Lecture Notes: Earth's Energy Balance and the Greenhouse Effect

Introduction

• Earth's Surface Heating
  • The Earth is primarily heated by the sun.
  • Geothermal energy (from within the Earth) is negligible compared to solar energy.
  • Earth's surface temperature is determined by the sun, not internal geothermal heat.

Solar Radiation

• Reflection and Absorption
  • Part of the sun's energy is reflected back to space (more from ice-covered areas and regions with many clouds).
  • The rest is absorbed by the Earth, primarily in the visible spectrum.

Earth's Radiation

• Heat Absorption and Emission
  • Absorption of solar energy heats the Earth.
  • Heated objects emit radiation at longer wavelengths (infrared).
  • This emitted radiation heads back to space.

Atmospheric Influence

• Role of the Atmosphere
  • Without an atmosphere, Earth would be 30 degrees colder.
  • The atmosphere, with greenhouse gases, makes Earth habitable.

Greenhouse Gases

• Types and Functions
  • Main greenhouse gases: carbon dioxide, methane, and water vapor (most significant).
  • Water Vapor
    • Often overlooked as a greenhouse gas.
    • Acts as an amplifier due to its quick turnover (hours to weeks).
    • Enhances the effect of other greenhouse gases (like a stereo amplifier).

The Greenhouse Effect

• Mechanism
  • Greenhouse gases absorb and re-radiate infrared radiation, acting like a thermal blanket.
  • They keep Earth's surface warmer than it would be without them.
  • Example: Venus is 460°C due to a strong greenhouse effect.

Conclusion

• Empirical Evidence
  • The greenhouse effect is not just theoretical; it's observed on Venus and measured in labs (e.g., CO2's absorption of long-wave radiation).