3.1 Solar radiation and Earth's energy budget

Solar radiation, the driving force of Earth's climate, originates from the Sun's core. This electromagnetic energy spans ultraviolet, visible, and infrared wavelengths, with intensity peaking in the visible spectrum. The solar constant, averaging 1,361 W/m², fluctuates with Earth-Sun distance and solar activity.

Earth's energy budget balances incoming solar radiation with outgoing terrestrial radiation. Factors like albedo, greenhouse gases, and energy transfer processes influence this equilibrium. The distribution of solar radiation varies with latitude, seasons, atmospheric conditions, and surface characteristics, shaping our planet's climate patterns.

Solar Radiation

Characteristics of solar radiation

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• Originates from nuclear fusion reactions in the Sun's core converting hydrogen into helium
• Continuous spectrum of electromagnetic waves spanning ultraviolet (UV) (< 400 nm wavelength), visible light (400-700 nm), and infrared (IR) (> 700 nm)
• Intensity varies with wavelength peaking in the visible light range (sunlight)
• Solar constant represents the average intensity of solar radiation reaching Earth's atmosphere at $1,361 W/m^2$
• Intensity fluctuates with the Earth-Sun distance (perihelion vs aphelion) and solar activity (sunspot cycles)

Components of Earth's energy budget

• Incoming solar radiation (insolation) consists of shortwave radiation from the Sun
  • Partially reflected by clouds, aerosols (dust, smoke), and Earth's surface (albedo)
• Outgoing terrestrial radiation comprises longwave radiation emitted by Earth's surface and atmosphere
  • Partially absorbed by greenhouse gases (CO2, water vapor) in the atmosphere
• Energy transfer processes include absorption, reflection, and scattering of radiation
  • Latent heat release (evaporation) and sensible heat transfer (conduction)
  • Conduction and convection in the atmosphere and oceans (heat transport)

Earth's Energy Balance

Solar vs terrestrial radiation balance

• Earth's energy balance: incoming solar radiation = outgoing terrestrial radiation
  • Incoming solar radiation: $(1 - \alpha) \times S$
    1. $\alpha$: Earth's albedo with an average value of 0.3 (30% reflected)
    2. $S$: Solar constant at $1,361 W/m^2$
  • Outgoing terrestrial radiation: $\varepsilon \times \sigma \times T^4$
    1. $\varepsilon$: Earth's emissivity with an average value of 0.95 (95% emitted)
    2. $\sigma$: Stefan-Boltzmann constant at $5.67 \times 10^{-8} W/(m^2 \cdot K^4)$
    3. $T$: Earth's average surface temperature in Kelvin (K)
• Equilibrium temperature occurs when incoming and outgoing radiation are balanced
  • Calculated using the equation: $(1 - \alpha) \times S = \varepsilon \times \sigma \times T^4$

Distribution factors of solar radiation

• Latitude serves as the primary factor determining the amount of solar radiation received
  • Higher latitudes (poles) receive less solar radiation per unit area due to the oblique angle of the Sun
  • Lower latitudes (equator) receive more solar radiation per unit area due to the more direct angle of the Sun
• Seasonal variations caused by Earth's axial tilt (23.5°) and orbital motion (revolution)
  • Northern Hemisphere receives more solar radiation during June solstice (summer)
  • Southern Hemisphere receives more solar radiation during December solstice (summer)
• Atmospheric factors like clouds, aerosols, and gases affect the amount of solar radiation reaching the surface
  • Reflection (clouds), scattering (air molecules), and absorption (ozone) of solar radiation
• Surface characteristics influence the absorption and reflection of solar radiation
  • Albedo represents the fraction of solar radiation reflected by a surface
    • Snow and ice have high albedo (0.8-0.9) reflecting more solar radiation
    • Oceans and forests have low albedo (0.1-0.2) absorbing more solar radiation
  • Elevation affects solar radiation received with higher elevations receiving more due to thinner atmosphere (less attenuation)