Since humans are at present entirely dependent upon the terrestrial ecosystem, a transfer to artificial and sustainable habitats will ensure long term survival. This will serve to protect the human species from large-scale disasters, such as nuclear conflict, asteroid impacts, or an outbreak of a devastating plague.
Under the Earth Park concept, virtually the entire human population of the world will emigrate to artificial habitats, leaving the earth as a wildlife refuge that will gradually recover its proper ecological balance and diversity. To be successful over the long term this must be a universal migration, and no pockets of reproductive humanity can remain on the surface of the earth.
Certainly the restoration of a natural ecosystem will be greatly accelerated by the removal of various constructions, such as buildings, fences, dams, and roads. However even if left to recover on its own, the planet will inexorably return to a completely wild state over a geological time scale, provided we have not inflicted excessive damage before the general exodus is underway.
Any planetary cleanup programs will need to be managed from orbit, but most of the labor can be performed using robotics.
In all probability there will also be habitats created specifically to provide recreation for people living in the vicinity. These can be seeded with life forms from the earth, and will provide an opportunity for people to experience the outdoors without requiring a return to the planet. Such habitats also provide a safe location for the introduction of experiemntal crops and other life forms that might endanger the ecosystem of the Earth.
The biggest obstacle to the Earth Park program will be political. It is highly unlikely to be implemented until the situation is sufficiently dire to warrant such a measure. The exodus will also require universal concurance by all nations and peoples. Given the circumstances under which this is likely to be implemented, the available time for the exodus will be short, and the condition of the Earth is likely to be grim.
If an emigration plan can be implemented during the available window and the necessary habitats constructed and inhabited, there is every reason to believe that the solar system can sustain a large human population at a high quality of life while preserving the earth as a natural heritage.
Constructing the habitats will require self-replicating technology on a massive scale. Solar energy can be used to supply the power requirements, but large quantities of materials will need to be relocated and processed. The only realistic method of implementing such construction in a short time frame is through robotics and/or nanotechnology. Once the template for a suitable habitat is completed, the construction can proceed at an exponentially increasing rate until all required settlements have been completed.
Those remaining on the Earth in underground habitats will need to have their power requirements met through efficient geothermal energy. All other needs can be met through recycling of materials using nanotechnology.
Self-replicating robotics could conceivably be used to accelerate the restoration of the terrestrial ecosystem by eliminating most artificial constructs from the surface. The robots can also be used to construct any required underground habitats needed by those who decline to leave the planet.
Any materials that can not be directly recycled could be destroyed by sinking them into the ground at the foot of various subduction zones. This step can most likely be achieved by large scale application of current underwater robotics technology.
Finally all domesticated species may need to be removed, either through species-tailored sterlizing diseases or through a robotic search and collect program. Extinct species can only be restored by means of advanted technology and the collection and preservation of genetic materials. Thus libraries of such samples will need to be maintained up until the Earth Park program is implemented.