Abstract: The Paris Agreement of 2015 uses Nationally Determined Contributions (NDCs) to achieve its goal to limit climate change to well below 2∘C. Article 6 allows countries to cooperatively implement NDCs provided they do not double-count mitigation. We estimate that economic efficiency gains from cooperative implementation of existing NDC goals using Article 6 could reduce the cost of achieving NDC goals in 2030 to all parties by ∼$300×10^9, which if reinvested in additional emissions mitigation could add 9 billion tons CO2/year mitigation, beyond the 8 billion tons CO2/year currently pledged in 2030. We estimate that more than half of the 2030 gains could come from nature-based measures, but long-term potential for nature-based measures is more limited. How much or even if this economic potential can be realized is uncertain and will depend on both the rules and their implementation.