To evaluate a contrarian claim, it helps to break down the contentious issue into its contentious sub-issues. For example, contrarians deny that global warming is caused primarily by humans, an issue which can be broken down into the following sub-issues:

Have solar cycles significantly affected earth's recent climate?
Does cosmic radiation significantly affect earth's climate?
Has earth's orbit significantly affected its recent climate?
Does atmospheric CO2 cause significant global warming?
Do negative feedback loops mostly cushion the effect of atmospheric CO2 increases?
Are recent climatic changes consistent with the AGW hypothesis?
Is it possible to accurately predict climate?
Have climate models made good predictions so far?
Are the causes of climate change well understood?
Has CO2 passively lagged temperature in past climates?
Are climate records (of temperature, CO2, etc.) reliable?
Is the Anthropogenic Global Warming hypothesis falsifiable?
Does unpredictable weather imply unpredictable climate?

It's much easier to assess the liklihood of a position once you've assessed the liklihood of each of its supporting positions. In this particular case, I found that the contrarians made a very weak case indeed.