I believe aluminum bikes are the best for the environment. In 1971 John Holdren, who went on to be the director of the White House Office of Science and Technology Policy from '09-'17, said a new ice age was likely, and bikes were made from steel. The in the late 80's aluminum bikes started coming out and they dominated in the 90's. I believe that stopped the cooling. But then carbon bikes came out and suddenly we got global warming. Just buy aluminum.