>>1
My review of internet sources suggests a very low probability of a bolide impact or volcanism and a very high probability of atmospheric gas concentration changes causing a mean surface temperature increase of 6C in a relatively short period of several million years. Basically, a bolide impact or volcanism would have the opposite effect, and even in the absence of ostensible craters (hypothesized by some to be filled in by magma) the shocked quartz evidence for bolide impacts is lacking, as is the atmospheric pollution and lava deposit evidence for volcanism. Fungus was reputedly the dominant form of life for millions of years after the Permian-Triassic mass extinction event.
The estimates I've seen say that a mean surface temperature increase of 10C would be sufficient to disrupt the carbon cycle necessary for the continued existence of multicellular life, and that increases in the luminosity of the sun over time will raise the mean surface temperature of the earth to 100C over the course of somewhere between 500 million and 1 billion years from now, this being even without any major transitions in the sun's stellar life cycle, which aren't expected to occur for billions of years after that.
There are other tidbits also worth noting. The carbon dioxide content of the atmosphere was significantly higher in the past, even with pollution of human origin contributing to current levels. Dinosaurs lived in Antarctica. The rate of extinction is now higher than even the Permian-Triassic extinction event, and this is called the "ongoing holocene extinction event" ("ongoing" is to distinguish it from the other use of "holocene extinction event" for the extinction of the megafauna [woolly mammoths, sabre-toothed tigers, giant elk and bears, etc.]). It's believed that the ongoing holocene extinction event is of anthropogenic origin. Also, as continents converge into Pangaea Ultima (the last Pangaea before the evaporation of oceans dominates the picture, though the timescales suggest there should actually be one or two more continental convergences after that before the oceans dry up), hurricanes are expected to become progressively more violent as the amount of contiguous ocean over which their winds can accelerate increases to the majority of the circumference of the earth; it's been speculated that hurricanes formed in such arrangements could achieve supersonic wind speeds. Ice ages correspond to a cycle of what's believed to be solar activity somewhere around 0.5 million years in length, and we're still relatively close to the end of the most recent ice age.
I suppose politics and economics could still be considered more immediate problems, but it certainly appears that our distant descendants will have to look to space travel in a more geologically near-term future than we anticipated. Not that it looks particularly likely that there will be anywhere for them to go. Then again, looking at current events, maybe it's for the best.
"What a piece of work is a man. How terrible in reason? How finite in faculty? In action, how like a demon? In apprehension, how like the Devil? The ugliness of the world, the worst of all animals; and yet to the world, what is this mote of dust?"