There are a couple of lines in IPCC Working Group I («New coupled climate - carbon models (Betts et al., 2004; Huntingford et al., 2004) demonstrate the possibility of large feedbacks between future climate change and vegetation change, discussed further in Section 7.3.5 (i.e., a die back of Amazon vegetation and reductions in Amazon precipitation).»). (realclimate.org)
Determining which types of cloud cover will increase or decrease, whether that will result in a net positive or negative feedback, and how large the feedback will be, are major challenges. (skepticalscience.com)
However, this high degree of internal variability (suggesting a positive and large feedback ratio, or «climate sensitivity» value) doesn't negate external forcing by any stretch, as Swanson and Tsonis pointed out in their conclusion: (scholarsandrogues.com)