Consider the following statements :

Statement-I : The atmosphere is heated more by incoming solar radiation than by terrestrial radiation.

Statement-II : Carbon dioxide and other greenhouse gases in the atmosphere are good absorbers of long wave radiation.

Which one of the following is correct in respect of the above statements ?

The Correct answer is Statement I is incorrect, but Statement II is correct.

Key Points

• The primary mechanism of atmospheric heating is the absorption of terrestrial radiation by greenhouse gases, not the direct absorption of incoming solar radiation. Hence, statement 1 is incorrect.
  • Incoming Solar Radiation (Short-Wave Radiation): The Sun emits energy primarily in the form of short-wave radiation, including visible light, ultraviolet light, and some infrared radiation. When solar radiation reaches the Earth, a portion of it is reflected back into space, and this reflected portion is called the "albedo" of the Earth; essentially, it's the amount of sunlight that is bounced back rather than absorbed by the planet's surface and atmosphere. Clouds, snow, ice, and light-colored surfaces tend to reflect more sunlight, while the rest is absorbed by the Earth's surface, warming it.
  • Terrestrial Radiation (Long-Wave Radiation): The Earth's surface, once warmed by the absorbed solar radiation, emits energy in the form of long-wave infrared radiation. This process is crucial because the Earth's surface is cooler than the Sun, so it emits energy at longer wavelengths.
• Atmospheric Heating Mechanism:
  • The atmosphere is relatively transparent to incoming short-wave solar radiation, meaning much of it passes through and reaches the Earth's surface.
  • The Earth's surface absorbs this solar energy and then re-emits it as long-wave infrared radiation.
  • The atmosphere contains greenhouse gases such as carbon dioxide (CO₂), methane (CH₄), water vapor (H₂O), and others, which are good absorbers of long-wave radiation.
• Primary Source of Atmospheric Heating:
  • ​The atmosphere is heated more by the absorption of long-wave terrestrial radiation than by the direct absorption of incoming solar radiation. This re-emitted terrestrial radiation is absorbed by greenhouse gases, leading to the warming of the atmosphere through the greenhouse effect.
  • Hence, statement 2 is correct.
    • Greenhouse Gases: Greenhouse gases include carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), water vapor (H₂O), and ozone (O₃). These gases have molecular structures that allow them to absorb and emit infrared radiation effectively.
    • Long-Wave Radiation: Long-wave radiation refers to the infrared radiation emitted by the Earth's surface as it cools.
• Absorption Mechanism:
  • When the Earth's surface emits long-wave radiation, greenhouse gases absorb this energy. Upon absorbing the energy, the molecules of these gases become excited and then re-emit the radiation in all directions, including back toward the Earth's surface.
  • This process traps heat in the atmosphere, creating a warming effect known as the greenhouse effect.
  • Carbon dioxide and other greenhouse gases are indeed effective absorbers of long-wave radiation, playing a crucial role in the warming of the atmosphere through the greenhouse effect.