-the desktop computer dies out (because smartphones, tablets, and laptops are all that's needed)
-The U.S. keeps going further into debt
-The U.S. finishes becoming a theocracy
-American scientific progress is stalled due to the above
>We discover life on other planets (almost certainly microscopic, e.g. bacteria)
>Non-American science (probably Japan) develops humanlike AI and/or the ability to copy the human brain
>Physics takes a giant leap forward and is able to explain basically everything that it can't already (which isn't a lot)
>Fossil fuels abruptly run out or get MUCH harder to find
idk the order in which those 4 things will occur, but they probably all will eventually
-When the last happens, Yanks get pissed at the government for not preparing for the future, the U.S. collapses, and there's a revolution after which the country's all green-powered, not a world power anymore and starts slowly paying off its debts. This causes other developed nations that are least-prepared for the future (China maybe? I really have no idea) to have similar revolutions, while everyone else makes the change without massive upheaval. Global green energy, hooray!
-Once two of the other three happen, Yanks get pissed at the government for being structured around some stupid 2000+ year-old book (read: Christianity. It's the U.S., remember) that everyone now knows is filled with bullshit (because these developments would disprove all religion for good), causing a revolution after which the U.S. is the least-religious country in the world...until the rest of the world makes the change more gradually.
-While the U.S. is turning itself inside-out, everyone inside, say, the G20 nations, is struggling to deal with climate change and can't help those outside the G20. By the time the U.S. has had both revolutions (or if it takes all of the above motivators to cause any revolution, after the one revolution), all of said less-fortunate nations have basically disappeared because everyone said "screw this" and migrated to richer nations.
-Global food prices skyrocket because the countries people abandoned ran mostly on agriculture. Developed nations annex the abandoned land and provide huge financial incentives for starting farms there. Countries start talking about policies limiting the number of children a family can have, or giving bigger families higher taxes, or whatever the least-evil population-overload-prevention plan anyone can think of is.
-Half of science shifts its focus toward discovering methods of more-efficient food production. The other half starts planning to colonize the moon.
-All foods become severely genetically modified and even more loaded with pesticides than they are now (if the thought doesn't make you nauseous, you need to learn more about what you eat. Watch Supersize Me and Food, Inc.). We start colonizing the moon.
-Food prices go way back down, except real (read: organic, non-GM) food.
-Somebody finally does a study showing just how incredibly bad pesticides and GM foods are.
-Real food gets more popular.
-The price of real food goes down somewhat. Population-overload-prevention plans are adopted (because real food is harder to grow).
-We colonize all of the moon. Population-overload-prevention becomes more strict while we figure out how to terraform Mars.
-We start colonizing Mars. Population-overload-prevention eases back off.
**jump ahead a century or two**
-Once we've spread across multiple planets, begin never-ending cycle on a national/continental/whatevertheequivalentisonotherplanets scale everywhere:
a) person in power becomes corrupt
b) person in power starts becoming more tyrannical
c) people get fed up and cause a revolution
d) new person gets into power
e) life's good for several years
f) go to a
Unless someone comes up with a political system that doesn't have the problems democracy has; mainly, that most people aren't smart enough to vote properly and most politicians know this. But I don't think that'll ever happen. /blisteringcynicism
Meanwhile, we're constantly expanding to new solar systems. Repeat for literally the foreseeable future.