Examine the distribution and balance of energy in the Earth's atmosphere system.

Distribution of Incoming Solar Radiation

Solar radiation, primarily in the form of shortwave radiation (ultraviolet, visible light, and near-infrared), is the primary energy source for the Earth's atmosphere system. The distribution of this incoming energy is uneven across the globe and through the atmosphere.

• Uneven Latitudinal Distribution: The total annual incoming radiation is greatest at the equator and decreases towards the poles. This is due to the spherical shape of the Earth and the angle at which the sun's rays strike the surface. Equatorial regions receive more direct, concentrated sunlight, leading to an energy surplus, while polar regions receive oblique, less concentrated sunlight, resulting in an energy deficit.
• Atmospheric Absorption and Scattering: As solar radiation enters the atmosphere, a significant portion is affected before reaching the surface.
  • Approximately 23% of incoming solar energy is absorbed by atmospheric gases (like ozone in the stratosphere absorbing UV radiation), water vapor, and dust.
  • Around 7% is reflected back to space by the Earth's surface (e.g., snow, ice, deserts).
  • About 23% is reflected back to space by clouds, which are highly reflective.
• Surface Absorption: On average, about 47% of the incoming solar radiation is absorbed by the Earth's land and ocean surfaces, leading to their heating. This absorption varies significantly based on surface type; dark surfaces absorb more, while lighter surfaces reflect more.

Components of Earth's Energy Balance

The energy balance is maintained by various processes that redistribute and release absorbed energy back to space. This outgoing energy is primarily in the form of longwave infrared radiation emitted by the Earth's surface and atmosphere.

1. Albedo: Earth's Reflectivity

Albedo is the fraction of incoming solar radiation reflected by a surface. It plays a crucial role in the initial distribution and balance of energy.

• High albedo surfaces (like fresh snow and ice) reflect a large percentage of solar radiation (40-80%), contributing to cooling.
• Low albedo surfaces (like oceans and dark forests) absorb a large percentage (4-10% reflectivity for water), leading to warming.
• The Earth's average planetary albedo is approximately 0.3 (30%), meaning about 30% of incoming solar radiation is reflected back to space.

Changes in surface albedo, such as due to melting ice caps or deforestation, can significantly impact the Earth's energy budget and contribute to feedback loops in climate change.

2. Terrestrial Radiation and the Greenhouse Effect

The absorbed solar energy warms the Earth's surface, which then emits energy back into the atmosphere and space as longwave infrared radiation. This process is profoundly influenced by the greenhouse effect.

• Emission: The Earth's surface emits longwave radiation. Without an atmosphere, Earth's average temperature would be around -18°C.
• Greenhouse Gases: Atmospheric gases such as water vapor ($\text{H}_2\text{O}$), carbon dioxide ($\text{CO}_2$), methane ($\text{CH}_4$), nitrous oxide ($\text{N}_2\text{O}$), and fluorinated gases absorb a significant portion of this outgoing longwave radiation.
• Re-emission: These greenhouse gases then re-emit the absorbed energy in all directions, some back towards the Earth's surface, further warming it. This natural process is vital for maintaining Earth's average temperature at approximately +15°C, making it habitable.
• Atmospheric Window: Not all longwave radiation is absorbed; some passes directly through the atmosphere into space, constituting the "atmospheric window."

3. Heat Transfer Mechanisms

Beyond radiation, energy is transferred within the Earth-atmosphere system through:

• Conduction: Direct transfer of heat between the Earth's surface and the lowest layer of the atmosphere. Air is a poor conductor, so this accounts for a small portion of energy transfer (around 7% of incoming solar energy).
• Convection: Transfer of heat through the movement of fluids (air and water). Warm air rises, carrying heat upwards, forming convection currents. This process, along with sensible heat transfer, removes heat from the surface.
• Evaporation and Latent Heat: When water evaporates from the Earth's surface (oceans, lakes, soil, plants), it absorbs latent heat. This energy is carried into the atmosphere as water vapor. When the water vapor condenses to form clouds or precipitation, this latent heat is released into the atmosphere, warming it.

Overall Energy Budget and Balance

In a state of equilibrium, the total incoming solar radiation should equal the total outgoing energy (reflected solar radiation plus emitted terrestrial radiation). However, the Earth's energy budget is a dynamic system, and a perfect balance is rarely sustained over short periods.

Energy Component	Percentage of Incoming Solar Radiation (approx.)	Description
Reflected by clouds	23%	Shortwave radiation reflected back to space.
Reflected by surface	7%	Shortwave radiation reflected by bright surfaces (e.g., snow, ice).
Absorbed by atmosphere	23%	Shortwave radiation absorbed by gases, dust, and clouds.
Absorbed by surface	47%	Shortwave radiation absorbed by land and oceans, converting to heat.
Outgoing Longwave Radiation (Direct to space)	12-17%	Infrared radiation emitted directly from surface/atmosphere to space.
Outgoing Longwave Radiation (Absorbed by GHG)	~34% re-emitted to surface, ~49% to space from atmosphere	Infrared radiation absorbed and re-emitted by greenhouse gases.

Ideally, at the top of the atmosphere, incoming solar radiation (100 units) is balanced by reflected solar radiation (30 units) and outgoing longwave radiation (70 units). However, human activities, primarily the emission of greenhouse gases, have led to an enhanced greenhouse effect, trapping more outgoing longwave radiation and causing an energy imbalance. This imbalance is characterized by more energy being absorbed than emitted, leading to a net accumulation of heat in the Earth system, manifesting as global warming.