Climate change requires international cooperation across national and regional scales to effectively mitigate greenhouse gas emissions and impacts. As greenhouse gases mix globally, uncoordinated independent action among countries will not solve this global problem. International agreements have potential to address challenges like diverse perceptions of costs and benefits, unevenly distributed emissions sources, and uncertain distant impacts. Strong multilateral agreements, harmonized national policies, and coordinated decentralized national policies could enhance cooperation. Limiting future warming involves substantial technological, economic and institutional challenges that also require cooperation.