Initiatives which catalyse climate action are now recognised increasingly as playing an important role in mitigating greenhouse gas emissions (GHG) and bridging the global emissions gap. The number and range of these initiatives is growing rapidly. There are several open questions about these initiatives at a global scale, including what contribution they can make to closing the emissions gap, but also what makes a successful initiative and how can this be replicated and scaled up. This paper focuses on the first of these questions. Quantifying the emission reduction contribution these initiatives can (or are likely to) make is now critically important to understanding their overall impact on international climate mitigation efforts. By demonstrating what is already being achieved through these initiatives, such analysis could also play an important role in encouraging national governments to pledge more ambitious commitments through the international negotiations. In this report, we present a quantitative assessment of the total GHG emissions mitigation impact in 2020 of current important non-state climate action.