Global warming reshuffle

[Corbeau]

The GW mechanism in Freeciv is just plain wrong, realism-wise and gameplay-wise. It has a period of accumulation where nothing happens and then it blows up and creates massive damage in one turn. Firstly, this is not how climate change works and, secondly, it creates discrete and abrupt havoc on players' lands.

Instead, I propose the following:

1. Population pollution (PP) and industrial pollution (IP) are completely separated.

2. PP influences the rate of creation of polluted tiles on the map and ends there.

3. What is now global "Pollution rate" is renamed to "Excess CO2" and is no longer influenced by the polluted tiles on the map. Instead, it is a direct consequence of the sum of all IP across the map.

4. "Chance of catastrophic warming each turn" is renamed to "Climate change rate" and works in the following way:
- it is now the chance that ONE TILE somewhere will suffer change on one turn (grass to swamp, swamp to water, plains to desert).
- once this happens, the rate does not go back to zero, but remains the same / grows further with increasing IP
- once the rate exceeds 100, one tile suffers change automatically and the remainder (P-100) is the probability for an additional tile suffering change; once it exceeds 200, it's 2 tiles + probability and so on

Both PP and IP conversions to effective events should be controlled by server/ruleset settings; probably the ones already in place can be used.

A simpler solution would be to not separate IP and PP, but for both of them to work on both polluting tiles and excess CO2.

OR to leave most things as they are, only apply #4 and ignore #1, #2 and #3.

[Corbeau]

Second iteration of the idea:

1. Pollution baseline per city already exists in ruleset files. It can be adjusted.

2. Any excess pollution (beyond baseline) adds to ExcessCO2, add all cities.

3. every turn: climate_change_rate (was "chance of catastrophic warming") = climate_rate_change + ExcessCO2 * globalwarming_percent (the latter is a server setting)

4. For every 100 climate_change_rate, 1 tile somewhere in the world is automatically changed every turn, the remainder goes as probability for an additional tile.

This creates the mechanism and calibration is fully up to the ruleset. Ruleset testing is needed in order to get the desired effect but that is no longer a concern of the developers.

For added realism:

5. Every forest/jungle tile in the world decreases total ExcessCO2 by 1.