Historical clue formulations

• Environmental problem addressed in the Clean Air Act — 2013-01-23
• Deleterious precipitation — 2010-06-06
• One cause of deforestation — 2009-09-08
• Environmental awareness topic — 2007-06-01
• Environmental woe — 2006-08-26
• Environmentalist's concern — 2004-07-17
• Environmental problem — 2002-12-24
• Environmental hazard — 2002-01-05
• Harmful precipitation — 1998-09-08
• Environmental problem — 1996-08-06