One of the most exciting areas of advanced manufacturing is 3-D printing. While it has been around for many years to produce crude prototypes, 3-D printing is now being used to make everything from jet engines and complex machine parts to bridges and buildings, artificial limbs and biomedical tissue. One company is even producing 3-D printing machines for use by NASA in space to avoid costly space flights to supply the International Space Station. It is still too early to determine the full potential of 3-D printing, but the technology is advancing quickly. Yet the environmental impacts of 3-D printing have been little studied, and may cut the other way too, writes Jason Bordoff in the Wall Street Journal.