[bmcm]

Nice article on the “tipping point” concept; but it doesn’t explicitly mention what I think is the most decisive aspect – irreversibility. That is, once you cross a tipping point not only do you get a relatively “large” change in the “output” variable of interest (possibly after a lag in time, which may be long), but simply reversing the input variable back below the tipping point cannot fix this. In this sense, although “tipping point” conjures up the image of a balance scales (or see-saw), that is completely the wrong idea: because with a scales, just adjusting the relative weights back will tip the balance back the other way. That doesn’t work with real “tipping points” – or at least, not the kind of concern in climate change.
The distinction is clearly seen in the metaphor of the camel’s back: removing that last straw can’t make the camel whole again. Now mind you, sometimes irreversibility is more limited that that: it may be possible to get back to where you were, but simply bringing the input back below the original (“upward”) tipping point will not suffice. Instead, it is necessary to bring the input back below some lower value – the “downward tipping point”. This is called hysteresis. One of the simplest and most familiar example of a system with hysteresis is the typical light switch: once you push past the “tipping point” for “on”, it clicks or locks into that state and you then have to push a good bit in the opposite direction to get back to the “off” state. Hysteresis is typical of systems involving positive feedback – like the climate system.
So in the context, it might be better not to talk about “tipping points”, but rather “points of no (practical) return”. In even a moderately complex system it is typically hard to be sure when such a point is crossed; but in the case of the current situation with climate, I would not be optimistic…
Regards – Barry.

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