Is the composition of the atmosphere changing?
Carbon dioxide levels and water vapour levels are increasing largely due to our industrialised societies and fossil fuel consumption as well as volcanic activity.
Yes, the atmosphere is changing, and especially in the last 250 years since the Industrial Revolution. Pollution from human activity is changing the atmosphere. Burning of fossil fuels (coal, oil and natural gas) in industry, transport and the generation of electricity, which releases carbon dioxide (CO2). Bad agricultural practices emit methane (CH4), a powerful greenhouse gas. Deforestation has removed the trees that used to take carbon dioxide from the atmosphere.