Greenhouse gases play a crucial role in Earth's climate system. These gases, including carbon dioxide, methane, and water vapor, trap heat in the atmosphere, maintaining our planet's habitable temperature. Without them, Earth would be a frigid -18°C instead of its current average of 15°C.

Human activities have dramatically increased greenhouse gas concentrations, intensifying the natural greenhouse effect. This enhancement leads to global warming and climate change, with far-reaching consequences for our planet's ecosystems and human societies.

Greenhouse Gases and Their Role in Earth's Climate

Main greenhouse gases and formulas

• Carbon dioxide $CO_2$ primary greenhouse gas contributing to global warming
• Methane $CH_4$ potent greenhouse gas with shorter atmospheric lifetime than $CO_2$
• Nitrous oxide $N_2O$ long-lived greenhouse gas with high global warming potential
• Water vapor $H_2O$ most abundant greenhouse gas acts as a feedback to climate change
• Ozone $O_3$ greenhouse gas formed by photochemical reactions in the atmosphere
• Chlorofluorocarbons CFCs synthetic compounds with extremely high global warming potential (refrigerants, aerosol propellants)

Sources of greenhouse gases

• Carbon dioxide $CO_2$
  • Natural sources
    • Respiration by living organisms (humans, animals, plants)
    • Volcanic eruptions release $CO_2$ from Earth's interior
    • Decomposition of organic matter (dead plants and animals)
  • Anthropogenic sources
    • Burning of fossil fuels (coal, oil, natural gas) for energy production and transportation
    • Deforestation and land-use changes reduce carbon sinks and release stored carbon
    • Cement production involves calcination of limestone releasing $CO_2$
• Methane $CH_4$
  • Natural sources
    • Wetlands produce $CH_4$ through anaerobic decomposition of organic matter
    • Termites emit $CH_4$ during digestion of plant material
    • Oceans release small amounts of $CH_4$ from microbial activity and hydrates
  • Anthropogenic sources
    • Agriculture practices (rice cultivation, livestock farming) produce $CH_4$ from anaerobic conditions and enteric fermentation
    • Landfills generate $CH_4$ from decomposing organic waste
    • Fossil fuel extraction and distribution (natural gas leaks, coal mining) release trapped $CH_4$
• Nitrous oxide $N_2O$
  • Natural sources
    • Soil and ocean bacteria produce $N_2O$ through nitrification and denitrification processes
    • Tropical soils release $N_2O$ due to high rates of microbial activity
  • Anthropogenic sources
    • Agricultural practices (nitrogen fertilizers) increase $N_2O$ emissions from soil
    • Industrial processes (nylon production) generate $N_2O$ as a byproduct
    • Biomass burning (forest fires, crop residue burning) releases $N_2O$
• Water vapor $H_2O$
  • Natural sources
    • Evaporation from oceans, lakes, rivers increases atmospheric water vapor
    • Transpiration from plants releases water vapor through leaf stomata
• Ozone $O_3$
  • Natural sources
    • Photochemical reactions in the stratosphere produce ozone from oxygen molecules
  • Anthropogenic sources
    • Photochemical smog from nitrogen oxides and volatile organic compounds forms ground-level ozone
• Chlorofluorocarbons CFCs
  • Anthropogenic sources
    • Refrigerants in air conditioners and refrigerators
    • Aerosol propellants in spray cans
    • Solvents used in industrial cleaning processes

Role in Earth's energy balance

• Greenhouse gases absorb and re-emit infrared radiation from the Earth's surface
  • Traps heat in the lower atmosphere leading to warming of the planet
• Natural greenhouse effect maintains Earth's average temperature at ~15°C (59°F)
  • Without greenhouse gases average temperature would be around -18°C (0°F)
• Increasing concentrations of greenhouse gases due to human activities enhance the greenhouse effect
  • Leads to global warming and climate change (rising temperatures, sea level rise, extreme weather events)

Warming potential comparisons

• Global Warming Potential GWP measures relative warming effect of a greenhouse gas compared to $CO_2$ over 100 years
  • $CO_2$ has a GWP of 1 by definition serves as the reference gas
• Methane $CH_4$
  • GWP of 28-36 over 100 years
  • Shorter atmospheric lifetime than $CO_2$ but more potent heat-trapping ability
• Nitrous oxide $N_2O$
  • GWP of 265-298 over 100 years
  • Long atmospheric lifetime and high warming potential per molecule
• Chlorofluorocarbons CFCs
  • GWP ranging from 1,000 to 10,000 over 100 years
  • Extremely potent greenhouse gases regulated by the Montreal Protocol due to their ozone-depleting properties