A world without clouds

you ever get that sense you might be living in the end times of humanity? well... if this scenario plays out, it's not unreasonable that you might be. to be clear, it is just a model--but it's one that, like literally everything else, suggests that things are going to go badly very quick in this century unless things start changing yesterday and in this particular scenario, likely lead us to the brink of extinction.

To imagine 12 degrees of warming, think of crocodiles swimming in the Arctic and of the scorched, mostly lifeless equatorial regions during the PETM. If carbon emissions aren’t curbed quickly enough and the tipping point is breached, “that would be truly devastating climate change,” said Caltech’s Tapio Schneider, who performed the new simulation with Colleen Kaul and Kyle Pressel.

like, there's really no other way to state it. even a 3C or 4C change in climate (which we are most likely on track for at this point) would probably lead to hundreds of millions of climate refugees and the upending of large parts of human civilization as we currently know it. 12C would be genuinely apocalyptic.

At the 4-degree end of the range, we would see not only “the destruction of the world’s coral reefs, massive loss of animal species, and catastrophic extreme weather events,” Mann said, but also “meters of sea-level rise that would challenge our capacity for adaptation. It would mean the end of human civilization in its current form.”
It is difficult to imagine what might happen if, a century or more from now, stratocumulus clouds were to suddenly disappear altogether, initiating something like an 8-degree jump on top of the warming that will already have occurred. “I hope we’ll never get there,” Tapio Schneider said in his Pasadena office last year.

the threshold they come to is somewhere around 1200ppm for this scenario in the model, which is about three times the current level.

Countervailing forces and effects eventually get overpowered; when the CO2 level reaches about 1,200 parts per million in the simulation — which could happen in 100 to 150 years, if emissions aren’t curbed — more entrainment and less cooling conspire to break up the stratocumulus cloud altogether.
Schneider emphasized an important caveat to the study, which will need to be addressed by future work: The simplified climate model he and his colleagues created assumed that global wind currents would stay as they are now. However, there is some evidence that these circulations might weaken in a way that would make stratocumulus clouds more robust, raising the threshold for their disappearance from 1,200 ppm to some higher level. Other changes could do the opposite, or the tipping point could vary by region.

but fret not, there's still hope and we are, currently, a long way away from that cataclysm (although things will alter how fast we get to that point). with a ton of effort, carbon neutrality--or at least, significant carbon reduction--is very much possible:

There’s a long way to go before we reach 1,200 parts per million, or thereabouts. Ultimate disaster can be averted if net carbon emissions can be reduced to zero — which doesn’t mean humans can’t release any carbon into the sky. We currently pump out 10 billion tons of it each year, and scientists estimate that Earth can absorb about 2 billion tons of it a year, in addition to what’s naturally emitted and absorbed. If fossil fuel emissions can be reduced to 2 billion tons annually through the expansion of solar, wind, nuclear and geothermal energy, changes in the agricultural sector, and the use of carbon-capture technology, anthropogenic global warming will slow to a halt.

what this underscores, though, is people really need to work and push for climate-friendly policies and developments everywhere immediately. we might not just be talking mass-migration and mass-displacement anymore as would probably be the case with just a 2C or 3C or 4C rise.