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Earth's Energy Balance and Feedback Loops

Earth's energy balance is the equilibrium between the energy Earth receives from the Sun and the energy it emits back into space. This balance determines Earth's climate and temperature. When the incoming solar energy equals the outgoing energy, Earth remains in a stable state. However, changes in factors like greenhouse gases, clouds, and surface reflectivity can disrupt this balance, leading to global warming or cooling. Understanding this balance is crucial for predicting climate changes and their impacts on the planet.

Figures (9)

As the temperature of a black body decreases, the emitted thermal radiation decreases in intensity and its maximum moves to longer wavelengths. Shown for comparison is the classical Rayleigh–Jeans law and its ultraviolet catastrophe.
Color of a black body from 800 K to 12200 K. This range of colors approximates the range of colors of stars of different temperatures, as seen or photographed in the night sky.
The color (chromaticity) of black-body radiation scales inversely with the temperature of the black body; the locus of such colors, shown here in CIE 1931 x,y space, is known as the Planckian locus.
The temperature of a Pāhoehoe lava flow can be estimated by observing its color. The result agrees well with other measurements of temperatures of lava flows at about 1,000 to 1,200 °C (1,830 to 2,190 °F).
Nine-year WMAP image (2012) of the cosmic microwave background radiation across the universe.[22][23]
Photo of person in the visible spectrum
Photo of a person in the infrared spectrum, shifted to the visible
Earth's longwave thermal radiation intensity, from clouds, atmosphere and ground
The Earth only has an absorbing area equal to a two dimensional disk, rather than the surface of a sphere.

Earth's energy balance is about keeping the planet's temperature stable by matching the energy it gets from the Sun with the energy it sends back into space. Imagine Earth as a giant scale: one side holds the sunlight it receives, and the other side holds the heat it radiates away. If more energy comes in than goes out, Earth warms up.

If less energy comes in, it cools down. This balance is what shapes Earth's climate and weather. Several factors can tip this scale.

Greenhouse gases, like carbon dioxide, trap heat in the atmosphere and reduce how much energy escapes. Clouds and ice reflect sunlight, cooling the planet. Scientists track these changes to understand how the climate might shift over time.

Feedback loops are natural responses that either make changes bigger or smaller. For example, melting ice reduces Earth's reflectivity, causing more heat to be absorbed, which leads to more ice melting. This is a positive feedback loop that speeds up warming.

On the other hand, some processes, like increased cloud cover, can reflect more sunlight and slow warming. These loops help explain why small changes in energy balance can lead to big climate shifts over time.

Key Points

  • Albedo is the fraction of solar radiation reflected by a surface, with Earth's average albedo being about 0.30, meaning 30% of incoming solar radiation is reflected back to space.
  • Blackbody radiation is the thermal electromagnetic radiation emitted by a body in thermodynamic equilibrium with its environment, characterized by a continuous spectrum that depends only on the body's temperature.
  • Radiative forcing is the change in the net radiative flux at the top of the atmosphere due to an external driver of climate change, such as changes in greenhouse gas concentrations or solar irradiance.
  • Planetary equilibrium temperature is the temperature a planet would have if it radiated as a perfect black body in the infrared, assuming an unchanging albedo and ignoring greenhouse effects.
  • Wien's displacement law states that the wavelength at which the intensity per unit wavelength of black-body radiation has a local maximum is inversely proportional to the temperature of the body.
  • The Stefan–Boltzmann law states that the total radiant heat power emitted from a surface of a black body is proportional to the fourth power of its absolute temperature.

Terms

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