Greenhouse Gas

Summary: A gas that absorbs infrared radiation in the atmosphere, contributing to the greenhouse effect. CO₂, CH₄, H₂O vapour. Tags: igcse chemistry definition environmental-chem Created: 2026-07-14 Last Updated: 2026-07-14

A greenhouse gas is any gaseous compound in the atmosphere that can absorb and re-emit infrared (thermal) radiation, trapping heat within the Earth’s atmosphere in a phenomenon known as the greenhouse effect. The most significant greenhouse gases covered at IGCSE level are carbon dioxide (CO₂), produced primarily through the combustion of fossil fuels and deforestation; methane (CH₄), released by agriculture (cattle farming), landfill decomposition of organic waste, and natural gas leaks; water vapour (H₂O), which acts as a natural greenhouse gas but also creates a positive feedback loop as global warming increases evaporation; and nitrous oxide (N₂O), emitted from nitrogen-based fertilisers. Without the natural greenhouse effect, Earth’s average surface temperature would be approximately -18 degrees Celsius instead of the current +15 degrees Celsius — but the enhanced (anthropogenic) greenhouse effect caused by human activities is driving global warming and climate change. The molecular mechanism behind the effect is that greenhouse gas molecules absorb specific wavelengths of infrared radiation emitted by the Earth’s surface, causing their bonds to vibrate more vigorously, and then re-radiate the energy in all directions, some of which returns to warm the surface. IGCSE exam questions often ask students to name the major greenhouse gases, link their sources to specific human activities, and describe the environmental consequences of an enhanced greenhouse effect, including rising sea levels, extreme weather events, and ecosystem disruption.

Key term — IGCSE Chemistry (0620/0971) glossary.