The Intergovernmental Panel on Climate Change 5th Assessment Report (IPCC AR5) concluded that cumulative emissions of CO₂ largely determine global mean surface warming by the late 21st century and beyond. The IPCC AR5 also highlighted the positive feedback between climate change and the carbon cycle: climate change will affect carbon cycle processes in a way that will exacerbate the increase of CO₂ in the atmosphere. These findings highlight the central role of the carbon cycle in the global climate system.

Although the CO₂-carbon cycle and climate-carbon cycle feedbacks have been identified for more than a decade, we still have limited abilities in quantifying these feedbacks, as well as confidently attributing past changes of the carbon cycle and hence anticipating its future evolution. Therefore, there is an urgent need to better understand and better model the processes that drive the observed variability in atmospheric CO₂ at seasonal to century time-scales, in order to improve climate projections and inform climate mitigation and adaptation.

Reducing the current uncertainties in our understanding of the carbon cycle is an essential contribution of the 4C project to the implementation of the Paris Agreement. So far, few research projects have attempted to predict the near-term evolution of the carbon cycle, and in particular what would be the near-term growth rate of atmospheric CO₂ in the next decade if all countries follow their Paris agreement ambitions on emission reduction. Thus, there is an urgent need to develop the capability to simulate and assess the near-term evolution of the global carbon cycle and the climate system in response to different near-term emission trajectories.

The Transient Climate Response to Cumulative Carbon Emissions (TCRE) is the Earth system metric that quantifies the global average surface warming for a given cumulative emission of CO₂ and can be used to infer the carbon emissions consistent with a given climate target. TCRE is an attractive metric for policymakers as it directly links CO₂ emissions to global warming. However, the uncertainty in carbon cycle feedbacks severely undermines attempts to estimate the climate response for a chosen emission scenario, and to quantify the anthropogenic CO₂ emissions that would be consistent with a stabilization of global warming at a chosen level. Resolving key carbon cycle uncertainties is essential in order to provide greater clarity on necessary mitigation actions required to meet the Paris Agreement long-term temperature goal of “limiting warming to well below 2°C, and pursuing efforts to 1.5°C”, hence limiting the effects of dangerous climate change. Thus, there is an urgent need to provide useful constraints on TCRE to support policy-making.