Besides carbon dioxide, which one of the following gases is also an important greenhouse gas contributing to the greenhouse effect in the Earth atmosphere?

Introduction / Context:
This question concerns environmental chemistry and climate science. It asks you to identify another greenhouse gas besides carbon dioxide among the listed gases. Greenhouse gases absorb and emit infrared radiation, thereby trapping heat in the Earth atmosphere and contributing to global warming. Knowing which gases are greenhouse gases is important for understanding climate change and for answering general knowledge questions in competitive examinations.

Given Data / Assumptions:
Carbon dioxide is recognised as a major greenhouse gas, but the question explicitly says besides CO2 and then lists other gases.
CH4 denotes methane, a simple hydrocarbon with one carbon and four hydrogen atoms.
N2 denotes nitrogen gas, which makes up most of the atmosphere by volume.
Ar stands for argon, a noble gas present in small but significant amounts in air.
O2 denotes oxygen gas, the second most abundant gas in the atmosphere by volume.

Concept / Approach:
Greenhouse gases are characterised by their ability to absorb infrared radiation at wavelengths emitted by the Earth surface. Methane, nitrous oxide, ozone and water vapour are some key greenhouse gases in addition to carbon dioxide. Nitrogen and oxygen, despite being abundant, are symmetric diatomic molecules with limited interaction with infrared radiation in the relevant range, and argon is a noble gas that is monoatomic and essentially infrared inactive under normal conditions. Therefore, among the options, methane is the gas that significantly contributes to the greenhouse effect besides carbon dioxide.

Step-by-Step Solution:
Step 1: Recall the main greenhouse gases: water vapour, carbon dioxide, methane, nitrous oxide and ozone are commonly listed. Step 2: Identify methane among the options. Methane (CH4) is a greenhouse gas that is much more potent than carbon dioxide on a per molecule basis, although it is present in lower concentrations. Step 3: Consider nitrogen. N2 is a homonuclear diatomic molecule with very limited infrared absorption and is not classified as a major greenhouse gas. Step 4: Consider argon. Ar is a noble gas with a closed shell electronic configuration and does not have vibrational modes that would interact strongly with infrared radiation. Step 5: Consider oxygen. O2 is also homonuclear diatomic and similarly has very limited infrared activity, so it is not regarded as a key greenhouse gas compared with methane.

Verification / Alternative check:
Climate science resources and reports from bodies such as the Intergovernmental Panel on Climate Change list methane as a significant greenhouse gas with a high global warming potential over shorter time scales. They also specify that while nitrogen and oxygen dominate the atmosphere by volume, their contribution to the greenhouse effect is minimal. Argon, being inert and monoatomic, likewise does not contribute significantly to infrared absorption. This independent confirmation from authoritative climate sources supports the conclusion that methane is the correct answer.

Why Other Options Are Wrong:
Nitrogen is wrong because it is largely infrared inactive and does not significantly trap heat in the atmosphere despite its high concentration.
Argon is wrong because as a noble gas it is chemically inert and does not have vibrational modes that interact with infrared radiation in the way greenhouse gases do.
Oxygen is wrong because, like nitrogen, it is a homonuclear diatomic molecule and has limited involvement in infrared absorption relevant to the greenhouse effect.

Common Pitfalls:
Students sometimes assume that the most abundant gases must be the most important for every atmospheric process, including greenhouse warming. This leads to choosing nitrogen or oxygen incorrectly. Another pitfall is confusion between ozone and oxygen; ozone is a greenhouse gas and also protects against ultraviolet radiation, but it is not listed among the options. Remembering that methane is one of the key greenhouse gases and that abundance alone does not determine greenhouse behaviour helps to avoid these errors.

Final Answer:
Besides carbon dioxide, an important greenhouse gas in the Earth atmosphere is CH4, that is methane.