With reference to the Earth's atmosphere, which one of the following statements is correct?

Question

Accepted Answer

Infrared waves are largely absorbed by water vapor that is concentrated in the lower atmosphere Quick Verdict: Water vapor, heavily concentrated in the lower troposphere, is a powerful greenhouse gas that actively absorbs infrared radiation. The other options contain factual inaccuracies regarding the distribution and electromagnetic properties of solar radiation. Thematic Discussion: Insolation and Infrared Radiation Option a - Incorrect: The equator does receive significantly more solar radiation than the poles due to the direct angle of the sun's rays, but the total amount of insolation received at the equator is roughly 2.4 times that received at the poles, not 10 times. Options b & d - Incorrect: Incoming solar radiation comprises a mix of different wavelengths along the electromagnetic spectrum. The infrared (IR) radiation constitutes roughly 49% of incoming solar radiation, visible light about 44%, and ultraviolet about 7%. In other words, visible light and infrared are distinct parts of  electromagnetic spectrum. Visible light consists of wavelengths that the human eye can see (roughly 400 to 700 nanometres). Infrared waves have longer wavelengths that lie just beyond the red edge of the visible light, making them invisible to the naked eye. Option c - Correct: Water vapor is the most abundant greenhouse gas in Earth's atmosphere, and is heavily concentrated in its troposphere. Water vapour’s complex molecular structure makes it highly effective at absorbing infrared radiation—both the near-infrared from the Sun and the far-infrared radiation emitted by the Earth's surface. This absorption is a critical mechanism for maintaining the Earth's heat budget. 💡 The Examiner’s Mindset: The "Extreme Multiplier" and "Definitional" Traps The examiner is testing two common UPSC traps here. First, Option (a) relies on an "extreme multiplier." While it is logically true that the equator receives more insolation, "10 times" is an absurdly high exaggeration. If a metric looks cartoonishly large in a UPSC statement, be highly suspicious. Second, Option (d) is a definitional trap. The examiner is testing if you know the literal prefix meaning of "infra-" (which means below or further on). By definition, infrared exists just outside the visible spectrum. Mains Concept Builder: The Water Vapor Positive Feedback Loop While Carbon Dioxide (CO₂) gets most of the media attention, water vapor is actually the most abundant greenhouse gas. To understand why CO₂ is still blamed for global warming, students must distinguish between a "climate forcing" and a "climate feedback." The Trigger (CO₂ Forcing): CO₂ acts as the initial driver. Human emissions trap excess heat, causing a baseline rise in global temperatures. The Amplifier (Water Vapor Feedback): Warmer air physically holds more moisture. As CO₂ heats the planet, evaporation increases, loading the lower atmosphere with more water vapor. Because water vapor is a highly potent greenhouse gas, it traps even more infrared radiation. This raises temperatures further, which causes even more evaporation. This continuous, self-amplifying cycle is why water vapor cannot start global warming on its own, but acts as the most powerful multiplier of human-caused CO₂ emissions. 📝 Mains Practice Question (GS Paper 1 / 3): Although water vapor is the most abundant greenhouse gas in the Earth's atmosphere, carbon dioxide is cited as the primary culprit for anthropogenic global warming. Explain this paradox using the concept of climate feedback loops. (150 words, 10 marks)