Reacción a "Reactions: the available carbon budget to meet the Paris Agreement goals is smaller than previously thought"

Above all, the study shows that the calculation of remaining CO₂ budgets depends on many assumptions and that these are not immutable quantities. Most importantly, it is necessary to understand that the budgets only include carbon dioxide and that these CO₂ quantities are dependent on assumptions about future mitigation of other important greenhouse gases – such as methane and nitrous oxide – and it is these assumptions that have changed. The problem is that the budgets in the Working Group I contribution to the IPCC Assessment Report, published in 2021, were still calculated without knowledge of the emission reduction pathways from the Working Group III contribution, published in 2022. In the IPCC synthesis report, these two strands of knowledge were not allowed to be combined due to strict IPCC regulations. Basically, this current study now catches up with that. A similar recalculation can already be found in [1]. That is why the downward readjustment of the CO₂ budgets does not come as a surprise to experts." 

"The accompanying communication states, this calculation method halves the remaining CO₂ budget. That is grossly misleading. The budget in the IPCC report of 500 gigatons of CO₂ starts counting on Jan. 1, 2020, while the budget recalculated in this current study starts counting on Jan. 1, 2023, three years later. A rough estimate shows that 120 to 125 gigatons of CO₂ emissions have been emitted in between. The difference between this study and the result of the IPCC Report is therefore only 125 to 130 gigatons. So of the 375 to 380 gigatons of the IPCC budget that was left at the beginning of 2023, it's about one-third." 

"Without attracting particular attention, the remaining CO₂ budgets during the preparation of the IPCC reports, the 1.5-degree Special Report and the Working Group I contribution to the Assessment Report, have grown strongly due to methodological improvements, by several hundred gigatons – in [2] you will find graphs showing this vividly. For the 2 degree target, even in the current study from 2023 the budget is still about the same as calculated in the previous one, the Fifth IPCC Assessment Report for the period from 2011: the famous 1000 gigatons left for the world. All these changes of direction show that while the calculation of remaining CO₂ budgets provides important indications, the figures themselves must not be set in absolute terms. Due to their permanent recalculation, global residual budgets are also not suitable for deriving national or European residual budgets from them, because these changes in direction and size cannot be implemented in the short term for German and European climate policy and undermine any climate policy certainty of expectation.