If Jupiter is warming/cooling or Mars is warming/cooling, then something has to be responsible for it, and it sure isn't human activity.
If not the sun, then what? Something about planetary environments we're likely interested in learning more about, right?
If other planets are warming/cooling then something is responsible for it. If it's not the sun, then it's something else related to planetary environments with which we are not familiar. Studying that would be A Good Idea, regardless of where people stand on global warming, because examining Earth alone just gives us a single data point.
What's happening on Mars? Does it parallel the changes we're seeing on Earth to some degree? What's happening in Jupiter's atmosphere to bring about such a dramatic change in a short period of time?
Could we see a similar dramatic change on this planet?