What was done
This Viewpoint article in Science examines the complexities involved in modeling future climate.

What was learned
Earth's climate is described as a complex system composed of both ordered forcing and chaotic behavior, the latter of which is much more difficult, if not impossible, to model, due to its nonlinear mathematical behavior.  The full impact of the chaotic element on climate is not fully understood at present, although the author cites one model study that concluded chaotic fluctuations are indeed significant, demonstrating that temperatures in the mid-latitudes were altered by the chaotic element to a degree twice as large as the predicted greenhouse gas forcing there over the past two decades.

What it means
According to the author, if nonlinear chaotic elements of climate make up a significant part of the whole, there is a need to incorporate such effects into current GCMs.  To do so, he states, future climate models will need to improve their resolution or scale, as most nonlinear components of the climate system operate on scales much smaller than are presently resolved in current GCMs.  Even then, however, it will still be a difficult task to predict earth's climatic future; for although earth's complex climate exhibits some degree of "determinism in the midst of chaos," it also manifests "unpredictability in the midst of understanding."

Our Conclusion?  Don't bet the economies of the world's nations on the predictions of current climate models.  The premise of the Kyoto Protocol is not a sure thing.