* Former vice president proud of channel, didn't see only as investment * Gore touts Al Jazeera's climate coverage * He praises Obama's pledge to address environment By Susan Heavey WASHINGTON, Jan 29 (Reuters) - Al Gore said on Tuesday that tough competition from major U.S. television networks forced him to sell Current TV, a struggling progressive cable channel, to Al Jazeera, and he praised the Arabic news broadcaster's coverage of climate change.