Because of deforestation there are less trees to absorb man made and naturally occurring carbon. What exacerbates deforestation is when the slash and burn technique is implemented. This not only means less trees for absorbing CO2, but when trees die and/or burned they too release carbon dioxide in the air. So there is more CO2 being created and less being absorbed at the same time.
As i mentioned above, the biggest concern that comes with deforestation is that of balancing CO2 levels. But a plethora of other problems arise with deforestation. Without trees to protect the soil from erosional processes like wind and rain, the soil has been depleting. The end result of this erosional process is desertification; where the soil gets to the point that it is no longer fertile. Another problem arises in the form of the loss of natural habitats for the animals that live in these areas. The Amazon rainforest is home to one of the most diversified places on earth in terms of the number of different species living there. There may seem to be plenty of land still left in the Amazon covered by forests, but with less forest means less food for the animals.
Article found at: http://www.earthtimes.org/encyclopaedia/environmental-issues/deforestation/
Picture of Amazon time lapse