Fairness and uncertainty in determining national emission reduction targets

As 2030 approaches and current Nationally Determined Contributions (NDCs) are not sufficient to meet the Paris Agreement climate goals, setting more stringent national emission reduction targets in 2040 and 2050 become more and more relevant. The basis of these targets, however, can be manyfold. In this research, we present a comprehensive framework to quantify national emission reduction targets by distinguishing and varying three types of considerations. First, the interpretation of the Paris Agreement and how this boils down to, for example, global emissions in 2040: not only in terms of temperature target, but also involving uncertainty on climate sensitivity and non-CO2 assumptions. Second, we consider normative decisions: the allocation of global emissions is done using six different effort-sharing allocation rules following different principles of fairness. Third, socio-economic and technical uncertainties also impact these calculations, such as population projections and the potential for negative emissions. These considerations and their impact on national emission reduction targets are shown on a new open online tool, the Carbon Budget Explorer, facilitating navigation through the effects of each of these considerations. We compare these national emission reduction targets to current NDCs and to cost-optimal model outcomes, shedding light on the shortcomings and importance of international collaboration in reaching global goals.