Biodegradable plastic bags in themselves can have an adverse environmental impact by creating greenhouse gases within landfill sites

The amount of CO2 released by a decomposing plastic bag is not that high. Rotting vegetable matter or decaying refuse is going to produce CO2, methane and other gases anyway. If the source of the starch used to make the biodegradable plastic is fast growing vegetation then the carbon can be offset and the actual overall CO2 footprint would be negligible.

See http://en.wikipedia.org/wiki/Biodegradable_polythene_film

People should be focused on the reduction of CO2 gases from larger sources and reducing the amount of energy wasted…