Writing in another thread, I described a proposal to make a dent in our energy needs by roofing every parking area in the country with solar panels/shingles -- supposedly it would reduce our imports by somewhere around 8%.
I envision all the parking around sports stadiums and malls and universities and government buildings turning into sparkling expanses of silicon (or whatever they're making panels with these days). Then I thought that this is a wonderful idea in two big ways: that's a lot of energy, and lots of people wouldn't have to worry any longer about being rained on while getting in and out of their cars. The second one there is what we call a side effect, i.e. a secondary consequence.
But there's another secondary consequence that could be big: in all those places, rain would no longer be hitting the ground. Where those parking areas aren't paved (for example around many stadiums), that's going to change the whole ground water and runoff equation. That could be a bad consequence, depending on the area, but it lends itself to something else: since it's being intercepted above the ground, that water is subject to collection, and could be used as a source for water supplies.
Of course that would require storage, which would bring further effects....
So the topic here is issues today, and proposed solutions which would have secondary effects, some good and some bad. Comment on the one above, or add your own.
As to why, I think it's a good idea to stop and realize how liunked things are, given how we spend so much time looking at things in isolation.