Sunspot do go in cycles, but since they're cyclical they are not a cause of global warming (at least not the global warming that we are talking about) because the duration of the cycle is only 11 years, while the warming that is generally discussed is over the course of the last century or so. As was stated above, such global temperature phenomina have occured in the past over several centuries (if not millenia), so you could argue that even an entire century is simply too short a time-scale to tell. Remember: a century may seem like a long time to us, but it's almost negligible compared to the age of the Sun or the Earth. On those scales, an 11 year sunspot cycle is practically the blink of an eye.