I still retain my antipathy to the idea. TL has now switched from “tipping points” to “tipping elements” to describe the components of the Earth system that can be switched – under particular conditions – into a qualitatively different state by small perturbations. This is broad and vague, but he produces a longer but also very broad and vague definition later in his article. He wants to include slow changes “analogous to passing the points on a railway track”.
TL’s first example is the Great Oxidation 2.4byr ago, and the “tipping element” is the entire earth system. Which seems to me to stretch things so far as to make the whole concept meaningless. The GO was – presumably – very slow by any kind of timescales we’re used to – millions of years I’d guess at least. It only looks fast because it was so distant. If a “tipping point” really includes changes on this timescale, then saying “we are approaching a tipping point” should be met with yawns.
D-O events is a much better example, though unfortunately we still don’t understand these so its not clear at all what the “tipper” was.
Probably the clearest example of a tipping element that has already been triggered by human activities is one that is not primarily related to climate change; the Antarctic ozone hole This isn’t clear at all – because the CFC perturbation probably wasn’t “small”. Of course, since there is no meaningful definition of “small” there is no way to decide.
A case can be made that climate warming may have caused the Arctic sea-ice to pass a tipping point… analysis has shown that positive ice-albedo feedback (the warming due to changing from reflective ice to dark ocean surface) dominates over external forcing (the global warming signal) in causing the thinning and shrinkage since around 1988 so this at last may be a real tipping point? Possible, though I’m not sure what analysis he has in mind (my own work shows that, at least i a GCM, if you take the sea ic away it grows back again, so the ice-albedo doesn’t dominate but needs external forcing).
And then, inevitably, we come to the Greenland ice sheet. Is it a tipping element? TL discusses what happens under a 3 oC warming. Is that, too, a “small” perturbation? Seems quite large to me.