End Times: Too Smart?
Anybody who thinks about the prospects of other intelligent life in the galaxy eventually runs into Fermi's famous question: "Where are they?" If other technological civilizations have developed, why haven't any of them showed up here? One possibility is that technological civilizations rarely develop. Another is that they develop but destroy themselves quickly.
That latter possibility seems very plausible. There seems to be an increasing number of ways that we could manage that fate for ourselves, and it looks increasingly unlikely that we can manage to avoid some of them. One probably effective way would be a CO2 catastrophe.
The biggest mass extinction event for which we have information was the end Permian extinction. The details of causation are unclear, but it seems to have deeply involved a big increase in CO2 concentration. One somewhat plausible scenario is that the Siberian traps volcanism set fire to massive coal beds and released massive amounts of methane from undersea clathrates.
Not that that should remind us of anything happening now.