Earth's atmosphere is a complex mixture of gases, each playing a unique role. Nitrogen and oxygen dominate, but trace gases like carbon dioxide and methane have outsized impacts. These greenhouse gases trap heat, warming our planet and shaping our climate.

Greenhouse gases come from both natural and human sources. Burning fossil fuels releases CO2, while agriculture produces methane. Some gases linger for centuries, while others break down quickly. Understanding these differences helps us tackle climate change effectively.

Atmospheric Composition and Greenhouse Gases

Composition of Earth's atmosphere

• Nitrogen ($N_2$) makes up approximately 78% of the atmosphere by volume is the most abundant gas
• Oxygen ($O_2$) is the second most abundant gas at around 21% by volume is essential for life on Earth
• Argon (Ar) is the third most abundant gas constituting about 0.93% by volume is an inert noble gas
• Water vapor ($H_2O$) varies in concentration spatially and temporally typically ranging from 0.1% to 4% by volume influences weather and climate
• Carbon dioxide ($CO_2$) currently makes up approximately 0.04% by volume (as of 2021) concentration has been increasing due to human activities like fossil fuel combustion and deforestation
• Trace gases include neon (Ne), helium (He), methane ($CH_4$), krypton (Kr), and xenon (Xe) each constitutes less than 0.002% by volume but can have significant impacts on the atmosphere and climate

Role of greenhouse gases

• Greenhouse gases are atmospheric gases that absorb and emit infrared radiation primary ones include water vapor ($H_2O$), carbon dioxide ($CO_2$), methane ($CH_4$), nitrous oxide ($N_2O$), and ozone ($O_3$)
• Trap heat in the atmosphere by absorbing outgoing infrared radiation emitted by Earth's surface and re-emitting this energy in all directions including back towards the surface leading to warming
• The greenhouse effect warms Earth's surface and lower atmosphere without greenhouse gases Earth's average temperature would be around -18°C (0°F) much colder than current conditions
• Increasing concentrations of greenhouse gases lead to an enhanced greenhouse effect causing global warming and climate change impacts ecosystems, sea levels, and weather patterns

Sources of greenhouse gases

• Carbon dioxide ($CO_2$)
  • Anthropogenic sources include fossil fuel combustion (coal, oil, natural gas), deforestation, and cement production
  • Natural sources include respiration, volcanic eruptions, and decomposition of organic matter
• Methane ($CH_4$)
  • Anthropogenic sources include agriculture (livestock and rice cultivation), landfills, and fossil fuel extraction and distribution (natural gas leaks)
  • Natural sources include wetlands, termites, and wildfires
• Nitrous oxide ($N_2O$)
  • Anthropogenic sources include agricultural soil management (fertilizers), industrial processes, and biomass burning
  • Natural sources include microbial processes in soils and oceans
• Water vapor ($H_2O$)
  • Anthropogenic sources include irrigation and combustion of fossil fuels
  • Natural sources include evaporation from oceans, lakes, and rivers as well as transpiration from plants
• Ozone ($O_3$)
  • Anthropogenic sources include photochemical reactions involving nitrogen oxides and volatile organic compounds from vehicle emissions and industrial processes
  • Natural sources include the stratospheric ozone layer and lightning

Atmospheric lifetime of gases

• Atmospheric lifetime is the average time a molecule of a gas remains in the atmosphere before being removed or chemically altered through processes like chemical reactions, photolysis (breakdown by sunlight), deposition on Earth's surface, or uptake by oceans or vegetation
• Greenhouse gases with longer atmospheric lifetimes have a more persistent impact on the greenhouse effect carbon dioxide ($CO_2$) has an atmospheric lifetime of centuries to millennia
• Greenhouse gases with shorter atmospheric lifetimes can still have significant impacts if their emissions are large and continuous methane ($CH_4$) has an atmospheric lifetime of about 12 years but has a strong greenhouse effect
• Understanding atmospheric lifetimes helps predict the long-term effects of greenhouse gas emissions on climate change and develop mitigation strategies reducing emissions of long-lived gases is crucial for limiting long-term warming