[hmd_imputer]

So, in the best-case scenario, there will be a mere 1.0degree Celcius increase and in the worst-case scenario, the average increase will be 6.5degree Celcius. I don't know how it is possible to model technological breakthroughs. I mean if we have a working Nuclear Fusion technology by 2100, that could drastically change everything - (almost) free energy = unlimited desalinated water = full aforestation, even in the desert areas.

[pharke]

The knock on effects from unlimited energy go a lot further.

We could use technology we have today for CO2 capture since it is heavily energy dependent and simply scale it massively. The bulk of industry processes that currently require coal, oil, or gas to provide cheap heat could be fully electrified. Prices of a lot of commodities would drop dramatically. Indoor farming would be a cinch. People and governments would abandon internal combustion for the cheaper alternative.

We could do absurd things that no one would even consider today like greening deserts as you mentioned or settling the arctic. You might be able to replace concrete with basalt if you could cleverly design a system for melting stone and casting the resulting lava in a form. Hell, you could banish winter by putting radiant heat in everything, walls, sidewalks, streets, and adding supplemental lighting like Moonlight Towers[0] except I suppose they would be Sunlight Towers in this case. The list goes on.

The world would change so much it would be like living through the first half of the 20th century again. From horses, coal, and brick buildings to jet aircraft, nuclear power, and glass skyscrapers.

[0] https://en.wikipedia.org/wiki/Moonlight_tower

[lopmotr]

Nuclear fusion won't be free energy at all. The plants will still cost money to build and operate. If you're only counting fuel cost, then solar and wind are already free energy. Even conventional nuclear fission is also almost free by that standard, and indeed people used to imagine it would be "too cheap to meter".