Two gases to focus on besides CO2 are methane and nitrous oxide. Methane is released in large amounts by enteric fermentation in ruminant animals like cows and sheep, and it also comes from the decay of organic waste in landfills. Nitrous oxide mainly comes from soil and manure management, where microbial processes in soils and manure handling release N2O—especially when fertilizers and manure are used and managed. This combination matches human activities and the major sources scientists widely identify. Why this pairing stands out: both gases have significant warming effects per molecule and are substantially influenced by human practices, making them two of the most important non-CO2 greenhouse gases to understand in environmental chemistry. The other options mix gases that aren’t primarily linked to distinct human-source activities in the same way, such as water vapor (a climate feedback more than a direct emission) or nitrogen gas (not a greenhouse gas).