Critical Thresholds in Earth's Ecosystem...

Feedbacks, Tipping Points, Planetary Boundaries, Ecological Footprint, Irreversible Changes, Ecosystem Collapse are mentioned frequently. 
Lot of these are matters of opinion and the time scales associated with such predictions are important.  
When will we reach a tipping point and how long after that the ecosystem will change in a way never to return to the current state are relevant questions.  How long is 'never'?
In planning and making decisions, human time scales range from a decade to a millenium. 
Politicians and policy makers largely work on a timescale determined by the next general election - and that is most unfortunate in the present context.

In relation to the Earth's Ecosystem, Tipping points or Critical Thresholds are real.  They have occurred in the past.  Great extinctions have happened when the world's living species have been reduced to a small fraction of their pre-event numbers.  Even as recently as 12,000 years ago, global temperatures had risen by a massive 5 or 6 degrees in a matter of decades making a transition from the ice age to the warmer interglacial that we enjoy just now.

As human population grows, the rate at which we consume Earth resources and produce waste increases.  Ecosystem services (worth a cool $33 Trillion) have limits.  Earth can produce services we demad and clean up the waste we produce only up to an extent.  We are already using resources of the earth 50% faster than they may be replenished.
There are signs that Earth's capacity is being adversely affected by our overuse and may be weakening.  Increasing consumption will only quicken the weakening setting up a spiral towards collapse.  One can see how a tipping point may be reached if business as usual (increasing population and overconsumption) continues.

Scientists in Journal Nature (2009) have identified nine processes in the ecosystem which have critical thresholds or tipping points (subject to limits or planetary boundaries).  According to the study, three of these processes have already passed the tipping points with the others under serious stress.  Remember, however, it is difficult to identify the actual point when a prcess passes its critical threshold or even to know what the critical threshold is.
National Economic Council Report (January 2010) discusses these processes in deatil.  
At this stage we shall only list them.

1.  Climate Change
2.  Rate of biodiversity loss (terrestrial and marine)
3.  Interference with the nitrogen and phosphorus cycle
4.  Stratospheric ozone dpeletion
5.  Ocean acidification
6.  Global freshwater use
7.  Change of land use
8.  Chemical Pollution
9.  Atmospheric aerosol loading

We shall look at some of these in future blogs.