Nature Doesn't Care.
The only threat to life on Earth that I can think of is the time when the Sun starts running out of fuel and expands into a red giant. Then the Earth will be scorched and probably all life will become extinct. Of course that won't be for another 4 or 5 billion years.
That summing up is excellent; nature does not care what we do.
Regarding how long humans will survive 'on earth', I have a different opinion. In my view, (based on my hypotheses, the 'Finiteness Theory'), it is the expansion that controls the evolution. According to my theory, the period of expansion is 25.7 billion years, and the middle one third period of 8.5 billion years (excluding the extreme conditions)will be suitable for complex organic molecules to exist in the universe. Out of this, a middle period of 2.8 million years represents the 'vegetative period' during which plant life will be present. A middle period of 900 million years represents the 'period of intelligence' during which animals having brain like structures will be present. A 300 million period represents the the 'period of consciousness' when life forms that are conciseness of their existence will exist, and the middle 100 million year period represents the zenith of evolution, the 'period of civilization', when life forms in various isolated regions of the universe will acquire real scientific knowledge.
At exact halfway, the average temperature of the universe will be 0K. The present temperature is 2.7K (temperature of the background radiation), and it will take nearly 7 million years for the expansion to reach halfway. Thus we are at the zenith of evolution, and this condition will exist for some 55 million years. After that, there will be gradual devolution. The maximum period that humans can expect to live here is less than 900 million years and that also with a redundant brain towards the last.
