The atmospheric burden of carbon dioxide (CO2) and methane (CH4) has been steadily increasing
since the start of the industrial age.  The observed increases are undoubtedly due to human activities.
The figures below illustrate these changes.  While increasing CO2 warms the lower atmosphere
(greenhouse effect),  it cools the upper atmosphere due to direct radiative loss to space.  Oxidation
of CH4 in the stratosphere produces H2O,  which then diffuses into the upper atmosphere.   Since
cooler temperatures and more humidity should enhance NLC formation,  NLCs are an indication
of atmospheric change.