The earth climate system is not impossibly complex. The global temperature is at it's core determined by an energy balance. Visible light from the sun enters, infrared radiation leaves. Some of that infrared radiation is blocked, then temperature has to rise before the balance is restored. Simple as that, and completely uncontroversial, because proven by spectrum analysis of our neighboring planets (and the moon).
Now I know that doesn't give you hard answers to questions policymakers want, like 'will New Orleans flood from increased tropical storms? Will the Middle East and India dry out?' We don't know that, and yes, that sucks.
But we do know the effect of increased amounts of greenhouse gases. (And, for what it's worth, we do also know that carbon dioxide taxes work much better than an emission trading system does, as proven by e.g. Norway vs. European Union, and as proven by the fact that industry and fossil fuel companies actually prefer it).
And the fact that the top comment on HN is about climate change denial rather than the actual article is excellent proof of the study's truth.