The point where a system irreversibly starts to change from one state of equilibrium into another. As opposed to a threshold, which is a point that when crossed brings rapid change.
Shooting for the tipping point. A 1962 Life-Magazine ad for Humble Oil (which later merged with Standard Oil to become Exxon).
1. Here's arch-skeptic blogger Chiefio on the proposition that human emissions could produce CO2 levels that act as a tipping point for a runaway greenhouse effect:
"The whole ‘tipping point' thesis is simply and demonstrably false. For most of the history of the planet, CO2 has been much much higher. 10 times or more. We are actually at historic low levels. (Plants respond to CO2 as they do to any nutrient that is lower than their ideal value, up to about a 1000 ppm value. This implies they evolved expecting that much, and that is what the geological record shows.) Why are we trying to reduce CO2 to levels that restrict plant growth? Why are we trying to make the planet colder when that reduces food production? The potential harm here is stupendous. Why have we never ‘tipped' before?"
2. Here's Wallace Broecker on tipping points:
Skeptic claim: "The Earth's climate system has a built-in tendency to compensate any impetus to change.
Rebuttal: The records kept in ice from Greenland, in sediments from the ocean floor, and in stalagmites from caves provide firm evidence that this is definitely not the case. Rather, the climate system appears to have overreacted to the nudges it has received.
These nudges have three known origins: tiny changes in the Sun's luminosity, cyclic changes in the Earth's orbital characteristics and reorganizations of the ocean's circulation. Indisputable evidence exists in the records of past climates that the Earth has, in a sense, over-responded to all three. Models suggest that a nudge to our climate system by a tripling of atmospheric CO2 will be more than an order of magnitude stronger than that from these natural nudges. Hence, it would be prudent for us to establish a cap on the extent of this CO2 increase."