Solar radiation is the main energy source for Earth's climate system. It powers atmospheric and oceanic circulation, and drives the biosphere. The Sun emits a spectrum of electromagnetic radiation, with peak intensity in visible light wavelengths.

Earth's energy budget balances incoming solar radiation with outgoing terrestrial radiation. This balance maintains Earth's average temperature. The atmosphere, greenhouse gases, and surface features all play crucial roles in this delicate equilibrium.

Solar Radiation Reaching Earth

Characteristics of Solar Radiation

• Solar radiation is the primary source of energy for Earth's climate system
  • Drives atmospheric and oceanic circulation
  • Powers the biosphere
• The Sun emits a continuous spectrum of electromagnetic radiation
  • Wavelengths range from short-wave ultraviolet to long-wave infrared
• Peak intensity of solar radiation occurs in the visible light portion of the spectrum
  • Corresponds to the wavelengths that the human eye can detect (380-700 nm)
• The amount of solar radiation reaching Earth's surface is affected by the distance between the Earth and the Sun
  • Varies throughout the year due to Earth's elliptical orbit
  • Earth is closest to the Sun in January (perihelion) and farthest in July (aphelion)

Attenuation of Solar Radiation in Earth's Atmosphere

• Solar radiation is attenuated as it passes through Earth's atmosphere
  • Some wavelengths are absorbed or scattered by atmospheric gases, aerosols, and clouds
  • Ozone absorbs most of the ultraviolet radiation
  • Water vapor, carbon dioxide, and other greenhouse gases absorb infrared radiation
• The solar radiation that reaches Earth's surface is composed of two components
  • Direct radiation: travels in a straight line from the Sun
  • Diffuse radiation: scattered by the atmosphere and arrives from all directions
• Atmospheric scattering is caused by molecules, dust particles, and other aerosols
  • Rayleigh scattering: occurs when the wavelength of light is larger than the particle size (responsible for the blue color of the sky)
  • Mie scattering: occurs when the wavelength of light is similar to the particle size (responsible for the white color of clouds)

Factors Influencing Solar Radiation

Latitude and Angle of Incidence

• The amount of solar radiation received at a given location on Earth's surface is primarily determined by the angle of incidence
  • Angle of incidence: the angle at which the Sun's rays strike the surface
• The angle of incidence varies with latitude
  • Locations near the equator receive more direct solar radiation than locations at higher latitudes
  • At the equator, the Sun's rays are perpendicular to the surface, resulting in a higher concentration of energy per unit area
• The amount of solar radiation received also varies with the time of day
  • Maximum amount occurs at solar noon when the Sun is highest in the sky
• Earth's axial tilt causes seasonal variations in the amount of solar radiation received at different latitudes
  • Northern Hemisphere receives more radiation during the Northern Hemisphere summer (June solstice)
  • Southern Hemisphere receives more radiation during the Southern Hemisphere summer (December solstice)

Atmospheric Conditions and Surface Features

• Atmospheric conditions can reduce the amount of solar radiation reaching the surface
  • Clouds, dust, and pollutants absorb or scatter incoming radiation
  • Clouds have a high albedo and reflect a significant portion of incoming solar radiation
• The elevation of a location affects the amount of solar radiation received
  • Higher elevations receive more radiation due to the thinner atmosphere overhead
  • The atmosphere attenuates solar radiation through absorption and scattering
• Surface features can create local variations in the amount of solar radiation received
  • Mountains and valleys affect the angle of incidence and the amount of shading
  • South-facing slopes in the Northern Hemisphere receive more radiation than north-facing slopes
  • Urban areas can create heat islands due to the low albedo of buildings and pavement

Earth's Energy Budget

Components of Earth's Energy Budget

• Earth's energy budget refers to the balance between incoming solar radiation and outgoing terrestrial radiation
• The primary components of Earth's energy budget are:
  1. Incoming solar radiation: total amount of solar energy that reaches the top of Earth's atmosphere (~340 W/m²)
  2. Reflected solar radiation: portion of incoming solar radiation reflected back to space by clouds, aerosols, and Earth's surface (~30% of total incoming radiation)
  3. Absorbed solar radiation: portion of incoming solar radiation absorbed by Earth's surface and atmosphere (~70% of total incoming radiation)
  4. Terrestrial radiation: long-wave infrared radiation emitted by Earth's surface and atmosphere
  5. Atmospheric absorption and emission of radiation

Interactions Between Energy Budget Components

• Absorbed solar radiation is balanced by terrestrial radiation emitted by Earth's surface
  • This balance maintains Earth's average temperature
• The atmosphere absorbs and emits long-wave radiation
  • Greenhouse gases (water vapor, carbon dioxide) play a key role in this process
  • Greenhouse gases absorb terrestrial radiation and re-emit it back to the surface, causing the greenhouse effect
• The interactions between these components drive the climate system
  • Changes in the balance between incoming and outgoing radiation can lead to climate change
  • Positive radiative forcing (more incoming than outgoing radiation) leads to warming
  • Negative radiative forcing (more outgoing than incoming radiation) leads to cooling

Albedo and Energy Balance

Concept of Albedo

• Albedo: the proportion of incoming solar radiation that is reflected by a surface
  • Expressed as a value between 0 and 1
  • High albedo surfaces (fresh snow, ice) reflect a large portion of incoming solar radiation
  • Low albedo surfaces (dark soil, water) absorb more radiation
• Earth's average albedo is about 0.3
  • ~30% of incoming solar radiation is reflected back to space
• The albedo of different surfaces on Earth varies widely
  • Deserts, grasslands, and forests have lower albedo than snow and ice-covered regions
  • Ocean albedo varies with the angle of incidence and the presence of waves and foam

Albedo's Role in Earth's Energy Balance

• Changes in Earth's surface albedo can have significant impacts on the energy balance and climate
  • Decreases in albedo lead to increased absorption of solar radiation and warming
  • Increases in albedo have a cooling effect
• The albedo feedback is an important climate feedback mechanism
  • Changes in surface albedo due to melting snow and ice can amplify or dampen climate change
  • As snow and ice melt, the exposed darker surfaces absorb more radiation, leading to further warming (positive feedback)
• Human activities can alter the albedo of Earth's surface
  • Deforestation and urbanization generally decrease albedo, leading to increased absorption of solar radiation
  • Land use changes (agriculture, afforestation) can increase or decrease albedo depending on the specific practices
• Changes in Earth's albedo can contribute to long-term climate change
  • Variations in Earth's orbit and axial tilt (Milankovitch cycles) can change the global distribution of albedo
  • Volcanic eruptions can increase Earth's albedo by injecting reflective aerosols into the stratosphere, causing short-term cooling