What fact supports the opinion that paper bags are less harmful than plastic bags

Paper bags can decompose in the environment in about a month meanwhile plastic can take hundreds of years. Plastic is harmful in many other ways too, for instance they release toxic chemicals into the earths soil and land and sea creatures can get caught in plastic or eat it and later die.

they float because they are less dense than the water itself

something denser than water, like a rock, will sink to the bottom