Équilibre Radiatif Terrestre

Introduction

• The topic of study is the Earth's terrestrial radiative balance.
• The main question is: Why does the average surface temperature of the Earth remain constant?

Solar Power Reaching Earth

• The entire amount of energy emitted by the Sun does not reach Earth.
• This study aims to determine the proportion of the Sun's emitted power that is actually received by Earth and its atmosphere.
• The process is done in steps.
• Calculate the area of the sphere centered on the Sun with the Earth-Sun distance as radius (R = 150 x 10⁹ m).
• Calculate the power per square meter received from the Sun at the Earth-Sun distance (solar power = 3.84 x 10²⁶ W).
• Determine the area of the non-illuminated disk in document 2 using Earth's radius (r = 6380 km).
• Calculate the solar power intercepted by Earth and its atmosphere.

Radiative Balance

• In the example provided, Earth is shown to receive only approximately 1% of the total solar radiation emitted by the sun.
• The Earth's surface emits radiation as a result of this gained energy.
• This radiation influences the planet's temperature, creating a balance.
• Calculations show the average power available per square meter on the surface of the Earth is 136 x 10³ W·m^-2.
• The calculated power intercepted by Earth and its atmosphere is 1.73 x 10¹⁷ W.

Earth's Energy Balance

• Earth receives solar energy.
• Part is reflected by the atmosphere and Earth's surface.
• The rest is absorbed and warms the planet.
• Earth's average temperature is 15°C, but if there were no atmosphere it would be around -18°C.
• The presence of an atmosphere affects Earth's temperature.
• Greenhouse gases absorb infrared radiation emitted by Earth.
• This re-radiated energy warms the planet further.

Factors Affecting Solar Power Received on Earth

• Earth's electromagnetic radiation outputs also influence this balance.
• Calculations and examples are given to demonstrate this.

Greenhouse Effect

• Earth radiates primarily in the infrared range.
• This radiation is absorbed by atmospheric gases (greenhouse gases).
• Re-emitted radiation warms the planet.
• Natural greenhouse effect is essential for life on Earth.

Composition of the Atmosphere

• The atmosphere has several important components, including nitrogen (78%), oxygen (21%), argon (0.9%), and carbon dioxide (0.033%).
• Water vapor and dust are present in varying amounts.
• Some gases (like H₂O, CO₂) are strongly involved in the greenhouse effect and absorb most radiated infrared radiation, thereby influencing the Earth's temperature.

Summary

• Earth's average surface temperature remains relatively constant due to the complex interplay between incoming solar radiation, outgoing terrestrial radiation, atmospheric composition, and the natural greenhouse effect.