A new report from the UN's top scientists says so-called "biodegradable plastics", marketed as part of a potential solution to the vast amounts of waste in the oceans, don't break down well in marine environments. 

The authors wrote:  "There is a moral argument that we should not allow the ocean to become further polluted with plastic waste, and that marine littering should be considered a 'common concern of humankind'."

The report (.pdf link) says that the rate at which they break down depends heavily on environmental conditions, and there's no evidence that biodegradable plastics break down any faster than unmodified polyethylene, which is non-biodegradable.  The principle weathering agent that causes this is UV radiation, which is more prevelent in coastal regions.  But once plastics are submerged, they're more or less protected from UV rays by the water.

The report also says these plastics require temperatures of at least 50 degrees Celsius "to breakdown completely into its constituent components of water, carbon dioxide, methane, on a reasonable or practical timescale".  The oceans are warming, but not that much - 50 degrees Celsius is about the operating temperature of large scale commercial composters.

And the worst part is that the misnomer of being "biodegradable" actually makes people careless and more likely to pollute.