'Hysteresis' seems to be the word we were looking for, a fancy way of saying tipping point.

https://arstechnica.com/science/2019/02/striking-study-finds-a-climate-tipping-point-in...

"A new study led by Caltech's Tapio Schneider may have identified a disturbing hysteresis in Earth's climate—a shift in cloud patterns in response to warming that could quickly heat the planet much further."

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"Up to about 1,000 parts per million, there were no major surprises. Things got around 4°C warmer and numbers changed for things like water vapor and cloud altitude. But the cloud deck generally looked familiar.

At about 1,200 parts per million, however, the simulated clouds suddenly dissipated. And without that shade reflecting sunlight, the world warmed another 8°C."

One question that this raises is, what might cause the CO2 to rise to 1,200ppm? According to the article, it's now at 400ppm:

"Are we doomed to see this play out soon? There's a good case to be made for, if not optimism, at least holding off on pessimism. It would take around a century of continued emissions growth to hit the equivalent of 1,200 parts per million CO2. Even the emissions-reduction pledges that have already been made can prevent that."

The article goes on to basically raise more questions than it answers, but that's worrying, really. Basically it's saying that there are all sorts of events that have happened in the distant past, are happening now, and could happen theoretically, none of which we really understand.