19. Global warming is the long-term heating of Earth’sclimate system due to human activities, primarily fossilfuel burning, which increases heat-trapping greenhouse gas levels in Earth’s atmosphere. The reason for heating is
    (1) Absorption of UV by Ozone
    (2) Absorption of IR by CO₂
    (3) Absorption of IR by ozone
    (4) Absorption of UV by CO₂ 

Global warming is the long-term heating of Earth’s climate system, primarily caused by human activities that increase the concentration of heat-trapping greenhouse gases in the atmosphere. The most notable of these activities is the burning of fossil fuels, which releases large amounts of carbon dioxide (CO₂), methane (CH₄), and other gases. But what is the actual mechanism that causes this heating? Understanding the science behind global warming is essential for addressing climate change and protecting our planet.

How the Greenhouse Effect Works

The Earth receives energy from the sun in the form of sunlight, much of which is visible light. This sunlight passes through the atmosphere and warms the planet’s surface. The warmed surface then radiates energy back into space, but not all of this energy escapes. Greenhouse gases in the atmosphere—such as CO₂, methane, water vapor, and others—absorb some of this outgoing energy and re-emit it in all directions, including back toward the Earth’s surface. This process is called the greenhouse effect, and it is responsible for keeping the planet warm enough to support life.

Without the greenhouse effect, Earth’s average temperature would be about -18°C (0°F), making the planet inhospitable for most forms of life. However, human activities have increased the concentration of greenhouse gases, enhancing the natural greenhouse effect and leading to global warming.

The Role of Different Types of Radiation

To understand why certain gases cause global warming, it’s important to distinguish between different types of solar radiation:

• Ultraviolet (UV) Radiation: High-energy, short-wavelength radiation from the sun. Most UV radiation is absorbed by the ozone layer in the stratosphere, protecting life on Earth from its harmful effects.

• Visible Light: The part of the solar spectrum that we can see. It passes through the atmosphere and warms the Earth’s surface.

• Infrared (IR) Radiation: Lower-energy, longer-wavelength radiation emitted by the Earth’s surface as it cools down. Greenhouse gases are particularly effective at absorbing and re-emitting IR radiation, trapping heat in the atmosphere.

Why CO₂ and Other Greenhouse Gases Cause Global Warming

Greenhouse gases like CO₂, methane, and water vapor have molecules that can absorb infrared radiation. When the Earth’s surface emits IR radiation as it cools, these gases absorb the energy and re-emit it in all directions, including back toward the surface. This process traps heat in the lower atmosphere, raising the planet’s average temperature.

Absorption of Infrared Radiation by CO₂

Carbon dioxide is especially effective at absorbing infrared radiation. When CO₂ molecules in the atmosphere encounter IR radiation emitted by the Earth’s surface, they vibrate and absorb the energy. This energy is then re-emitted, with some of it returning to the surface and warming the planet further.

What About Ozone and UV Absorption?

Ozone (O₃) in the stratosphere absorbs most of the sun’s harmful UV radiation, protecting life on Earth from its damaging effects. However, ozone does not play a significant role in trapping heat in the lower atmosphere or in the greenhouse effect that causes global warming. Ozone in the troposphere (the lower atmosphere) can act as a greenhouse gas, but its contribution to global warming is much smaller than that of CO₂ or methane.

Evaluating the Options

Let’s examine each option in the context of global warming:

• Absorption of UV by Ozone: Ozone absorbs UV radiation in the stratosphere, protecting life from its harmful effects. This process does not cause global warming.

• Absorption of IR by CO₂: Correct. CO₂ absorbs infrared radiation emitted by the Earth’s surface, trapping heat in the atmosphere and causing global warming.

• Absorption of IR by Ozone: Ozone can absorb some IR radiation, but its contribution to the greenhouse effect is minor compared to CO₂ and methane.

• Absorption of UV by CO₂: CO₂ does not absorb significant amounts of UV radiation. Its role in global warming is due to its absorption of infrared radiation.

The Science Behind Heat Trapping

The greenhouse effect is a well-understood physical process:

1. Sunlight Passes Through the Atmosphere: Visible light from the sun passes through the atmosphere and heats the Earth’s surface.

2. Earth Emits Infrared Radiation: The warmed surface emits infrared (heat) radiation back toward space.

3. Greenhouse Gases Absorb IR Radiation: Greenhouse gases like CO₂ absorb this outgoing IR radiation.

4. Heat Is Trapped: The absorbed energy is re-emitted in all directions, including back toward the Earth’s surface, trapping heat and warming the planet.

The Enhanced Greenhouse Effect

Before the Industrial Revolution, the greenhouse effect was a natural process that kept Earth’s temperature stable. However, human activities—especially the burning of fossil fuels—have increased the concentration of greenhouse gases, enhancing the greenhouse effect and leading to global warming. This is known as the enhanced greenhouse effect.

Other Greenhouse Gases

While CO₂ is the most significant greenhouse gas in terms of total emissions, other gases also contribute to global warming:

• Methane (CH₄): More effective at trapping heat than CO₂ on a molecule-for-molecule basis, but present in much smaller concentrations.

• Nitrous Oxide (N₂O): A potent greenhouse gas, mainly from agricultural activities.

• Water Vapor (H₂O): The most abundant greenhouse gas, but its concentration is controlled by natural processes and is not directly increased by human activities.

Key Takeaways

• Global warming is caused by the absorption of infrared radiation by greenhouse gases like CO₂, which traps heat in the atmosphere.

• Ozone absorbs UV radiation in the stratosphere, protecting life from harmful UV, but does not cause global warming.

• CO₂ does not absorb significant amounts of UV radiation; its role in global warming is due to its absorption of infrared radiation.

• The enhanced greenhouse effect, driven by human activities, is responsible for the current trend of global warming.

Summary Table

Conclusion

Global warming is the result of the enhanced greenhouse effect, primarily caused by the absorption of infrared radiation by CO₂ and other greenhouse gases in the atmosphere. Ozone’s absorption of UV radiation is important for protecting life on Earth, but it does not contribute to global warming. The correct answer to the question is:

Absorption of IR by CO₂.

Understanding this fundamental process is essential for addressing climate change and reducing the impacts of global warming on our planet. By reducing greenhouse gas emissions, especially from fossil fuel burning, we can help slow the pace of global warming and protect the environment for future generations.