There's another concept that's discussed both in the systems literature and the popular press: the tipping point. That's the claim (loosely) that systems may reach a point where a slight addititional change will lead to a qualitatively different state. Often the definition includes something about irreversibility (you can't go back) or at least about the difficulty of reversing a change due to the reaching of a tipping point.
Now John Schellnhuber and his colleagues have published Tipping elements in the Earth's climate system in the February 12, 2008 issue of PNAS. They've also published appendices, not in the printed version, that provide their formal definition of a tipping point, evaluate other potential tipping elements, and describe how they elicited the information for the main article.
Those of you interested in tying systems concepts to the real world (hopefully that's just about anyone doing systems work, although some may be more interested in some applications than others) might find this of real interest. Those of you wondering about the risks of climate change might find it informative.
For a report on Schellnhuber's lecture at the annual meeting of the AAAS, see John Schellnhuber’s Third Industrial Revolution, a New Approach to Addressing the Hazards of Global Warming by Julia Whitty in Mother Jones.