You caught me on the reference to "terraforming". Looks like we need to start by terraforming our own planet to sustain its suitability for human life. Not so funny.
My suggestion along these lines would be a network of large controllable mirrors in orbit. The individual sections could be aimed, essentially by rotating them with gyroscopes. Some region is too hot? Adjust more mirrors to give it more shade and reduce its temperature. Another area is too cold? Add the appropriate amount of reflected sunlight and warm it right up. Might as well send some extra sunlight to the polar regions and cultivate crops there, too. Surplus light for electricity generation on the side.
Expensive? Yes, but basically within the capabilities of existing technologies. I actually think the largest technical hurdle would be sufficiently accurate weather modeling. We'd essentially need to micromanage the weather all over the world. I don't think the launch capacity would be unsolvable. The early launches would focus on the power generation, and the power would be used to crack sea water for the hydrogen that would be used to boost more mirror satellites into orbit.
Okay, so it would also be potentially dangerous, but I'm hoping that the security problems could be solved, and all technology is morally neutral. Any power to do good is also a power to do harm. (Unfortunately, this is not a balanced relationship. There are some powers that can do nothing but harm... But that's getting off the focus--which can be risky with such large mirrors.)