Revising carbon budgets in a 1.5 degree world

The concept of a remaining carbon budget associated with global warming levels has underpinned mitigation efforts since the Paris Agreement.  However, as observed temperatures near 1.5 degrees, a number of challenges have emerged for the continued use of carbon budgets to frame mitigation needs.  Firstly, while the transient response to cumulative emissions describes the temperature response to constant emissions - Paris-compatible pathways require deep emissions cuts and potentially extended periods of negative emissions, the temperature outcome of which is complicated by zero emissions commitments and non-CO2 responses.  Understanding of Zero emissions commitments has been thus far been primarily informed by idealised experiments which terminate emissions during an idealised concentration ramp - but these metrics are subject to unrealistic termination shocks and model-specific emissions pathways.  Second, non-CO2 responses remain highly uncertain, and recent satellite observations of global radiative imbalance raise further questions on the adequacy of current modeling platforms to describe the warming which should be expected due to aerosol phaseout. 

Here, we consider how two novel developments impact carbon budgets beyond estimates presented in the IPCC 6th Assessment.  Firstly, we present an ESM ensemble of climate reversibility experiments which provides a more realistic proxy for non-TCRE carbon dynamics during a net zero transition for use in carbon budgets.  Secondly, we consider how the inclusion of recent global mean temperature measurements and CERES top of atmosphere radiative flux measurements would impacts the calibration of simple climate models - with subsequent impacts on both estimates of both TCRE and expected warming due to non-CO2 effects.  Synthesising this information, we provide an updated estimate of carbon budgets and timing with respect to the 1.5 and 2 degree thresholds.