Even if there is no mechanic that would dynamically change the terrain into a colder or warmer Earth, I would argue we can emulate a somewhat “cold” / “warm” map through the Advanced Setup.
- For a colder Earth, we can pick Cold temperature, Arid rainfall, Low sea level and a lower disaster level.
- For a warmer Earth, we can pick Hot temperature, Wet rainfall, High sea level and a higher disaster level.
Cold temperature setting doesn't seem to increase the number of Polar Ice tiles (I need to check that). You can also change the World Age, for New (more Hills and Mountains) or Old (more Flatland).
About Nuclear Power, I need to talk about the Power system and CO2 system.
With Gathering Storm, some buildings have been updated to require Power such as the Factory, the Research Lab, Broadcast Center, the Stock Exchange, the Food Market, the Shopping Mall, the Airport, the Stadium and the Aquatic Center. Powering those buildings will increase the yields given by them.
To generate Power, you can build a Power Pant. There is 3 different kind: a Coal Power Plant, an Oil Power Plant, and a Nuclear Power Plant. They consumes Strategic Resources and convert it into Power to cities within 6 tiles range. They also generates yields by themselves:
- Coal Power Plant: Grants Production equal to the Industrial Zone adjacency bonus.
- Oil Power Plant: +3 Production to all City Centers within 6 tiles (cannot stack).
- Nuclear Power Plant: +4 Production and +3 Science to all City Centers with 6 tiles (cannot stack). Randomly, it can: self pillage itself; pillage the tile improvements, districts and building adjacent to the Industrial Zone; damage the defense of adjacent City-Center and Encampment; kill adjacent citizens, civilians and military units; and create an area of effect draining 50 HP per turn for units finishing the turn there. The risk starts 10 turns after the Nuclear Power Plant has been build (doesn't scale with Speed...), but can be reset by the Recommission Nuclear Reactor project, a 400 Production project.
When Coal, Oil and Uranium are consumed, they generates CO2. The amount per Strategic Resources is different: High for Coal, Medium for Oil, and somehow, Uranium generates some CO2 too. It applies to units too (like Ironclad, Tank and Giant Death Robot), or when putting Railroad (as it requires Coal).
Oil Power Plant have its use when nearby cities doesn't have an Industrial Zone, so that you can power them and give them +3 Production. Nuclear Power Plant has drawbacks that outweigh its marginal bonuses. I don't why they implemented the Nuclear Power Plant like this, but can be used the same way the Oil Power Plant. As the bonus given by Power Plants cannot stack, there is no use to build Oil or Nuclear Power Plants everywhere.
It may depend on the playstyle, but for me, the Coal Power Plant is vastly superior. Coal is more handy as no end-game units requires Coal, so we can freely use it without competition. It is improved by Mines, so easier than Oil that may require an Offshore Oil Rig, unlocked later. Oil is also highly demanded for late-game unit. Uranium is quite rare and allows to maintain Giant Death Robot.
As I tend to build high adjacency for my Industrial Zones, I use a lot Craftsmen and Five-Year Plan that double the Industrial Zone's adjacency bonus, making Coal Power Plant even better. I use Craftsmen even more since it has been converted into a Military policy card. I would argue that Natural Philosophy is the new Rationalism (more ways to gain Science adjacency, Rationalism being harder to trigger), and Five-Year Plan is the fusion of Craftsmen and Natural Philosophy, so that card is going to be slot as well.
There are also buildings and tile improvements that generate Power with no Strategic consumption (so: no CO2), such as an Hydroelectric Dam (a Dam's building), Geothermal Plants, Solar Farms and Wind Farms.
CO2 emitted in the world stockpiles. When it goes beyond some threshold it dynamically remove Polar Ice tiles, flood or remove some coastal tiles. There is a total of 8 phases of sea level rise, going from +0 meter to +3.5 meters that gradually flood and remove coastal tiles. Improvement, Districts and Wonders can be pillaged by flood or removed due to the rising sea level. It also increases the likelihood of Natural Disasters. Flood Barrier can be built to counter it.