Step 1: Greenhouse gases trap the heat (infrared radiation) radiated by the Earth, warming the lower atmosphere. Step 2: The main greenhouse gases are carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), water vapour and CFCs. Step 3: Nitrogen (N₂) makes up about 78% of air, but its molecule does not absorb infrared radiation, so it is not a greenhouse gas. Step 4: Do not confuse N₂ (nitrogen) with N₂O (nitrous oxide), which is a greenhouse gas. Answer: Nitrogen (N₂).