Examining the weather of other planets would help explain if solar activity or other factors are affecting planetary temperatures more than recent human actions.
If any planets ARE warming because of an increase in solar activity, the ones we should expect to see a change on are Venus, Earth, and Mars.
Venus is a good example of what happens if the greenhouse effects run amok, and Mars is a good example of the opposite. We're in the middle, at least for now.
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?