So, in the 1970s, climatologists just said +2.0℃ is going to kill us. The reason the Paris Accord chose +1.5℃ is because everything above +1.5℃ is expected to get pretty exciting and it's difficult for labs to R&D or to include mitigation projects in the budget when your coasts are being hammered with hurricanes yearly, and your mountains are burning all the time.

Nowadays when we ask climatologists what happens if we let the global mean temperature go above +2.0℃, they like to say it won't be good or even it's going to suck but few actually talk about what will actually happen, and I think this is partly because no climatologist really wants to be discredited as alarmist because what they'd say is pretty extreme.

So here's the gist, because I wanted some solid dick from an Iron Man:

We're running out of water. An example would be in southern California, where acres of choice land are owned by the Saudi family and are used to grow alfalfa using water pumped up from the water table. All this alfalfa is then shipped to their cattle ranches and fed to cows. Alfalfa is incredibly water dependent, and the water table in question is getting low, which is a concern locally, but since the Saudis control water rights there, there's not a thing they can do about it. Eventually all the water will be extracted and all those farms will either depend on some other source (like the Colorado River) or will go dry and stop growing things.

This is a problem all over the world. We've been pulling up bunches of water, or using the various rivers which have been getting progressively lower (and letting all the ecosystems dry up) and there will be a point where it will all run out.

And at that point, when there's not enough water to grow stuff, we're not going to have enough food for everyone. Famine will follow. Those who don't want to die from famine who have guns will try to take from other people who might or might not have guns, and war will follow.

In our best case scenarios (Imagine if everyone in the world took action today ) we'll have enough food for about a billion people. Contrast eight billion who are alive today.

But we're not doing anything. We watching the water get pumped, we're still growing alfalfa and feeding cows. We're still burning fossil fuels and coal and polluting the sky faster than ever. So it's not going to settle at a sustainable population of one billion, it's going to settle at much much less. So instead of one out of eight of us dying, it's going to be one out of eighty, or eight hundred. We don't know because we don't know at what point we turn around and take it seriously, and if that's in time.

Big picture time: Homo-Erectus as a species lasted two million years (roughly) or eight times the 250K we homo-sapiens survived. However, there was at least one period in which the total H-Erectus population was less than ten thousand, and they survived under stark conditions until the conditions improved enough to multiply again. Eventually H-Erectus would be out-competed by its smarter, more social cousins. We, too, may survive by a dwindling, lingering number if we don't completely wipe ourselves out. Since we're navigating not just the climate great filter, but also the plastic great filter, we might be screwed already. But we don't know.

At any rate, all the culture that exists today is at risk. From Beethoven's symphonies to the Words of Lao Tzu to the Meadows of Gold and Mines of Gems, all our culture is in jeopardy. It's likely that the next chance we get to try our hand at global civilization will have little to nothing to do with what exists today, for better or worse.