Quantifying process-level uncertainty contributions to TCRE and carbon budgets for meeting Paris Agreement climate targets

To achieve the goals of the Paris Agreement requires deep and rapid reductions in anthropogenic CO2 emissions, but uncertainty surrounds the magnitude and depth of reductions. Using the concept of TCRE—the transient climate response to cumulative carbon emissions—we can estimate the remaining carbon budget to achieve 1.5 or 2 °C. But the uncertainty is large, and this hinders the usefulness of the concept.We are also entering an era where some of the regular metrics to monitor climate and carbon cycle change are changing if/when emissions stop increasing, begin to decline and may one day reach net zero or even globally net negative. The past behaviour of the global carbon cycle has seen a remarkably constant fraction (the airborne fraction) of CO2 emissions remain in the atmosphere each year – approximately half. But how will the Earth system behave under a new regime of decreasing and negative emissions? And is the TCRE relationship reversible – does the same gradient hold for negative emissions? We also need to understand the sequence of events which will be visible and detectable in observations of the carbon cycle if/when we achieve net zero.Here we explore uncertainty in carbon budgets associated with a given global temperature rise as determined by the physical feedbacks in the Earth system and also by the carbon cycle response to elevated temperatures and CO2 levels.  Earth system models provide a means to quantify the link from emissions to global climate change, and here we explore multi-model carbon cycle simulations across three generations of Earth system models to quantitatively assess the sources of uncertainty which propagate through to TCRE.We examine the sequence of changes in observational metrics such as the airborne fraction and sink rate and the eventual reversal of land and ocean carbon sinks as CO2 levels decline. Quantitative understanding of this sequence is vital as we enter an era where the qualitative behaviour of the climate-carbon cycle system may be fundamentally different.