In 20,000 years lets see...
1. catastrophic global warming unless we change our ways
2. melting ice caps/hotter temperatures changes population geo-distribution
3. over-population and denser living increases disease epidemics
4. increasing pollution, toxic environments
5. possibly fallout and radiation from severe nuclear wars
6. food shortages, synthetic foods, maybe Soylent Green
Sounds like the prequel to Planet of The Apes.
![Laughing :-DD](https://www.eevblog.com/forum/Smileys/default/smiley_laughing.gif)
If we even survive 20,000 more years as a species we will probably be living in underground bunkers that can withstand crazy weather events, probably high in the mountains, not the coasts which will be flooded by rising sea levels (at least until the levels stabilize and establish new stable coasts). We will have to have solved a whole pile of issues by then, like renewable energy sources, over-population issues, food supply, global warming, etc. We may look a bit like the Borg... everyone will be implanted at birth with some biochip-interconnected interface to access all the world's knowledge and be part of the collective.
Alternative to the dystopian future, we may also imagine that we manage to responsibly tackle these issues:
1. over-population trend (which in itself will kill our planet for everyone or lead to massive wars, famine, disease, ecological destruction, pollution, etc)
2. once populations plateau and remain at a sustainable level, energy resource stability
Basically all components of our planet, from environmental, energy, food, etc... have to come to a sustainable level based on the technology available. The system will have to remain in BALANCE and any change in population or behaviour of people or other activity that perturbs the balance will have to be offset by an EQUAL AND OPPOSITE technological or behavioural change to continue balancing the system, leading to sustainability.
For example, if there is a trend to start building wooden houses again, we would need to plant trees to sustain the increased demand on wood (and also compensate for the reduction in Carbon sequestration that trees provide and oxygen production). If there is a limit to how many trees or how fast they grow, there would need to be a quota on number of trees that can be culled, and number of wooden houses that can be built. Same goes for everything. That would also be a sort of dystopia ruled by some master sustainability A.I. to protect and preserve Earth, as it would restrict many of the freedoms we enjoy today.