The physical science portion of the Summary for Policymakers of the latest assessment report of the Intergovernmental Panel on Climate Change states that the best estimate of climate sensitivity, defined as the warming expected at equilibrium from doubling the amount of carbon dioxide in the atmosphere, is 3 degrees Celsius (or 5.4 degrees Fahrenheit). It also states that the global average surface temperature will continue to increase until at least the middle of the current (twenty-first) century. Recalling that the Paris Agreement of 2015 specified a goal or target of limiting warming to 2 degrees Celsius (or 3.6 degrees Fahrenheit), with an even more ambitious aspirational goal of 1.5 degrees Celsius (or 2.7 degrees Fahrenheit), this Summary for Policymakers states that neither of these goals will be met during the twenty-first century unless deep reductions in emissions occur. The report also states that many of the climate changes caused by greenhouse gas emissions cannot be reversed and will persist for hundreds or thousands of years. It cites global sea level rise and the loss of ice sheets and glaciers as examples of changes that will be irreversible on human time scales.