We aren't worried about the "planet's" temperature. Our concern is with the thin layer of the planet's surface in which we live - about equivalent to the layer of water on a wet apple, or the morning frost on the roof of a car. It's fairly easy to alter the temperature of, say, the upper 10 meters of the soil and water along with the 10 kilometers of gas above it. Just slightly changing the balance of absorption and emission of radiant energy from the sun in that layer will do it, for example. It does take some effort, but we have been putting the work in and we have significant achievement to show for that work.