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It is unequivocal that human activities have heated our climate. Recent changes are rapid, intensifying, and unprecedented over centuries to thousands of years. With each additional increment of warming, these changes will become larger, resulting in long-lasting, irreversible implications, in particular for sea level rise. United Nations Secretary-General António Guterres has stated that ‘the evidence is irrefutable’and ‘we see the warning signs in every continent and region’.

The Working Group I contribution to the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report provides a reality check on climate change. We now have a much clearer picture of the past, present and possible future climates, and this information is essential for understanding where we are headed, what can be done, and the multiple facets of a changing climate to prepare for, in every region. Unless deep reductions in greenhouse gas emissions occur in the coming decades, global warming of 1.5 C and 2 C above pre-industrial levels will be exceeded during the 21st century.