Climate change will upend the 21st century world order. It will redefine how we live and work, and change the systems of production, trade, economics, and finance. Yet, the world is woefully underprepared for climate change’s cascading impacts. COVID-19 has only underscored the inadequacy of our responses to global crises and heightened the urgency of this call to action. 21st century diplomacy will have to raise climate ambition, shape the transformative systems change needed, and promote and facilitate new modes of multilateral collaboration.