Like many of its predecessors, the 2021 United Nations Climate Change Conference (COP26) in Glasgow, Scotland concluded with bold promises on international climate action aimed at keeping global warming well below 2 degrees Celsius, but few concrete plans to ensure that those promises will be kept. While it's not too late for the Paris Agreement's nearly 200 signatory nations to take concerted action to cap global warming at 2 C—if not 1.5 C—there is simply no guarantee that they will do so. If they fail, how much warming is the Earth likely to see in the 21st century and beyond?