Returning the whale population to "historical levels" as a benign form of geo-engineering
Before there was petroleum, there was whale oil, until there weren't enough whales (Moby Dick). Apparently, whales took millions of tons of CO2 out of the atmosphere a year. What if killing so many of them was the trigger for anthrogenic climate change? What if the system would have been in balance otherwise?
I don't see how we know, but as usual, the earth is more complicated and more beautiful than we know, in our simplicity. Perhaps the "Save the whales!" crowd got it right, though they didn't know why. And perhaps it would be a good idea to leave that foul stuff, petroleum, in the ground, and only use it for important things, like iPod cases and #D printing thermoplastic.