Some of those signs:
- Earth is bubbling with supervolcanos.
- Invasive species are everywhere. They can eat all the food starving out local creatures who can't migrate easily.
- Climate by definition is a process of change. This change seems to be accelerating in the last decades, not allowing species to either adapt of migrate.
- Ocean acidification. We should also add overfishing and dumping of plastics and other toxic and nuclear waste.
Read more: http://io9.com/7-signs-we-are-heading-for-a-mass-extinction-5950630
We are familiar with iconic species disappearing due to human actions, such as the polar bear, of course, and elephants, rhinos, and bees. But there are countless other less popular disappearing species that play a key role in their ecosystems. Their dwindling or disappearance can precipitate a cascade of other extinctions.
Where is the tipping point? Who knows. But we should remember how living things have a huge influence on Earth's climate.
The Gaia Hypothesis http://www3.geosc.psu.edu/~dmb53/DaveSTELLA/Daisyworld/gaia.htm
argues that it is the biosphere that regulates Earth's climate so as to make it more hospitable to life. If we mess with natural ecological equilibrium, we harvest disaster.
Mass extinctions - http://science.nationalgeographic.com/science/prehistoric-world/mass-extinction/