James Hansen has long used the idea of a climate tipping point in his public arguments for urgent and drastic action to combat climate change (For an extended exposition of his idea, see a pdf of one of his papers here.)
Note: in response to some comments about the term "tipping point" being vague, I have added some quotes below to illustrate what Hansen himself seems to believe. These are from a Huffington Post article by Hansen himself in 2008. I think it is fair to say he is talking about an irreversible catastrophe involving runaway change (my emphasis in the quotes to highlight this idea).
...we have used up all slack in the schedule for actions needed to defuse the global warming time bomb. The next president and Congress must define a course next year in which the United States exerts leadership commensurate with our responsibility for the present dangerous situation.
Otherwise it will become impractical to constrain atmospheric carbon dioxide, the greenhouse gas produced in burning fossil fuels, to a level that prevents the climate system from passing tipping points that lead to disastrous climate changes that spiral dynamically out of humanity's control.
...climate is nearing dangerous tipping points. Elements of a "perfect storm," a global cataclysm, are assembled.
Climate can reach points such that amplifying feedbacks spur large rapid changes.
Not all climate scientists agree. This New York Times report, for example, points out that:
But other scientists say there is little hard evidence to back up specific predictions of catastrophe. They worry that the use of the term “tipping point” can be misleading and could backfire, fueling criticism of alarmism and threatening public support for reducing greenhouse gas emissions.
Moreover, the last interglacial didn't tip the world into irreversible warming despite passing the threshold 2°C warmer than pre-industrial temperatures (it was followed by the ice age that ended ~10,000 years ago).
Update Tipping points are back in the news. The BBC recently reported this:
Researchers believe we could soon cross a threshold leading to boiling hot temperatures and towering seas in the centuries to come.
Even if countries succeed in meeting their CO2 targets, we could still lurch on to this "irreversible pathway"...
An international team of climate researchers, writing in the journal, Proceedings of the National Academy of Sciences, says the warming expected in the next few decades could turn some of the Earth's natural forces - that currently protect us - into our enemies.
So are warnings that the earth's climate is close to a tipping point just alarmism designed to stir public action? Or is there concrete evidence from modelling or climate history that clearly signals an imminent tipping point?