I disagree.
I think this would be an interesting aspect to war when major nulcear fights start etc. One thing to add though is that i think by continent would be better so if the continent receiving the most nukes goes over that threshold then the continent goes into winter mode. I dont know if you covered that above, if you did i apologize.
Just think about the lower nations though, were in the beginning, that 10k is really nice. If you did 5% would take a bit for someone to actually get a bonus bigger than 10k. Maybe make it 5k + 5% to help with both situations?
So could we offer one for a newer nation with less benefits, and one for an older nation with more benefits but expensive? I get what you are saying though.
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^ With the financial programs you could make, those upper nations could boost the lower ones. Heck this thing is something i guess to reward longterm membership. That may be useful to keep members and to have guys look forward to being an older and larger nation.
What about nation age as well, i mean if a nation is older irl its going to have more older sites that will attract tourism, so unless there are improvements that are made just for tourism, i think age and polution should be a big factor.