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In previous versions, Global warming changed land tiles to jungles, swamps, or deserts one after another.
Therefore my nation was forced a decline of economic or productive powers.
I hated it, but on the other hand, I made an effort to reduce the amount of pollutions.

In latest versions, Global warming changes land tiles to oceans directly.
This just makes players happy and this is no longer a disaster.
Because with Harbor and Offshore Platform, oceans supply a sufficient amount of food, shield, and trade points.
Also roads disappear along with land tiles, Howitzers become completely useless.
I have no choice but to use Cruise Missiles, so wars become boring.

I preferred how global warming worked in the older versions.
It must be a mega disaster which should be avoided absolutely.

In latest versions, Global warming changes land tiles to oceans directly.

You can see it in the 2.4.x .../data/default/terrain.ruleset looking for warmer_wetter_result, affected were grassland, jungle, plains, swamp, and tundra. Changing the URL to S2_3 and then S2_2 I think you are talking about a modification in 2.3, i.e., it was different in 2.2.

Simple suggestion, copy your 2.5.4 .../data/classic folder to a new .../.freeciv/2.5/classic22 folder in your HOME directory, edit the five warmer_wetter_results in .../.freeciv/2.5/classic22/terrain.ruleset to whatever you like better, and create a text-file .../.freeciv/2.5/classic22.serv with one line rulesetdir classic22. Your HOME directory is wherever you find the .freeciv/2.5 stuff.