The Sixth Great Extinction
Five times in the past half-billion years, the fossil record shows us, living things have been wiped out over much of the earth. Catastrophic changes in climate, or the impact of an asteroid or a comet, are the likeliest causes for the five great extinctions which geologists and palaeobiologists have identified, ranging from the Ordovician-Silurian extinction, of about 439 million years ago, to the Cretaceous-Tertiary (K/T} extinction of 65 million years ago, when the dinosaurs disappeared . -- The Five Worst Extinctions in Earth's History. Some add another at the Cambrian-Ordovician boundary.
What is the rate of extinction? The normal "background rate of extinction is roughly 1-2 species per year." . Others calculate it at as 'two to five families of marine invertebrates and vertebrates every million years'.
"Paleontologists estimate the background rate of species extinction--the long-term extinction rate exhibited prior to humanity's influence--at between 1 and 10 extinctions each decade among every million fossil species. Assuming from a variety of estimates that 10 million species are alive today (Stork 1993 and 1997, May 1988, Hammond 1992), scientists can expect from 1 to 10 species to go extinct each year from all forms of life, visible and microscopic. In fact, species are exiting much faster. Based on records of extinction among the best- studied types of animals, ecologist Stuart Pimm and colleagues calculated extinction rates during the past century to range from 100 to 10,000 species per year (again, assuming 10 million species exist). That rate is between 100 and 1000 times faster than the background rate of species extinction (Pimm et al 1995)."  - Bold mine. Extinction rates in the tropics are based on rates of deforestation.
Unfortunately "the Earth needs, on average, about 10 million years to recover from global extinctions".
"If we substantially diminish biodiversity on Earth, we can't expect the biosphere to just bounce back. It doesn't do that. The process of diversification is too slow", "The planet would be biologically depleted for millions of years, with consequences extending not only beyond the lives of our children's children, but beyond the likely lifespan of the entire human species." Evolutionary "Speed Limit". For a graph of mass extinction events click here.
The current, or sixth mass extinction is entirely human caused. This is being accomplished through a variety of means including habitat destruction, hunting and poaching, the introduction of non-native species, pollution and climate change. In 1992 "some 1,700 of the world's leading scientists, including the majority of Nobel laureates in the sciences" warned that:
Human beings and the natural world are on a collision course. Human activities inflict harsh and often irreversible damage on the environment and on critical resources. If not checked, many of our current practices put at serious risk the future that we wish for human society and the plant and animal kingdoms, and may so alter the living world that it will be unable to sustain life in the manner that we know. Fundamental changes are urgent if we are to avoid the collision our present course will bring about.... WARNING we the undersigned, senior members of the world's scientific community, hereby warn all humanity of what lies ahead. A great change in our stewardship of the earth and the life on it, is required, if vast human misery is to be avoided and our global home on this planet is not to be irretrievably mutilated. World Scientists' Warning To Humanity
Concerning Global Warming alone we read that:
Per Feeling the heat: Climate change and biodiversity loss and Never Mind That Boiling Kettle, "International scientists from eight countries have warned that, based even on the most conservative estimates, rising temperatures will trigger a global mass extinction of unprecedented proportions. They said global warming will set in train a far bigger threat to terrestrial species than previously realised, at least on a par with the already well-documented destruction of natural habitats around the world. It is the first time such a powerful assessment has been made and its conclusions will shock even those environmentalists accustomed to "worst-case" scenarios." 
"the corporate media's long-term, stubborn refusal to address the real issues behind global warming - the corporations fighting with unrelenting ferocity to destroy not just the Kyoto protocol but the environment movement itself - represents the ultimate betrayal of us, our future, and our planet." 
Gaia Theory's James Lovelock predicts disaster .
Then: French explorer Pierre Esprit Radisson c1652 in the description of his journey (of what would later become the United States): "The further we sojourned the delightfuller the land was to us. I can say that in my lifetime I never saw a more incomparable country....The country was so pleasant, so beautiful and fruitful that it grieved me to see the world could not discover such enticing countries to live in. This I say because the europeans fight for a rock in the sea against each other, or for a sterile and horrid country. Contrariwise, these kingdoms are so delicious and under so temperate a climate, plentiful of all things, the earth bringing forth its fruit twice a year, the people live long and lusty and wise in their way."
Now: "We are predicting the extinction of about two-thirds of all bird, mammal, butterfly and plant species by the end of the next century, based on current trends." --Peter H. Raven (1999), former President of AAAS, the American Association for the Advancement of Science.
Before I flew I was already aware of how small and vulnerable our planet is; but only when I saw it from space, in all its ineffable beauty and fragility, did I realize that human kind's most urgent task is to cherish and preserve it for future generations. --Sigmund Jähn, Astronaut German Democratic Republic (More quotes from astronauts here)
Other SourceWatch Resources
- The IUCN Red List of Threatened Species
- Hundreds of links chronicling extinction news reports
- Correcting myths about biodiversity/endangered species
- Species Alliance nonprofit organization, producing film Call of Life: Facing the Mass Extinction
- The Sixth Extinction By Niles Eldredge
- The State of Major Ecosystems
- The Scale of our Presence
- Watch this clock!
- Human Population through History 1 A.D. to 2020