Ever consider that there might be a little of both? Arguing about to what extent each is responsible is more likely than totally eliminating one or the other. Human causes assume that greenhouse gases are responsible. http://meetingorganizer.copernicus.org/EGU2011/EGU2011-4505-1.pdf
This wiki explains why Warmists believe that humans are responsible for significant climate change: http://en.wikipedia.org/wiki/Greenhouse_gases
Dr Roy Spencer explains the Skeptic point of view: http://www.drroyspencer.com/global-warming-natural-or-manmade/
Nelson argues for a little of both http://www.randombio.com/co2.html
This newspaper article explains the net positive feedback theory that supports the notion that climate change is mostly natural: http://www.express.co.uk/posts/view/252230/An-uncomfortable-lie
This peer reviewed article notices that natural variation over time is enough to explain what we have seen so far: ftp://grads.iges.org/pub/delsole/dir_ipcc/dts_jclim_2010.pdf
Naturally, those that believe that humans are mostly responsible for the recent climate change have written computer models to show what they expect to happen. These models have not proven to be very good at predicting much of anything. http://pielkeclimatesci.files.wordpress.com/2010/09/r-358.pdf
Skeptic models using natural forces have fared no better.