I like this quotation from poet and essayist Gary Snyder*:
"The ending of the lines of so many creatures with whom we have traveled this far is an occasion of profound sorrow and grief. Death can be accepted and to some degree transformed. But the loss of lineages and all their future young is not something to accept. It must be rigorously and intelligently resisted. Defend all these plants, bugs, and animals equally ? Little invertebrates that have never been seen in a zoo or a wildlife magazine ? Species that are but a hair away from one another ? It isn't just a case of unique lineages but the lives of overall ecosystems (a larger sort of almost-organism) that are at stake. Some archly argue that extinction has always been the fate of species and communities alike. Some quote a Buddhist teaching back at us: 'all is impermanent'. Indeed. All the more reason to move gently and cause less harm. Large highly adapted vertebrates, once lost, will never return in the forms we have known them. Hundreds of millions of years might elapse before the equivalent of a whale or an elephant is seen again, if ever. The scale of loss is beyond any measure the planet has ever known. 'Death is one thing, an end to birth is something else.'"
* The practice of the Wild (Berkeley: North Point Press, 1990).
Quoted by Paul Hawken, The Ecology of Commerce. A Declaration of Sustainability. (Chapter "The death of birth").
Even if I don't agree with him that "the scale of loss is beyond any measure the planet has ever known", since there have been some mass extinctions in the geological past with fractions of species lost between 30 and 50 percent (see the wikipedia article on extinction), what we could experience is of this magnitude, according to a recent article in Science (Richard A. Kerr, Science, vol. 318, 23 November 2007), which reports findings from the fourth assessment reports of the Intergovernmental Panel on Climate Change: for a global temperature increase of 1.5 degrees above preindustrial level (we are already at more than 0.5 degrees), about 10 to 15 percent of species would be committed to extinction; and for an increase of 3 degrees, about 20 to 30 percent of species would be committed to extinction. Above 4 degrees, the study reports "major extinctions around the globe". The level that we will reach depends on how much more greenhouse gases we will continue to release into the atmosphere as a result of fossil fuel combustion. The optimist scenario stabilizes around 2.5 degrees at the end of this century...