. ♪ i don't know if you know the name yip harburg. he was a great lyricist of pop song.paper moon and brother, can you spare a dime and over the rainbow and his way of saying words make you think, music makes you feel, and a song makes it possible to feel a thought. interesting way of putting it. (crowd singing) ♪ we shall overcome. ♪ ♪ we shall overcome. ♪ ♪ we shall overcome someday. ♪ ♪ oh, deep in my heart, ♪ ♪ i do believe, ♪ ♪ we shall overcome someday. ♪ ♪ we shall overcome. ♪ ♪ we shall overcome. ♪ ♪ we shall overcome someday. ♪ funding for this program was provided by the annenberg/cpb project. this is pbs. funding for this program is provided by... narrator: the human species is engaging in a vast global experiment in climate change. each day, more greenhouse gases are being added to the atmosphere with unknown consequences. clues for what the future may hold are frozen deep within ice on the world's highest mountains.