One more question: How will the world end? Probably NOT with a whimper, but instead with a bang.
Q: Wow, a lot to un-pack there (by no means a criticism.) In fact, thanks very much for the extensive informative responses! My next question (maybe last, if I'm not pushing your patience too much here,) regards the truly catastrophic's. I'm 29 years old, which can't even be considered a mote in time when considering numbers like 13.7B or 4.5B, but nevertheless, here we are. Growing up it was accepted via the direction of our science teachers that dinosaurs were wiped out by an asteroid, and it seems to make sense. But alternative views like supervalcano's have been touted on science sounding TV channels as an alternative and I wonder about your thoughts on that. And minus dinosaurs, would we more statistically face worldwide threat from geology, a comet/asteroid, or a biological problem? We can leave out human stupidity towards ourselves for the sake of the argument. I guess cosmic factors too.
A: Most of humanity seems to gravitate towards a simple solution or answer to a complex problem. It's mentally easier. This is really obvious in the current political "debates" going on (Build a wall! Cut taxes! Increase spending on X!). The most difficult scientific problems to solve are the ones with more than one poorly-understood variable. Most science consists of trying to constrain down those variables to just one for your experiment. MOST problems, however, have complex causes. If my microwave stops working, I think oh: it must be the power is out. Closer inspection shows that the power is there. Darn, no simple answer. OK, what's next to check then? Heck, I’m gonna have to disassemble it…
The Chicxulub event certainly had a big impact (pun intended) on saurian life when it hit 66M years ago, but there IS evidence that Life for Large Saurians was getting more and more difficult before the major extinction event, with environmental degradation due to several things already underway, including volcanism (the extinction may have been accelerated by the formation of the Deccan Traps in India). However, make no mistake: a 2-cm layer of ash full of 1000-times-normal iridium in Gubbio, Italy, came from the Gulf of Mexico. A 10-km-diameter asteroid carries a ginormous amount of kinetic energy with it. The Earth's gravity well is pretty strong (a rock dropped from the Lagrangian point between Earth and Moon reaches about 8 km/second before it hits atmosphere). This is also the speed of a minimum Low Earth Orbit. The Chicxulub object certainly had a much higher velocity than that, because it came from the Asteroid Belt or the Oort Cloud, and energy is mass times velocity squared. Thus, double the velocity and you quadruple the energy delivered (it's a principle I teach to my Jujitsu students: the speed of a palm-heel strike is more important than putting your entire body-weight into it). From several studies there is a consensus that the Chicxulub object’s kinetic energy before atmospheric entry was about 5.4 x 10^23 Joules, or 130,000,000 Megatons of TNT equivalent. By comparison, the Tsar Bomba, the largest hydrogen bomb ever detonated (by the Soviets, at Novaya Zemlya in 1961), had a yield of "just" 55-60 Megatons.The bomb itself weighed 27 metric tons!
As far as future devastation goes, biologic threats tend to be self-limiting. Volcano and earthquake threats are pretty much steady state (with minor fluctuations) over time. The two unlimited threats are human interference (climate change is just one consequence) and asteroid/comet impacts. If there is one certainty, it is that there WILL be change.