This Summary for Policymakers (SPM) presents key findings of the Working Group I (WGI) ontribution to the IPCC’s Sixth Assessment Report (AR6)1on the physical science basis of climate change. The report builds upon the 2013 Working Group I contribution to the IPCC’s Fifth Assessment Report (AR5) and the 2018–2019 IPCC Special Reportsof the AR6 cycle and incorporates subsequent new evidence from climate science