When you were taught geography in school, they taught you about the forces that shape the landscape: erosion by wind, erosion by fresh water in rain and rivers, and erosion by the sea.
So, which is the most active geological force on planet Earth? What moves the most soil per year?
The answer is: none of the above. People, and their lackey machines do. A certain species of ape that wears clothes and drives bulldozers is the dominant geological force on the planet at present.
What uses more than half the fresh rainfall? What eats a quarter of the fish? Us again.
If you want to know how the Bengal Tiger, coral reefs or the anopheles mosquito will be faring in a few hundred years time - an instant in geological terms? You'll have to know what these swarms of apes are up to.
Want to know how the global climate will be changing over the next few thousand years? What's going to determine the radiation levels, CO2 levels, the ozone levels and oceanic pH? Again, you'll need to watch the monkeys. Will recent deposits of plastic and concrete build up over thousands of years into whole new strata? Maybe.
If you define "natural" as "not involving the actions of human civilisation" then the natural world does not exist any more. Human actions determine what happens on planet earth. Nature is dead and we're in charge.
Paul Crutzen, a German atmospheric chemist at the Max Planck institute, who got the Nobel prize in chemistry in 1995, and Eugene F. Stoermer, a researcher at the University of Michigan, proposed the word Anthropocene in the year 2000, to characterise the current geological epoch as the Anthropocene epoch, when the human race is the most important force acting upon the earth's ecology and geography. This can be regarded to start circa 1800 with the industrial revolution, though our influence was felt earlier; from the deforestation and desertification at the early Mediteranean and Near east civilisations a few thousand years ago, way back to the Pleistocene epoch summarised by Wikipedia as "Extinction of many large mammals. Evolution of fully modern humans" These two events happening together is not widely regarded as a co-incidence.
They said:
To assign a more specific date to the onset of the 'Anthropocene" seems somewhat arbitrary, but we propose the latter part of the 18th century, although we are aware that alternative proposals can be made (some may even want to include the entire holocene). However, we choose this date because, during the past two centuries, the global effects of human activities have become clearly noticeable. This is the period when data retrieved from glacial ice cores show the beginning of a growth in the atmospheric concentrations of several 'greenhouse gases", in particular C02 and CH4. Such a starting date also coincides with James Watt's invention of the steam engine in 1784. About at that time, biotic assemblages in most lakes began to show large changes.
Without major catastrophes like an enormous volcanic eruption, an unexpected epidemic, a large-scale nuclear war, an asteroid impact, a new ice age, or continued plundering of Earth's resources by partially still primitive technology (the last four dangers can, however, be prevented in a real functioning noosphere) mankind will remain a major geological force for many millennia, maybe millions of years, to come. To develop a world-wide accepted strategy leading to sustainability of ecosystems against human induced stresses will be one of the great future tasks of mankind, requiring intensive research efforts and wise application of the knowledge thus acquired in the noösphere, better known as knowledge or information society.
Sources:
Wikipedia article on Anthropocene,
http://www.mpch-mainz.mpg.de/~air/anthropocene/Text.html has the text of Crutzen and Stoermer's paper,
other news sites reporting on this paper, easily found via google, notably this.
Plastic found just about everywhere on earth: http://news.bbc.co.uk/1/hi/sci/tech/6218698.stm