I agree, heating a planet is much more difficult as you cannot readily increase the amount of solar energy a body is receiving. Because of this there is only so much you can do. You should be able to cool a planet to a vastly greater degree by either lowering atmospheric pressure or adding more reflective elements to the atmosphere. If you wanted to destroy the habitability of the planet there really is no reason why you should not be able to make any body 0K (within limits due to proximity of the parent star and level of solar energy it is receiving)
Yes and no. Terraforming, as currently implemented, is simply manipulation of gases, nothing more. The "reflectability" of a body is already managed by the albedo, which, I believe, cannot be changed. As such, there is a limit to how "reflective" an atmosphere you can make using only common gases that could be used for planetary terraforming (nitrogen, oxygen, the things).
If we start thinking in terms of industrial terraforming (ie, using manufactured. . . things) then it should work both ways. Increase or decrease albedo to heat or cool the amosphere. Just an example.
I'm not sure how many of you have read Mars Trilogy. I hope you won't consider this a spoiler.
Still here? Ok.
The major theme of the books was terraforming Mars. One of the things they did was to make enormous mirror that increased the amount of solar radiation on Mars to Earth levels. Later it was used as a shade to cool Venus (althought it was supposed to take several hundred years and was never shown in the books). Something similar is at works here. If you argue, that cooling a planet is "easy" by application of industrial terraforming (I just can't imagine of any gas or combination of gases, that would be reflective enough to lower the temperature of Mercury to freezing point) then the same applications can be used (in somewhat diffrent form) to heat the planet. Similar to the example from Mars Trilogy.
Of course this is all technobabble. In gameplay terms I think it simply imbalances the game, screwing certain elements in favour of lower temperatures (for example, becouse of how easy it is to cool a planet, creating species capable of living in higher temperatures is pretty pointless. Or the beforementioned case with cooling/heating of the Sun).
To be honest, for me the simplest, and satisfactory solution would be to implement the same kind of bottom line as with heating. The preferred solution would be (in addition to the limit) to divide terraforming into two groups: atmospheric and industrial. Atmospheric would be exactly the way it is, industrial would be much more expensive and would affect planetary albedo, allowing for the heating/cooling of a planet beyond the capabilities of "classical" terraforming. For most planets, it would make no diffrence, you would just change the atmosphere the same way as now. However, by changing albedo you could make other bodies terraformable, like Titan which is too cool right now, or Mercury which would be too hot if the limiting of anti-greenhouse effect would be implemented.
