From USEPA" Gases that trap heat in the atmosphere are called greenhouse gases.
Carbon dioxide (CO2) : Carbon dioxide enters the atmosphere through burning fossil fuels (coal, natural gas and oil), solid waste, trees and wood products, and also as a result of certain chemical reactions (e.G., manufacture of cement). Carbon dioxide is removed from the atmosphere (or "sequestered") when it is absorbed by plants as part of the biological carbon cycle."
Yes, marijuana smoke is indeed considered a greenhouse gas. Actually, for having the nickname green, it is not actually very "green" environmentally speaking. It produces a lot of carbon dioxide when burnt and smoked. Additionally, there is a lot of electricity used in the process of growing it (indoors) which adds to its carbon footprint.