Stupid Question...

[Elf of Avallónë]

I have a slightly stupid question... It isn't really all that important, but it's been annoying me that I can't figure it out:

Part way through the game (usually mid-Industiral Age) a yellow circle appears in the corner of the box showing your active unit, gov't, treasurey, &c. Does anyone know what this means? It doesn't seem to appear in a specific year or after I research a specific tech, it seems to appear just randomly.

 

[Tomoyo]

The redder it gets, the more global warming you have. It appears at the same time the first pollution appears.

 

[Blackbird_SR-71]

And watch out when it becomes to red then you will have terrain changing constantly and pollution everywhere