What evidence is there that shows that we are in the middle of a major mass extinction?

Here's a good article from this site http://www.pbs.org/wgbh/evolution/library/03/2/l_0...

Read the whole thing .. I am attaching only a bit.

Is the biosphere today on the verge of anything like the mass extinctions of the geological past? Could some equivalent of meteorite impacts or dramatic climate change be underway, as humankind's rapid destruction of natural habitats forces animals and plants out of existence?

Increasingly, researchers are doing the numbers, and saying, yes, if present trends continue, a mass extinction is very likely underway. The evidence is pieced together from details drawn from all over the world, but it adds up to a disturbing picture. This time, unlike the past, it's not a chance asteroid collision, nor a chain of climatic circumstances alone that's at fault. Instead, it is chiefly the activities of an ever-growing human population, in concert with long-term environmental change.

The background level of extinction known from the fossil record is about one species per million species per year, or between 10 and 100 species per year (counting all organisms such as insects, bacteria, and fungi, not just the large vertebrates we are most familiar with). In contrast, estimates based on the rate at which the area of tropical forests is being reduced, and their large numbers of specialized species, are that we may now be losing 27,000 species per year to extinction from those habitats alone.

here's another one

http://nonhumancommunications.blogspot.com/2007/08... says

So now we’re just discovering that we’re in the middle of a mass extinction. We happen to be in that moment when the worst thing that’s happened to the Earth in sixty-five million years is happening now. That’s number one. Number two, we are causing it. Number three, we’re now aware of it. There’s only a little splinter of humanity that’s aware of it. The numbers are this: At the minimum, twenty-five thousand species are going extinct every year. And if humans’activity were otherwise, or if humans weren’t here, there would be one species going extinct every five years. We’ve pushed up the natural extinction rate by the order of something like a hundred to a thousand times.

Here's a paper mentioning a theory about the mass extinction before our time. The reasons that they bring is similar to what we face now.

"Middle-Late Permian mass extinction on land " by Gregory J. Retallack et al.

The end-Permian mass extinction has been envisaged as the nadir of biodiversity decline due to increasing volcanic gas emissions over some 9 million years. We propose a different tempo and mechanism of extinction because we recognize two separate but geologically abrupt mass extinctions on land, one terminating the Middle Permian (Guadalupian) at 260.4 Ma and a later one ending the Permian Period at 251 Ma. Our evidence comes from new paleobotanical, paleopedological, and carbon isotopic studies of Portal Mountain, Antarctica, and comparable studies in the Karoo Basin, South Africa. Extinctions have long been apparent among marine invertebrates at both the end of the Guadalupian and end of the Permian, which were also times of warm-wet greenhouse climatic transients, marked soil erosion, transition from high- to low-sinuosity and braided streams, soil stagnation in wetlands, and profound negative carbon isotope anomalies. Both mass extinctions may have resulted from catastrophic methane outbursts to the atmosphere from coal intruded by feeder dikes to flood basalts, such as the end-Guadalupian Emeishan Basalt and end-Permian Siberian Traps.