This 2007 article describes how "an international research team is to study spectacular 'night-shining' clouds, thought by some to a harbinger of global environmental change. ...?? First spotted in Earth's atmosphere in 1885, the clouds have been increasing in frequency in recent decades and may be related to increases in carbon dioxide and methane emissions."
Includes an image of these noctilucent or Polar Mesospheric Clouds. From ScienceDaily.