[rwyinuse]

The main problem is agriculture. If rain patterns get severely disrupted in most of world's current breadbaskets, it takes time to increase production in areas that may now have more favourable climate. During that time lots of people would starve.

Rain patterns and extreme weather events are the things to really worry about. Temperature changes alone can be mostly dealt with by planting different crops.

[reverius42]

Oh, yeah, like even if it's survivable for humanity in general, it's going to kill billions of humans.

[apothegm]

It’s not just about time to increase production. A lot of crops that grow at lower latitudes won’t have enough time to mature in the short summers of higher latitudes and may suffer from weaker sunlight due to the lower angle resulting in more intervening atmosphere. We might eventually be able to breed or genetically modify crops to be hardier — but the former takes time and the latter requires sufficient remaining civilization and security to support the labs.

[timschmidt]

No doubt the transition period would likely involve more death than most catastrophes in history. In part because there are simply more people. Available sunlight is also less nearer the poles, which already affects agriculture in places like Greenland. Crops would shift. We'd be more dependent on energy and supplemental light for certain crops. Adjustment would be difficult. But quite a bit of land would still be habitable.

[Throaway199999]

Disease and parasitic life would also explode in all previously-habitable parts of the Earth due to increased temp.