It means we have just a few years to make significant cuts in greenhouse gas emissions, the primary cause of global warming. It’s not an end-of-the-world date. The genesis of the 12-year number was a 2018 report by the United Nations Intergovernmental Panel on Climate Change that drew a line in the sand. The message then was that within 12 years, the world must be on a path to net zero CO2 emissions by mid-century. Meeting that goal means keeping global warming to a maximum of 1.5 centigrade. Anything above that will greatly increase the risks of sea level rise, drought, floods, and extreme heat.