Historical clue formulations

• Rising concern? — 2021-01-04
• Something on the rise today — 2018-11-25
• It's affected by global warming — 2010-08-26
• Death Valley is below it — 2006-10-16
• Denver is way above it — 2005-08-23
• The Caspian is just below it — 1999-06-11
• 0, in some measures — 1997-12-19