The atmospheric burden of carbon dioxide (CO2) and methane (CH4) has been steadily increasing since the start of the industrial age.  The observed increases, illustrated below, are undoubtedly due to human activities. While increasing CO2 warms the lower atmosphere (the greenhouse effect), it cools the upper atmosphere through enhanced radiative loss to space. Oxidation of CH4 in the stratosphere produces H2O, which subsequently diffuses into the upper atmosphere. Since NLCs are controled by temperature and humidity, enhanced NLC activity provides is likely related to global atmospheric change.