Those abrupt changes, popularly known as "tipping points," are the low(?)-probability, high-impact catastrophes that keep climate scientists up at night: shifts in ocean circulation, release of methane from permafrost, sudden collapse of ice sheets. They're scary because they'd be both devastating and irreversible -- civilization-threatening stuff.
In the commentary, Paul Valdes reviews four examples of abrupt climate change from the historical record and finds that climate models don't do a good job simulating them. The models have to be retroactively tweaked to produce the conditions that ended up actually happening. Here's Valdes' conclusion:
According to the evidence from the past, the Earth's climate is sensitive to small changes, whereas the climate models seem to require a much bigger disturbance to produce abrupt change. Simulations of the coming century with the current generation of complex models may be giving us a false sense of security.
That moderate terror we were all feeling? Turns out that was a false sense of security. We should have been way more terrified! We're running an epic experiment on global biophysical systems with only the faintest clue what we're even doing, much less how to manage it. We know things could go rapidly, irreversibly, horribly wrong, but we're not sure how likely that is, or when it might happen. So we just blunder ahead at top speed. Because coal is cheap.
It's worth noting that the inability to grapple with long-tail risks also affects common economic models. They project climate-change costs rising linearly because, well, that's what makes the models tractable. That's why climate economics so often becomes "a knob-twiddling exercise in optimizing outcomes," in economist Martin Weitzman's memorable phrase. Fact is, radical uncertainty and volatility are the new normal. We have very little real grasp of the risks we face, we just know that some of them carry consequences so large -- potentially limitless -- that they completely short-circuit our models.
Anyway, for much more on the need to reform climate economic models, see this post. I've probably ruined your Thursday too by now, so I'll leave it there.