Johan Rockström is a person we should all listen to I feel. He and his colleagues identified and quantified a number of planetary boundaries that must not be transgressed in order to prevent unacceptable environmental change caused by human activity. They published their findings in Nature, an excerpt of which is provided below. Here is a video of Johan speaking, describing his work.
A safe operating space for humanity
To meet the challenge of maintaining the Holocene state, we propose a framework based on ‘planetary boundaries’. Theseboundaries define the safe operating space for humanity with respect to the Earth system and are associated with the planet’s biophysical subsystems or processes. Although Earth’s complex systems sometimes respond smoothly to changing pressures, it seems that this will prove to be the exception rather than the rule. Many subsystems of Earth react in a nonlinear, often abrupt, way, and are particularly sensitive around threshold levels of certain key variables. If these thresholds are crossed, then important subsystems, such as a monsoon system, could shift into a new state, often with deleterious or potentially even disastrous consequences for humans
NATURE|Vol 461|24 September 2009