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. 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.