The responses of the climate system are pretty well understood at this time, and nothing approaches 'existential', i.e. extinction-level - which I'd reserve for

The responses of the climate system are pretty well understood at this time, and nothing approaches 'existential', i.e. extinction-level - which I'd reserve for major asteroid strikes, supervolcanoes, and total nuclear war. Equating fossil-fueled global warming to such events is simply scientific exaggeration. It's a notch down the logarithmic scale of doom from them, is all I'm saying. What fossil-fueled global warming entails is more like a constant, steady grind with the grind getting noticeable worse over successive time periods (perhaps about a decade, maybe a little less). Conditions that span extreme weather are getting more frequent (mostly due to more water vapor in the atmosphere), record-breaking droughts in some regions are on the rise, 500-yr floods now take place every 5 years, etc. Fundamentally, however, none of this is going away - it takes 100 years at least for global climate to equilibrate to current forcing, and again, there are some feedback effects dumping ancient stores of carbon slowly into the atmosphere. Practically, this means as much energy and money are going to have to be spent on adapting to new conditions as on eliminating fossil fuels from the energy mix.