Imagine for a moment that the World is suddenly drained of all human beings who inhabit it. Everything else remains as it stands; the mechanism of this extermination has no importance, mass suicide for example.

So our planet would be freed of the anthropogenic stress placed on it for about 50,000 years, since the advent of human civilization. How would it evolve? Here is a possible scenario.

Cultivated fields return to wilderness; crop varieties are diluted among other species. The soil biological activity gets diversified in those areas where single crops were grown.
Approximate time: a few decades.

Farm animals and pets are spreading in nature and are subject to competition from wildlife. Some species survive and evolve, others disappear. Fish stocks are reestablished to a new level according to a new hierarchy of predation. There is not more or fewer species; neither are there more nor less appearances or disappearances of species. But local biodiversity is restored in those places where it had been reduced by human activity.
Approximate time: from 3 to 5 generations, or about one century.

Built-up areas, cities and transportation routes, are invaded by vegetation. Concrete buildings are deteriorating, and steel structures are rusting and dissolve. Biomass accumulates since it is no longer removed from the fields and forests. Constructions are slowly covered by soil that builds up by the action of soil organisms that fix and mineralize the biomass. The ruins become hidden.
Approximate duration: a few centuries (in tropical and temperate zones, following the example of Mayan ruins) to millennia (in cold areas).

Wastes degrade, the nuclear ones slower than others. Organic pollution is decomposed; mineral pollutions are either fixed in their state or slowly washed away by runoffs. Mines are closed and become clogged.

The electromagnetic cacophony is muted: radio, television, and other means of telecommunication. While most satellites are re-entering the atmosphere and destroyed, others remain in orbit or continue their journey into interstellar space; their energy capacities are diminishing slowly and turn off after perhaps several hundred years. These last signals sent by the humans continue to translate into the cosmos at the speed of 300,000 km per second.

Emissions of greenhouse gases are halted. While emissions of carbon dioxide loaded the atmosphere by an additional 3.8 ppm per year, only 2 ppm concentration increase was observed. This means that the absorption by water, soil and biomass was about 1.8 ppm of CO₂ each year. At this rate, assumed to be linear, 62 years will be necessary for the atmospheric concentration of CO₂ to decrease from  391 to 280 ppm, the pre-industrial level. Methane continues to be emitted by swamps and animals. Nitrous oxide is no more emitted by the decomposition of synthetic fertilizers. Chlorofluorocarbons disappear from the atmosphere in a few decades or centuries.

The weather continues to be changing and the climate evolves between ice ages.

Human knowledge is kept on paper, or on magnetic and optical media. It is likely that some copies remain available for tens of thousands of years. But it is also likely that neither the whole of knowledge nor the taste of human artistic expression will remain intact.

And then nothing else: the Earth is saved! Was she ever in danger?

She keeps a few scars that reflect an earlier human presence.

She can expect the emergence of a new species that possesses superior intelligence and consciousness of itself, apt to recreate a civilization.

This hypothetical heir will have to develop competencies in archeology to possibly find traces of human knowledge. But it will be of little use since the new civilization’s development state will already have been high enough to be able to interpret these traces. It will be interested, curious and amused at what it will understand about human wandering in prehistoric times. During its development it will also have problems and concerns about its sustainability. This is when an individual living in this civilization will speak on the subject of its disappearance and of the advent of a new dominant species.

Prometheus ending like Sisyphus, there will be nothing new under the sun until our star becomes a red giant, swallowing the Earth, and with it all traces of one or more civilizations.

A few more billions years of an ephemeral world.

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