No runaway---climate stabilizes at 4 degrees.

Thirdly, many of the feedbacks in the Hothouse Earth pathway are long-term, committing us to a Hothouse state in several centuries’ time rather than over the next few decades. Several tipping elements in the cascade figure above (e.g. ice sheets & the thermohaline circulation) will take centuries to millennia to complete their transitions even if tipped soon, while some of the others (e.g. permafrost thaw and Amazon/Boreal forest dieback) will still take decades to centuries. Stability of both the Greenland and West Antarctic Ice Sheets is projected to be at risk of tipping above 1.5-2°C (often mistakenly cited as evidence for a generic global tipping point rather than ice-specific). This would guarantee the eventual collapse of these ice sheets, and parts of West Antarctica may have already crossed this point. But ice sheets are so massive that this will still take far longer than our lifespans to play out. The Hothouse Earth trajectory also assumes temperatures stabilise and remain above these limits long-term – if we were to overshoot 2°C for a few decades but successfully draw down enough carbon to go back below, this might still stop these processes early enough to prevent a total collapse.

Finally, some of the coverage used the term “runaway warming” to describe the Hothouse Earth trajectory, but this is inaccurate. Runaway warming in scientific terms means when positive feedbacks surpass all negative feedbacks and uncontrollably amplifies warming until all water evaporates, creating a ‘moist greenhouse’ as likely happened on early Venus. But this is not what the Hothouse Earth paper is proposing – instead they are suggesting that there is an alternative stable climate state at around 4°C warmer than pre-industrial (shown in the stability landscape figure earlier on), and that the 2-4°C range is inherently unstable and leads to the Earth warming until it reaches this state. But this new state is potentially quite stable (as in the stability landscape image), and warming beyond it is unlikely without substantial further emissions. As the greenhouse effect is logarithmic (i.e. each 100ppm extra CO2 causes less warming than the 100ppm before) the effectiveness of positive feedbacks is gradually curtailed. True ‘runaway warming’ would take far more emissions (and probably much more than we are actually capable of) to surpass this, and so accelerating warming does not imply runaway warming that continues indefinitely. And even if people saying runaway is likely don’t mean a true Venusian runaway, the term still implies that a large amount of warming will soon become unstoppable, which calculations of feedback and tipping point impacts currently indicate is very unlikely.