The increasing amount of carbon dioxide in the air is slowly raising the temperature of the atmosphere, because it absorbs
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- AThe water vapour of the air and retains its heat.
- BThe ultraviolet part of the solar radiation.
- CAll the solar radiations.
- DThe infrared part of the solar radiation.
Show answer
Answer: (D) The infrared part of the solar radiation.
Carbon dioxide causes global warming because it absorbs infrared radiation (heat) emitted by the Earth's surface.
Here's the mechanism:
Sunlight passes through the atmosphere
→ heats the Earth's surface
→ Earth re-emits this energy as infrared (long-wave) radiation
→ CO₂ and other greenhouse gases absorb this infrared radiation
→ atmosphere warms up.
Option (a) is wrong — CO₂ doesn't absorb water vapour.
Option (b) is wrong — ozone absorbs UV radiation, not CO₂.
Option (c) is wrong — CO₂ doesn't absorb ALL solar radiation; it specifically absorbs infrared.
Source: NCERT Class 12 Biology.
Answer: (d).
Carbon dioxide absorbs infrared radiation emitted by Earth's surface, not incoming solar radiation, which is why it causes atmospheric warming.
The question tests whether students understand the specific mechanism of the greenhouse effect - that greenhouse gases trap outgoing heat rather than incoming sunlight.
Greenhouse Effect Mechanism
Environment carbon dioxide infrared solar radiation atmosphere
Greenhouse Effect: How CO₂ Traps Heat & Warms Earth
CO₂ absorbs infrared radiation emitted by Earth's surface
Sunlight passes through atmosphere but Earth's re-emitted heat gets trapped
CO₂ does NOT absorb UV radiation or water vapour
Greenhouse effect keeps Earth 33°C warmer than it would be otherwise
Greenhouse effect is Earth's natural warming mechanism where certain gases trap heat in the atmosphere. Carbon dioxide is the most important greenhouse gas after water vapour, specifically because it absorbs infrared radiation — not visible light or UV rays.
How CO₂ Causes Global Warming
%%{init: {"flowchart": {"wrappingWidth": 460}}}%%
flowchart TD
s1["`**Solar Energy Arrives**
**Shortwave radiation** from Sun passes through atmosphere`"]
s2["`**Earth's Surface Heats Up**
Land and oceans absorb solar energy and warm up`"]
s3["`**Earth Re-emits Heat**
Warm surface emits **longwave infrared radiation** back toward space`"]
s4["`**CO₂ Absorbs Infrared**
**Greenhouse gases** absorb this infrared radiation in atmosphere`"]
s5["`**Atmosphere Warms**
Trapped heat warms the atmosphere and Earth's surface`"]
s1 --> s2
s2 --> s3
s3 --> s4
s4 --> s5What Different Gases Absorb
Gas | Absorbs | Wavelength | Effect |
|---|---|---|---|
Carbon Dioxide | Infrared radiation | Longwave (8-15 μm) | Greenhouse warming |
Ozone (O₃) | UV radiation | Shortwave (0.2-0.3 μm) | Protects from UV |
Water Vapour | Infrared + some visible | Multiple bands | Strongest greenhouse gas |
Methane | Infrared radiation | Longwave (7-13 μm) | 28x stronger than CO₂ |
Question Analysis
This question tests the specific mechanism of CO₂ warming. The correct answer is (D) infrared radiation because Earth's surface re-emits solar energy as heat (infrared), which CO₂ then absorbs and traps.
Trap: Option (B) — Ozone absorbs UV, not CO₂
Trap: Option (A) — CO₂ doesn't absorb water vapour, it absorbs radiation
Trap: Option (C) — CO₂ only absorbs specific wavelengths, not all solar radiation
Memory: CO₂ = Infrared absorption; O₃ = UV absorption
Major Greenhouse Gases
Environment carbon dioxide
Major Greenhouse Gases: Properties & Global Warming Potential
Water vapour is strongest greenhouse gas but has short atmospheric lifetime
CO₂ contributes most to human-caused global warming (76% of emissions)
Methane is 28x stronger than CO₂ but lasts only 9 years in atmosphere
Nitrous oxide persists for 120+ years and depletes ozone too
Greenhouse Gas Comparison
Gas | Global Warming Potential | Atmospheric Lifetime | Main Sources | % of Emissions |
|---|---|---|---|---|
CO₂ | 1 (baseline) | 300-1000 years | Fossil fuels, deforestation | 76% |
Methane (CH₄) | 28x stronger | 9 years | Agriculture, landfills | 16% |
Nitrous Oxide (N₂O) | 265x stronger | 120+ years | Agriculture, industry | 6% |
F-gases | 100-25,000x stronger | 1-50,000 years | Refrigeration, industry | 2% |
Key Properties
Water vapour provides natural greenhouse effect but isn't directly emitted by humans
CO₂ accumulates because of its extremely long atmospheric lifetime
Short-lived gases like methane have immediate impact but fade quickly
F-gases (HFCs, PFCs, SF₆) are super-potent but used in small quantities
Trap: Water vapour is strongest but not counted in emission inventories
Trap: Methane is 28x stronger per molecule, but CO₂ dominates total impact
Memory: CO₂ = volume leader; CH₄ = potency leader
Solar Radiation & Earth's Energy Balance
Environment solar radiation ultraviolet infrared
Solar Spectrum & Earth's Radiation: Understanding Energy Flow
Sun emits shortwave radiation (UV, visible, near-infrared)
Earth re-emits longwave infrared radiation back to space
Greenhouse gases absorb Earth's longwave radiation, not incoming solar
Wien's Law: hotter objects emit shorter wavelengths
Radiation Types in Climate System
Radiation Type | Wavelength | Source | What Absorbs It | Climate Role |
|---|---|---|---|---|
Ultraviolet (UV) | 0.1-0.4 μm | Sun | Ozone in stratosphere | Protects life from UV |
Visible Light | 0.4-0.7 μm | Sun | Earth's surface | Heats land & oceans |
Near-Infrared | 0.7-3 μm | Sun | Earth's surface | Additional heating |
Longwave Infrared | 8-15 μm | Earth's surface | Greenhouse gases | Trapped heat warms atmosphere |
Earth's Energy Balance
%%{init: {"flowchart": {"wrappingWidth": 460}}}%%
flowchart TD
s1["`**Solar Input**
**342 W/m²** average solar energy reaches Earth`"]
s2["`**Reflection**
**30%** reflected by clouds, ice, and atmosphere`"]
s3["`**Absorption**
**70%** absorbed by land, oceans, and atmosphere`"]
s4["`**Re-emission**
Earth emits **longwave infrared** to balance energy`"]
s5["`**Greenhouse Trapping**
Some longwave radiation trapped by greenhouse gases`"]
s1 --> s2
s2 --> s3
s3 --> s4
s4 --> s5Why Wavelength Matters
Temperature determines wavelength: The hot Sun (5,800K) emits shortwave radiation, while cool Earth (288K) emits longwave infrared. Greenhouse gases are transparent to shortwave solar radiation but opaque to Earth's longwave emissions — this selective absorption creates the greenhouse effect.
Trap: Sun emits shortwave; Earth emits longwave — don't confuse the source
Trap: Greenhouse gases absorb outgoing Earth radiation, not incoming solar
Memory: Hot Sun = Short waves; Cool Earth = Long waves