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

1. Composition and Structure of the Atmosphere (basic)

Welcome to your first step in mastering climate science! To understand how our climate changes, we must first understand what the atmosphere is made of. Think of the atmosphere as a protective blanket of gases, water vapor, and dust particles held to the Earth by gravity. It didn't always look like this; it evolved through three stages: starting from the loss of the original hydrogen-rich "primordial" atmosphere, followed by "degassing" from the Earth's hot interior, and finally being modified by living organisms through photosynthesis FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), The Origin and Evolution of the Earth, p.15.

The composition of the modern atmosphere is dominated by Nitrogen (78.08%) and Oxygen (20.95%). While these are the most abundant, they are "permanent gases" because their proportion remains constant. However, the most critical players in climate science are the variable components like Carbon dioxide (CO₂), water vapor, and dust. Interestingly, these components are not spread evenly; for instance, oxygen becomes negligible at a height of 120 km, while CO₂ and water vapor are concentrated only up to 90 km from the surface FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Composition and Structure of Atmosphere, p.64.

One of the most vital concepts for you to grasp is how Carbon dioxide (CO₂) regulates our temperature. CO₂ acts like a one-way filter: it is transparent to incoming short-wave solar radiation (sunlight) but opaque to outgoing long-wave terrestrial radiation (heat radiated back from the Earth). This trapping of heat is known as the greenhouse effect. While naturally essential for keeping Earth warm enough for life, an excess of these gases due to human activity leads to global warming Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Climate Change, p.9.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), The Origin and Evolution of the Earth, p.15; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Composition and Structure of Atmosphere, p.64; Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Climate Change, p.9; Physical Geography by PMF IAS, Earths Atmosphere, p.271

2. Solar Insolation and Earth's Heat Budget (intermediate)

To understand why our planet doesn't just boil away or freeze solid, we must look at Solar Insolation (Incoming Solar Radiation). The Sun emits energy primarily in short-wave forms, including ultraviolet and visible light. As this energy travels toward Earth, it isn't distributed equally. Because of the Earth’s spherical shape and the angle of inclination of its axis (66½° with the orbital plane), the tropics receive more direct, intense rays than the poles FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 7, p.67. Interestingly, the maximum insolation isn't at the equator, but over subtropical deserts where cloud cover is minimal, allowing more radiation to reach the surface FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 7, p.68.

The Earth’s Heat Budget is the delicate accounting system that keeps our global temperature stable. While the Earth receives short-wave radiation, it cools itself by radiating energy back into space as long-wave terrestrial radiation (infrared). Think of it as a "give and take" relationship: if the Earth absorbs 100 units of energy, it must eventually release 100 units to maintain an equilibrium Physical Geography by PMF IAS, Horizontal Distribution of Temperature, p.293. This balance explains why the Earth, as a whole, neither significantly warms nor cools over long periods of time under natural conditions FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 7, p.69.

A critical player in this budget is Carbon Dioxide (CO₂). CO₂ acts like a one-way filter: it is transparent to incoming short-wave solar radiation but opaque to the outgoing long-wave terrestrial radiation. When Earth tries to radiate heat back to space, CO₂ molecules absorb that infrared energy, vibrating and trapping heat within the atmosphere—a process known as the Greenhouse Effect Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Chapter 7, p.9. While this effect is natural and necessary for life, increasing CO₂ concentrations from human activity are currently "trapping" more units than the budget allows for, leading to global warming.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 7: Solar Radiation, Heat Balance and Temperature, p.67-69; Physical Geography by PMF IAS, Horizontal Distribution of Temperature, p.293; Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Chapter 7: Climate Change, p.9

3. The Mechanism of the Greenhouse Effect (basic)

To understand the Greenhouse Effect, we must first look at how energy moves between the Sun, the Earth, and the atmosphere. The Sun emits energy primarily as short-wave solar radiation (visible light). Because of its high frequency and short wavelength, this radiation passes through the atmosphere relatively easily, as if the gases weren't even there. Once this energy reaches the Earth, it warms the surface. FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 7, p.64

The crucial shift happens next: the warmed Earth radiates that energy back toward space, but in a different form called long-wave terrestrial radiation (infrared or thermal energy). Unlike the incoming solar rays, these longer waves are easily captured by certain gases in our atmosphere, known as Greenhouse Gases (GHGs). Molecules like Carbon Dioxide (CO₂) and Water Vapor (H₂O) are physically structured to absorb these infrared wavelengths. After absorbing this heat, they re-emit it in all directions—including back down toward the Earth's surface. Environment and Ecology, Majid Hussain (3rd ed.), Chapter 7, p.9

This process is often compared to a glass greenhouse used for plants. In a glass house, the glass is transparent to incoming light but opaque to the heat trying to escape. While the atmospheric mechanism is slightly different—it involves radiation absorption rather than just trapping warm air physically—the result is the same: the atmosphere acts as a thermal blanket that keeps the planet habitable. Science, Class VIII NCERT (Revised ed 2025), Chapter 13, p.214

While the natural greenhouse effect is essential for life—without it, Earth would be a frozen -18°C—human activities like burning fossil fuels have significantly increased CO₂ levels. This "enhanced" greenhouse effect traps more heat than necessary, leading to the global temperature rise we call global warming. Environment, Shankar IAS Academy (10th ed.), Climate Change, p.254-255

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 7: Composition and Structure of Atmosphere, p.64; Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Chapter 7: Climate Change, p.9; Science, Class VIII NCERT (Revised ed 2025), Chapter 13: Our Home: Earth, a Unique Life Sustaining Planet, p.214; Environment, Shankar IAS Academy (ed 10th), Climate Change, p.254-255

4. Major Greenhouse Gases (GHGs) and Global Warming Potential (intermediate)

To understand global warming, we must first look at how our atmosphere acts like a thermal blanket. The sun emits short-wave solar radiation, which passes through the atmosphere relatively easily. However, once the Earth's surface absorbs this energy, it re-radiates it as long-wave terrestrial radiation (infrared). Greenhouse Gases (GHGs) are unique because they are transparent to incoming sunlight but opaque to this outgoing infrared heat. By absorbing these long wavelengths, GHGs induce vibrational motions in their molecules, trapping thermal energy that would otherwise escape into space FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 7, p.64.

While Carbon dioxide (CO₂) is the most discussed GHG due to the sheer volume of anthropogenic emissions from fossil fuel combustion, it is not the most potent on a molecule-by-molecule basis. To compare different gases, scientists use a metric called Global Warming Potential (GWP). GWP measures how much energy the emissions of 1 ton of a gas will absorb over a given period (usually 100 years), relative to the emissions of 1 ton of CO₂. A gas's GWP depends on two critical factors: its ability to absorb energy (radiative efficiency) and how long it remains in the atmosphere (lifetime) Environment, Shankar IAS Academy (ed 10th), Climate Change, p.260.

To simplify climate reporting, we often use the term CO₂ equivalent. This is calculated by multiplying the physical mass of a specific GHG by its GWP. For instance, emitting one ton of Methane is roughly equivalent to emitting 25 tons of Carbon dioxide in terms of its heat-trapping impact over a century Environment, Shankar IAS Academy (ed 10th), Environment Issues and Health Effects, p.425. This standardized unit allowed international frameworks like the Kyoto Protocol (1997) to set binding emission reduction targets for industrialised nations Contemporary World Politics, Textbook in political science for Class XII (NCERT 2025 ed.), Environment and Natural Resources, p.87.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 7: Composition and Structure of Atmosphere, p.64; Environment, Shankar IAS Academy (ed 10th), Climate Change, p.260; Environment, Shankar IAS Academy (ed 10th), Environment Issues and Health Effects, p.425; Contemporary World Politics, Textbook in political science for Class XII (NCERT 2025 ed.), Environment and Natural Resources, p.87

5. Ozone Layer and UV Radiation Absorption (intermediate)

The Ozone Layer, primarily located in the stratosphere (about 10 to 50 km above Earth), acts as our planet's biological shield. While gases like carbon dioxide are famous for trapping heat, ozone (O₃) performs a different, life-sustaining miracle: it filters out lethal ultraviolet (UV) radiation from the Sun. This absorption occurs specifically in the wavelength range of 0.1 to 0.3 microns Environment and Ecology, Majid Hussain, Chapter 6, p.11. Without this protective layer, the high-energy UV-B and UV-C rays would reach the surface, causing severe damage to DNA, increasing skin cancers, and disrupting marine ecosystems.

At the heart of this process is the Ozone-Oxygen Cycle. When a high-energy UV photon hits an ozone molecule, it splits the O₃ into an oxygen molecule (O₂) and a free oxygen atom (O). Almost immediately, these components can recombine to form ozone again, releasing energy in the process. This conversion is crucial: the ozone layer absorbs the energy of UV light and re-radiates it as longer-wavelength infrared (heat) energy Environment and Ecology, Majid Hussain, Chapter 1, p.8. This explains a unique feature of our atmosphere: unlike the troposphere where it gets colder as you go higher, the stratosphere actually gets warmer with altitude because of this heat-releasing chemical reaction.

The stability of this shield depends on a delicate equilibrium between ozone formation and destruction. However, human activities have introduced catalysts like chlorofluorocarbons (CFCs) and bromofluorocarbons. When these reach the stratosphere, solar radiation breaks them down to release chlorine and bromine radicals. These radicals are incredibly efficient killers of ozone; a single chlorine radical can initiate a chain reaction that destroys over 100,000 ozone molecules before it is eventually neutralized Physical Geography by PMF IAS, Earths Atmosphere, p.276. This imbalance is what we call Ozone Depletion, leading to a thinner shield that allows more harmful radiation to leak through.

Sources: Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.11; Environment and Ecology, Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.8; Physical Geography by PMF IAS, Earths Atmosphere, p.276; Environment, Shankar IAS Academy, Ozone Depletion, p.267

6. The Carbon Cycle and Anthropogenic Drivers (intermediate)

To understand climate change, we must first understand the Carbon Cycle—the process by which carbon is exchanged among the atmosphere, oceans, soil, and living organisms. Think of it as Earth's natural recycling system. Carbon exists in the atmosphere primarily as Carbon Dioxide (CO₂). In a natural, balanced state, plants take in CO₂ through photosynthesis to build tissue, and carbon returns to the atmosphere through respiration and the decomposition of organic matter. This is generally a short-term cycle. However, some carbon enters a long-term cycle, being stored for millions of years in deep ocean sediments or as fossil fuels beneath the Earth's crust Environment, Shankar IAS Academy, Functions of an Ecosystem, p.19.

Crucial to this cycle are the concepts of Sources and Sinks. A carbon sink is any reservoir (like a vast forest or the ocean) that absorbs more carbon than it releases. Conversely, a carbon source is anything that releases more carbon than it absorbs. For example, while a growing forest is a sink, a burning forest or a volcano is a source Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.57. Humans have also begun developing artificial sinks through technologies like Carbon Capture and Storage (CCS) to mimic these natural processes.

The problem today is that human activities, known as anthropogenic drivers, are disrupting this balance from two directions simultaneously:

• Increasing Sources: The massive combustion of fossil fuels (coal, oil, and gas) for electricity and transport releases carbon that was buried for millions of years back into the atmosphere in just a few decades Environment, Shankar IAS Academy, Climate Change, p.256. Additionally, industrial processes like cement production release CO₂ through chemical reactions of limestone.
• Reducing Sinks: Deforestation removes the very trees that would naturally absorb excess CO₂, effectively weakening the Earth's ability to clean its own atmosphere.

Why does this matter? CO₂ is unique because it is transparent to incoming solar radiation (short-wave) but opaque to outgoing terrestrial radiation (long-wave infrared). When the sun heats the Earth, the surface tries to radiate that heat back into space. CO₂ molecules absorb these infrared wavelengths, vibrating and trapping that thermal energy within our atmosphere Fundamentals of Physical Geography, NCERT Class XI, Composition and Structure of Atmosphere, p.64. This "greenhouse effect" is natural, but the sudden anthropogenic surge in CO₂ is trapping extra heat, leading to rapid global warming Exploring Society, NCERT Class VII, Climates of India, p.64.

Sources: Environment, Shankar IAS Academy, Functions of an Ecosystem, p.19; Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.57; Environment, Shankar IAS Academy, Climate Change, p.256; Fundamentals of Physical Geography, NCERT Class XI, Composition and Structure of Atmosphere, p.64; Exploring Society, NCERT Class VII, Climates of India, p.64

7. Infrared Absorption Properties of CO₂ (exam-level)

To understand why Carbon Dioxide (CO₂) is the primary driver of modern climate change, we must look at its role as a selective filter. The sun emits energy primarily as short-wave radiation (including visible light). The atmosphere, and CO₂ specifically, is largely transparent to these incoming short waves, allowing them to reach and warm the Earth's surface FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 7, p. 64. However, the Earth does not simply hold onto this heat; it cools itself by re-emitting energy back toward space. Because the Earth is much cooler than the sun, it emits energy at much longer, lower-energy wavelengths known as long-wave terrestrial radiation or infrared radiation.

This is where the unique molecular structure of CO₂ becomes critical. While simple diatomic molecules like Nitrogen (N₂) and Oxygen (O₂) allow infrared to pass through, CO₂ molecules are "tuned" to these specific wavelengths. When an infrared photon hits a CO₂ molecule, it induces vibrational motions within the molecule, effectively capturing the thermal energy Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Chapter 7, p. 9. Instead of the heat escaping into the vacuum of space, it is trapped within the lower atmosphere (the troposphere). It is important to distinguish this from the Ozone layer, which protects us by absorbing high-energy ultraviolet (UV) radiation in the stratosphere, whereas CO₂ handles the thermal infrared radiation in the lower atmosphere.

Once a CO₂ molecule absorbs this infrared energy, it doesn't just hold it forever; it re-emits the radiation in all directions Environment, Shankar IAS Academy (ed 10th), Chapter: Climate Change, p. 255. Some of this energy continues out to space, but a significant portion is radiated back down toward the Earth's surface. This process, known as the Greenhouse Effect, creates a second "blanket" of warmth. While this is a natural phenomenon that makes Earth habitable, the rapid increase in CO₂ from human activities like burning fossil fuels has thickened this blanket, leading to an enhanced greenhouse effect and global warming Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Chapter 6, p. 7.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 7: Composition and Structure of Atmosphere, p.64; Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Chapter 7: Climate Change, p.9; Environment, Shankar IAS Academy (ed 10th), Climate Change, p.255; Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Chapter 6: Environmental Degradation and Management, p.7

8. Solving the Original PYQ (exam-level)

In this question, we see the culmination of your study on the Earth's Heat Budget and Atmospheric Composition. You have learned that different gases act as filters for specific wavelengths. As noted in FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Carbon Dioxide (CO2) is meteorologically a very important gas because it is transparent to the incoming short-wave solar radiation but opaque to the outgoing long-wave radiation. This selective absorption is the fundamental building block of the Greenhouse Effect.

To arrive at the correct answer, follow this logical chain: the Sun emits high-energy, short-wave radiation which the atmosphere mostly allows to pass. The Earth's surface absorbs this energy and re-emits it as lower-energy, long-wave radiation, which we perceive as heat. CO2 molecules are specifically "tuned" to the vibrational frequency of this infrared radiation. By absorbing these wavelengths, CO2 traps thermal energy within the troposphere instead of allowing it to escape into space. Therefore, the correct answer is (D) the infrared part of the solar radiation (referring to the heat energy budget of the Earth-atmosphere system).

UPSC often includes distractors to test the precision of your conceptual clarity. Option (B) is a classic trap; Ultraviolet (UV) radiation is indeed absorbed in the atmosphere, but by the Ozone layer in the stratosphere, not by CO2. Option (A) mentions Water Vapour; while it is a potent greenhouse gas, the question specifically asks why the increasing concentration of CO2 is the driver of the current temperature rise. Finally, option (C) is logically incorrect because if the atmosphere absorbed "all" radiation, visible light would never reach the surface. Mastering these distinctions, as detailed in Environment and Ecology by Majid Hussain, ensures you won't fall for these common atmospheric science traps.