Which gas in the atmosphere contributes the most to the Greenhouse effect on Earth?

Introduction / Context:
Global warming and climate change topics often mention greenhouse gases. Students are usually familiar with carbon dioxide, but in terms of natural greenhouse effect, water vapour actually contributes the largest share. This question checks awareness of the relative roles of different atmospheric gases in trapping heat near the surface of Earth.

Given Data / Assumptions:

• The gases listed are water vapour, ozone, oxygen, and nitrogen.
• We are concerned with contribution to the greenhouse effect, not just abundance in the atmosphere.
• Greenhouse effect refers to trapping of outgoing infrared radiation by certain gases.

Concept / Approach:
Greenhouse gases are substances that strongly absorb infrared radiation emitted by Earth surface and atmosphere. While carbon dioxide, methane, nitrous oxide, and ozone are significant contributors, naturally occurring water vapour accounts for the largest part of the total greenhouse effect. Oxygen and nitrogen dominate by volume in the atmosphere but do not absorb infrared radiation efficiently. So we must identify which gas among the options behaves as a major greenhouse gas and contributes most in practice.

Step-by-Step Solution:
Step 1: Recall which gases are known greenhouse gases. Water vapour, carbon dioxide, methane, nitrous oxide, and ozone are greenhouse gases. Step 2: Compare relative contributions. Water vapour alone is responsible for a large fraction of the natural greenhouse effect, often quoted as more than half. Carbon dioxide and other gases contribute the rest but are not listed among the options here. Step 3: Analyse options. Ozone does contribute but in a smaller overall share compared to water vapour. Oxygen and nitrogen have almost no greenhouse activity. Thus the gas that contributes the most is water vapour.

Verification / Alternative Check:
Climate science sources commonly state that if all greenhouse gases were removed, Earth would be much colder and that water vapour accounts for the largest share of the natural greenhouse effect. However human activity mainly alters the concentration of gases like carbon dioxide and methane, which then indirectly affect water vapour levels. Since the question does not ask which gas is the most important anthropogenic greenhouse gas but which gas contributes most overall, water vapour remains the correct answer.

Why Other Options Are Wrong:
Option B: Ozone absorbs ultraviolet and some infrared radiation and is a greenhouse gas, but its total contribution to the greenhouse effect is smaller than that of water vapour. Option C: Oxygen is very poor at absorbing infrared radiation and is not considered a greenhouse gas in normal discussions. Option D: Nitrogen makes up most of the atmosphere by volume but does not contribute significantly to the greenhouse effect because it hardly absorbs infrared radiation.

Common Pitfalls:
Because media coverage focuses heavily on carbon dioxide, students may assume it is always the largest contributor in every sense. It is true that carbon dioxide is the most important greenhouse gas to control for climate policy, but that does not change the fact that, under natural conditions, water vapour provides the largest share of greenhouse warming. Carefully reading what the question specifically asks helps avoid this confusion.

Final Answer:
The gas that contributes most to the greenhouse effect is Water vapour.