These are not easy questions, and if you are the kind of person who thinks of yourself as "green in practice" for the little things you do (and I do, too), then this is absolutely necessary reading.
Update: Speaking of necessary reading, Natalie Bennett pointed me to David Morris' "The Once and Future Carbohydrate Economy". A teaser, the same quote she used
The first plastic was a bioplastic. In the mid-19th century, a British billiard ball company determined that at the rate African elephants were being killed, the supply of ivory could soon be exhausted. The firm offered a handsome prize for a product with properties similar to ivory, yet derived from a more abundant raw material. Two New Jersey printers, John and Isaiah Hyatt, won the prize for a cotton-derived product dubbed collodion.Morris notes that "the market" largely hasn't embraced bio-derived versions yet, in spite of government promotion, largely because of government spinelessness and government subsidies to petroleum economies. He also notes, going back to Anne's point above, that
Ironically, collodion never made it as a billiard ball: The plastic, whose scientific name is cellulose nitrate, is more popularly known as guncotton, a mild explosive. When a rack of cellulose nitrate pool balls was broken, a loud pop often resulted. Confusion and casualties ensued in saloons where patrons were not only drinking but sometimes armed.
Unlike most other renewable resources, biomass can be cultivated, harvested, and processed in nonsustainable ways. Soil erosion, fertilizer and pesticide runoff, and industrial pollution all can result from biomass inappropriately grown and processed. Public policy also needs to ensure that, when using biomass by-products such as cornstalks and wheat straw, farmland is not denuded of nutrients that nature needs to regenerate the land.There's a lot more there, including some fascinating ruminations on the renewal of local economies through bioenergy production, and some other warnings about what we could do wrong....