As we count down to the 4th International Conference on Carbon Dioxide Removal in Milano, we are hosting a series of discussions on the research that will be shaping our sessions this June! 
This research introduces the concept of “return carbon budgets” 
to estimate how much Carbon Dioxide Removal would be required to bring global temperatures back to a target level such as 1.5°C after an overshoot. 
Using Earth system modelling, the authors show that the amount of CO2 removal needed depends not only on cumulative emissions and cooling responses, 
but also strongly on the trajectory of non-CO2 climate forcing during the recovery period, 
highlighting the crucial importance of integrated mitigation strategies beyond CO2 alone! 
Full Abstract: Conceptualizing Return Carbon Budgets
Authors: Kirsten Zickfeld, Jayakrishnan Koramanghat Unnikrishnan
As anthropogenic greenhouse gas emissions continue to rise, limiting warming to 1.5°C above pre-industrial levels has become elusive. Emissions pathways seeking to return to 1.5°C after a period of temperature exceedance will therefore require net negative emissions. A crucial question in this context is how much CO2 would need to be removed from the atmosphere to return to a given warming level. The cumulative amount of CO2 that needs to be removed to return to a given warming level is here referred to as the return carbon budget.
Similarly to the remaining carbon budget, the return carbon budget can be conceptualized as a function of i) the relationship between cumulative CO2 removal and cooling (a metric referred to as the Transient Climate Response to Cumulative Carbon Removal or TCRR), ii) the Zero Emission Commitment after the time of peak warming, iii) the warming commitment after the time temperature returns to the target level, and iii) non-CO2 forcing. We test the accuracy of this conceptual framework by using simulations with an Earth system model of intermediate complexity forced by a set of scenarios with various levels of carbon budget exceedance and recovery. Preliminary results suggest that the proposed framework yields a reasonable approximation of return carbon budgets for different levels of peak warming, and, crucially, that return carbon budgets depend strongly on the trajectory of non-CO2 forcing between the time of peak warming and the time when the target warming level is reached.
If the amount of carbon removal we need to fix an overshoot depends heavily on non-CO₂ gases like methane and nitrous oxide, should global CDR policy and funding be reshaped to prioritize non-CO₂ mitigation? 
