Farhad Manjoo: There is only one existential threat. Let’s talk about it.

If you’re a supporter of that radical extremist group Keep America Habitable for Human Beings, you might have been encouraged by the 2020 presidential race.In 2016, climate change — the scientific fact of the Earth’s encroaching uninhabitability — was mostly ignored, including in the debates between Donald Trump and Hillary Clinton. This year, the changing climate and what to do about it got airtime in both presidential debates and the vice-presidential debate. Climate ch