Earth's atmosphere is a complex system of layers and gases that shape our planet's climate. From the weather-filled troposphere to the auroras in the thermosphere, each layer plays a unique role in protecting and regulating life on Earth.

The atmosphere interacts with other Earth systems to maintain a delicate balance. Through processes like the greenhouse effect, albedo, and atmospheric circulation, it regulates temperature and distributes heat and moisture around the globe, creating our diverse climates.

Atmospheric Structure and Composition

Structure of Earth's atmosphere

• Layers of the atmosphere
  • Troposphere
    • Lowest layer contains about 75% of the atmosphere's mass (air, water vapor, aerosols)
    • Weather phenomena occur here (clouds, precipitation, storms)
    • Temperature decreases with height at a rate of about 6.5℃ per km (lapse rate)
  • Stratosphere
    • Above the troposphere separated by the tropopause boundary
    • Contains the ozone layer which absorbs harmful UV radiation from the Sun
    • Temperature increases with height due to ozone absorbing UV
  • Mesosphere
    • Above the stratosphere separated by the stratopause boundary
    • Temperature decreases with height reaching as low as -90℃ at the mesopause
    • Meteors burn up here due to friction with the thin air (meteor showers)
  • Thermosphere
    • Above the mesosphere separated by the mesopause boundary
    • Temperature increases with height reaching up to 1,500℃ due to absorption of intense solar radiation
    • Auroras occur here when charged particles from the Sun interact with Earth's magnetic field (northern/southern lights)
  • Exosphere
    • Uppermost layer merges gradually with the vacuum of outer space
    • Extremely low density so gas molecules rarely collide and can escape Earth's gravity (helium, hydrogen)
• Composition of the atmosphere
  • Nitrogen ($N_2$): 78%, most abundant gas but chemically inert
  • Oxygen ($O_2$): 21%, second most abundant gas essential for aerobic life
  • Argon (Ar): 0.93%, third most abundant gas but chemically inert
  • Carbon dioxide ($CO_2$): 0.04%, small amount but important greenhouse gas
  • Water vapor ($H_2O$): 0-4%, highly variable amounts act as a powerful greenhouse gas (clouds, humidity)
  • Trace gases: methane ($CH_4$) from wetlands/livestock, nitrous oxide ($N_2O$) from fertilizers/natural sources, ozone ($O_3$) in the stratosphere

Atmospheric Interactions and Climate

Atmosphere's role in climate regulation

• Greenhouse effect
  • Greenhouse gases ($CO_2$, $H_2O$, $CH_4$) absorb and re-emit infrared radiation emitted by Earth's surface
  • Traps heat in the lower atmosphere warming Earth to a habitable average of 15℃ (without it Earth would be -18℃)
• Albedo
  • Fraction of incoming solar radiation reflected back to space by Earth's surface (snow, ice, deserts) and atmosphere (clouds, aerosols)
  • Higher albedo means more solar energy is reflected so Earth absorbs less heat (ice ages)
• Atmospheric circulation
  • Hadley, Ferrel and Polar cells form as warm air rises at the equator and cold air sinks at the poles redistributing heat
  • Jet streams, trade winds and westerlies steer weather systems and transport heat/moisture affecting regional climates (monsoons, hurricanes)
• Cloud formation and precipitation
  • Clouds reflect incoming sunlight cooling Earth (albedo) but also absorb outgoing infrared warming Earth (greenhouse effect)
  • Precipitation releases latent heat when water vapor condenses warming the air (thunderstorms, tropical cyclones)

Atmospheric interactions with Earth systems

• Hydrosphere
  • Evaporation from oceans, lakes and rivers adds water vapor (humidity) to the atmosphere
  • Precipitation (rain, snow) returns water to the surface recharging rivers, lakes and groundwater (water cycle)
• Cryosphere
  • Ice and snow cover have a high albedo cooling Earth by reflecting sunlight
  • Melting of glaciers and ice sheets due to warming raises sea levels and alters ocean circulation patterns (sea ice, permafrost)
• Biosphere
  • Photosynthesis by plants removes $CO_2$ from the air and releases $O_2$ (carbon cycle)
  • Respiration by plants and animals releases $CO_2$ back into the atmosphere
  • Changes in vegetation (deforestation, agriculture) alter regional albedo, evapotranspiration and carbon storage affecting local climates

Human impact on atmospheric system

• Greenhouse gas emissions
  1. Burning fossil fuels (coal, oil, gas) and deforestation have increased atmospheric $CO_2$ by 50% since the industrial revolution
  2. Enhanced greenhouse effect leads to global warming, rising sea levels, more extreme weather, shifting climates (climate change)
• Air pollution
  • Emissions of pollutants like sulfur dioxide ($SO_2$) and nitrogen oxides ($NO_x$) from industries and vehicles react to form acid rain harming ecosystems
  • Photochemical smog containing ground-level ozone irritates lungs and damages crops (Los Angeles, Beijing)
• Stratospheric ozone depletion
  • Chlorofluorocarbons (CFCs) once used as refrigerants and propellants break down ozone in the stratosphere creating the 'ozone hole'
  • Increases risk of skin cancer, cataracts and immune disorders in humans and damages phytoplankton and crops (Montreal Protocol)
• Land use changes
  • Deforestation and urbanization alter local albedo, evapotranspiration and roughness affecting regional temperature and rainfall patterns
  • Expansion of agriculture (irrigation, fertilizers) changes evapotranspiration and releases greenhouse gases ($N_2O$)