrestrial observational constraints of the carbon cycle to assess and improve carbon cycle models’ simulations of the current land and ocean carbon sinks.
2) 4C developed new Earth System Models capability to predict the evolution of global carbon cycle variability over the coming decade, predicting the annual growth rate of atmospheric CO2 and the strength of the land and ocean carbon sinks, accounting for the natural variability of the climate, in the context of the Paris Agreement.
3) 4C developed new emergent constrains on the land and ocean carbon fluxes to improve our understanding of climate-carbon feedbacks over the 21st century. 4C also developed new adaptive scenarios that allow to diagnose future greenhouse gases emissions compatible with a given climate target such as the 1.5°C limit as set in the Paris Agreement.
4) 4C ensured 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.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

We developed new observational constraints on the combined global and regional land and ocean CO2 fluxes to support quantitative understanding of the global carbon budget. We have developed global budgets of oxygen and carbon isotope (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, terrestrial soil water content and forest net ecosystem productivity. These new observations are being used to constrain historical simulations of the global carbon cycle to reduce uncertainties in the quantification of the historical land and ocean carbon sinks.
We developed new Earth System Models (ESMs) frameworks to predict the near-term evolution of the carbon cycle. We first used these ESMs to understand and quantify the potential predictability of the land and ocean carbon sinks. Next, we validated our modelling systems by quantifying their ability to predict the recent past once the observed state of the climate is assimilated into initial conditions. Finally, we used these models to perform actual predictions of the near future of the global carbon cycle.
We developed novel emergent constraints for the land carbon cycle, based on soil moisture observation, and on observed soil turnover time to constrain future changes in the land carbon cycle. We also developed 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 developed new adaptive scenarios for ESMs, allowing to diagnose the range of future CO2 (and non-CO2) global emissions that would be consistent with a global warming of 1.5°C or 2°C.
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.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

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.

The Global Carbon Cycle