How long can earth sustain life?
For life to be sustained on Earth mankind needs to learn to care for it.
Due to the pollution, shortage of food, poverty, wood clearing, and green houses gases increasing dramatically the chances of humankind and animals being able to live a good, comfortable life in the future is seen to be slim.
If the people on the planet do learn on how to care for the planet then it will be able to sustain life for another couple hundred million years.
The sun increases in luminosity by 10% every billion years, so Earth can only sustain life for the next 500 million years or even less.
Around 600 million years from now plants will all die off, or at least plants that use CO2. This will happen because the Sun will start getting brighter. This warms the Earth which increases weathering. More weathering means more sedimentary rocks. Which in turn means more CO2 is trapped. Eventually levels of CO2 will be so low plants can't live. Combined with climate change and increased radiation that's when most things will die.
The natural process of the earth can support us 1 to 2 billion years.
If we can survive that long on the earth we have evolved into something that don't need the earth support to survive --- something we cannot imagine now, that is the evolution distance from bacteria to human.
Though I am quite sure it wouldn't support us 5000 years as the way we live today. By then the air is toxic to breath, water is toxic to drink, ocean is toxic, seafood will not be edible, most animal extinct the remaining animal meat is toxic, there are more plastics in ocean than seaweeds...
By our present day reckless way of life and mode of production and consumption, the Earth ecosystem wouldn't live long. The Earth will still be there for a long time, but a dead planet.
For instances, when we quickly finish the fossil fuel, all human will turn to the geothermal energy.
Have to point out, unlike other forms of energy, geothermal energy has its crucial natural function in maintaining the Earth geophysical conditions to sustain the ecosystem, by natural process, the geothermal energy can maintain the ecosystem for 1 or 2 billion years but human use can shorten that to 200,000 years. So human should not use geothermal energy. Not even the shallow layer low temperature geothermal energy in the crust, because that heat is resisting the surface water from large scale leaking into the crust. By natural process the crust is still cooling but that rate will last the surface water for 1 billion years but human use will shorten it to 200,000 years or less. Mars is a planet that leaked out its surface water into its crust long time ago.
We simply cannot follow natural process; any natural process has an end. Nature wouldn't allow any exponential progression to proceed for long; it will stop it in one way or the other. And it is all cruel ways.
The Earth will be capable of sustaining life no matter what humans do to mess it up. It may not be life as we would like it to be, but something will always survive. Algae, cockroaches, carp...some things can survive the worst nature can throw at them.