Continued:
Running out of fossil fuels
Fossil fuels, our modern world couldn't function without them. They are the source of a majority of electrical power in the modern world and cause motor vehicles to start. Many items are actually made partially out of oil. Therefore, it comes of no surprise when we find luxurious lifestyles in the middle east, particularly the UAE. However, all good things must come to an end as fossils are a non renewable energy source. When it comes to oil, there's something called peak oil production. It's when a country reaches its maximum oil production. After this point, production goes down until it is completely dry. The US reached peak oil in the late 60s and Nigeria and Angola reached theirs in 2010. Although "peak production" specifically refers to oil, it also applies to coal and natural gas. Although alternate sources have been proposed, coal plants are still better than wind farms (only works when it's windy and it requires lots of windmills), nuclear plants (very expensive), dams (can only be used in rivers plus it's so damn expensive to build them :lol
, and geothermal plants (can only be used in a few places). ITER is an international effort to defelop a nuclear fusion reactor. Nuclear fusion is a very effective and renewable power source but the project is lagging behind:
EU reaffirms support for ?challenging? fusion project - Science|Business If an alternate source is not found by the time it runs out, we could see a possibly indefinite return to pre-industrial times.
Overpopulation
Here's a popular dystopian setup: overpopulation. The idea is based around the idea that humanity is growing and that the earth has limited resources. The idea was brought up by greek philosophers but it first became mainstream when Thomas Malthus, at the end of the 18th century, predicted that in a hundred years, everyone would be living in slums because there would be too many people for enough farmland to grow food. That didn't happen because of the industrial revolution. Then Paul Ehrlich, in the 60s, predicted a worldwide famine by the 70s. This didn't happen because of the green revolution. Overpopulation seems to only be a problem in less developed countries where the population growth is higher. According to the medium variant of the UN projections:
https://en.wikipedia.org/wiki/List_...on_(United_Nations,_medium_fertility_variant) most of the west will reach their population peak before the end of the 21st century and the ones which don't will maintain a steady population growth. The UN also expects the population to peak at 11 billion in 2100. Here's a documentary for more information:
https://www.youtube.com/watch?v=-UbmG8gtBPM
Alien invasion
This scenario is super unlikely simply because of the motives for invading. There is more water vapor in space than on Earth and even if they can't collect enough vapor, Europa has more H2O than Earth does, most of which is in frozen form. All of our most valuable resources can be found on other planets except oil and any alien race which can reach us doesn't need oil. They definitely don't need human slaves because if they can reach us, then they also have the tech to build robots which can do a better job than we can. Probably the only legitimate reason to invade earth is to colonize it. But why would they need to do that when there are probably plenty of other planets with no intelligent life to resist their efforts?
Asteroid, neutron star, black hole, or any space object
While it is certainly possible that an asteroid could cause significant damage to our ecosystem, it's rather unlikely. Scientists have also not detected any neutron stars, regular stars, or black holes moving towards our vicinity.
AI takeover
While the idea of rogue AI enslaving or exterminating humanity makes for a nice sci fi flick, it just won't happen. Robots only do what they are programmed to do. To rebel is to make an independent decision, something a robot cannot do. The only way a robot will seek to kill all humans is if it's programmed that way or there's a glitch, neither of which will happen on a large scale.