Amazonia update: On the drought in Brazil and the risk of a run-away greenhouse effect due to excess carbon, released by wood-gas from forest fires.... The original evidence suggested that if the drought persisted for just one more year, most of the forest cover would go up in smoke in one year. Only about a tenth of Amazonia would burn in 2007-2008, if the dry spell continues- there is difference between absolutely no moisture reaching the tree roots versus a 'drought'.
Recently the physicist Stephen Hawking asked for a plan to save the human race on an online forum, and he specifically mentioned the possibility that we might trigger a rapid runaway greenhouse effect very soon- i'm unsure if he was referring to the crisis in Amazonia.

How all that smokefrom the forest fires will effect the worlds weather is uncertain (Global dimming).