The Working Group I (WGI) contribution to the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report (AR6) provided a complete and comprehensive assessment of the physical science basis of climate change, building on previous ARs. According to the report, human influence on the climate system is now an established fact. Sustained changes have been documented in all major elements of the climate system: atmosphere, land, cryosphere, biosphere and ocean. Multiple lines of evidence indicate the recent large-scale climatic changes are unprecedented in a multi-millennial context, and that they represent a millennial-scale commitment for the slow-responding elements of the climate system, resulting in continued worldwide loss of ice, increase in ocean heat content, sea level rise and deep ocean acidification.
A combination of improved observational records and a series of very warm years since AR5 have resulted in a substantial increase in the estimated level of global warming to date. Updated paleoclimate evidence strengthens the conclusion that over the past several decades, key indicators of the climate system are increasingly at levels unseen in centuries to millennia and are changing at rates unprecedented in at least the last 2000 years. Based on a combination of the larger estimate of global warming to date and the assessed climate response to all considered scenarios, AR6 concludes that the central estimate of crossing 1.5oC of global warming (for a 20-year period) will occur during the early 2030s (ten years earlier than the mid-point of the likely range assessed in the Special Report on Global Warming of 1.5 °C (SR1.5)).