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?