The increasing amount of carbon dioxide in the air is slowly raising the temperature of the atmosphere, because it absorbs

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Accepted Answer

the infrared part of the solar radiation. Quick Verdict: While the official UPSC answer key marks (d) as correct, its wording contains a scientific imprecision. Carbon dioxide actually absorbs the infrared part of outgoing terrestrial radiation, rather than the incoming solar radiation. However, (d) is the only option that correctly identifies "infrared" as the heat-carrying wave trapped by CO₂. Thematic Discussion: Navigating the Infrared Discrepancy To understand why this question is technically flawed but still solvable, we must look at how the Greenhouse Effect actually works: The Scientific Reality: As discussed in our master explanation of atmospheric radiation (Question 7 in the 2025 PYQs on Physical Geography), incoming solar radiation consists mostly of shortwave visible and UV light, which passes right through greenhouse gases. The Earth absorbs this and re-emits it as longwave terrestrial infrared radiation. It is this outgoing terrestrial infrared that CO₂ absorbs and traps, keeping the Earth warm. The Enhanced Greenhouse Effect: The natural greenhouse effect maintains the Earth's average surface temperature at a comfortable +15°C instead of a freezing -18°C. However, when human activities pump excessive amounts of additional CO₂ into the atmosphere (via fossil fuels and deforestation), this natural heat-trapping mechanism intensifies. This means trapping excess terrestrial infrared heat, and causing the gradual rise in average global atmospheric temperatures known as "Global Warming." 💡 The Examiner’s Mindset: The "Best Available Option" Rule In competitive exams like the UPSC, you will occasionally encounter questions with slight technical errors. The secret is not to panic, but to eliminate the impossible. Option (a) is biologically/physically nonsensical. Option (b) is wrong because UV is absorbed by Ozone (O₃), not CO₂. Option (c) is completely false because the atmosphere is largely transparent to incoming solar waves. Option (d) mentions "infrared." Even though it mistakenly says "solar" instead of "terrestrial," infrared is the correct band of the electromagnetic spectrum associated with heat and CO₂ absorption. Therefore, by elimination, it is the only logical choice to bubble on your OMR sheet. Mains Concept Builder: The Anthropogenic Impact CO₂ is not the most abundant greenhouse gas (Water Vapor is), nor is it the most potent per molecule (Methane and Nitrous Oxide trap much more heat). However, CO₂ is the most significant driver of anthropogenic (human-caused) climate change because of the sheer volume we emit and its incredibly long atmospheric lifespan (it can linger for hundreds to thousands of years). 📝 Mains Practice Question (GS Paper 3 - Environment): Distinguish between the natural greenhouse effect and the enhanced greenhouse effect. Why is carbon dioxide considered the primary driver of anthropogenic climate change despite being less potent than other greenhouse gases? (150 words, 10 marks) 📚 Standard Source: NCERT Class 11 Fundamentals of Physical Geography, Chapter 9 (Solar Radiation, Heat Balance and Temperature).