9.1 Greenhouse gas emissions and radiative forcing

Greenhouse gases are heating up our planet. Carbon dioxide from burning fossil fuels is the main culprit, but methane and other gases also play a role. These emissions trap heat in the atmosphere, causing global temperatures to rise.

Radiative forcing measures how much these gases warm the Earth. It's key for understanding climate change and predicting future impacts. Scientists use this concept to compare different greenhouse gases and inform policy decisions.

Anthropogenic Greenhouse Gases and Sources

Carbon Dioxide and Methane Emissions

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• Carbon dioxide (CO2) serves as the primary anthropogenic greenhouse gas emitted through fossil fuel combustion (coal, oil, natural gas), deforestation (tropical rainforests), and industrial processes (cement production)
• Methane (CH4) ranks as the second most significant anthropogenic greenhouse gas with major sources including:
  • Agriculture (rice paddies)
  • Livestock (cattle, sheep)
  • Landfills (decomposing organic waste)
  • Natural gas production (leaks during extraction and transportation)
• Both CO2 and CH4 have increased significantly since the Industrial Revolution due to human activities

Other Significant Greenhouse Gases

• Nitrous oxide (N2O) emissions primarily result from:
  • Agricultural practices (synthetic fertilizer application, soil management)
  • Industrial processes (nylon production)
  • Fossil fuel combustion (vehicle emissions)
• Fluorinated gases encompass synthetic compounds with high global warming potentials:
  • Hydrofluorocarbons (HFCs) used in refrigerants and aerosols
  • Perfluorocarbons (PFCs) employed in electronics manufacturing
  • Sulfur hexafluoride (SF6) utilized in electrical insulation
• Water vapor acts as the most abundant greenhouse gas influenced indirectly by human activities through feedback mechanisms (increased evaporation from warmer oceans)

Radiative Forcing and Climate Change

Fundamentals of Radiative Forcing

• Radiative forcing quantifies the change in Earth's energy balance due to climate-altering factors measured in watts per square meter (W/m²)
• Positive radiative forcing leads to warming of the Earth's surface (increased greenhouse gas concentrations)
• Negative radiative forcing results in cooling (increased aerosol concentrations)
• Greenhouse gases contribute to positive radiative forcing by:
  • Absorbing longwave radiation emitted by Earth's surface
  • Re-emitting this radiation in all directions, including back towards the surface
  • Trapping heat in the Earth's atmosphere, raising global temperatures

Importance and Applications of Radiative Forcing

• Radiative forcing proves crucial for understanding the relative impact of different climate change drivers (greenhouse gases, aerosols, land-use changes)
• Scientists use radiative forcing to project future climate scenarios and inform policy decisions
• Changes in radiative forcing can trigger climate feedbacks:
  • Amplifying effects (ice-albedo feedback)
  • Dampening effects (increased cloud cover)
• The concept aids in comparing the warming potential of different greenhouse gases over various time scales

Greenhouse Gas Emissions Trends

Historical Concentration Trends

• Atmospheric CO2 concentrations have increased from pre-industrial levels:
  • Pre-industrial: ~280 parts per million (ppm)
  • Current: >410 ppm
  • Sharp rise observed since the mid-20th century due to industrialization and population growth
• Methane concentrations have more than doubled since pre-industrial times:
  • Pre-industrial: ~700 parts per billion (ppb)
  • Current: >1800 ppb
  • Significant increases in the 20th century with recent acceleration in growth rate
• Nitrous oxide levels have increased by about 20% since pre-industrial times:
  • Pre-industrial: ~270 ppb
  • Current: ~330 ppb
  • Relatively steady growth rate over the past few decades

Long-term Records and Recent Trends

• Ice core data and other proxy records provide evidence of greenhouse gas concentrations over millennia:
  • Antarctic ice cores (Vostok, EPICA Dome C)
  • Tree rings and sediment cores
• Current greenhouse gas levels appear unprecedented in at least the past 800,000 years based on these records
• Emission trends vary by gas and source:
  • Some sectors show recent stabilization or decline (improved energy efficiency in developed countries)
  • Others continue to increase (transportation emissions in developing countries)

Relative Contributions of Greenhouse Gases to Radiative Forcing

Major Contributors and Their Impacts

• Carbon dioxide stands as the largest contributor to anthropogenic radiative forcing due to:
  • High atmospheric concentration
  • Long atmospheric lifetime (hundreds to thousands of years)
  • Cumulative effect of historical emissions
• Methane ranks as the second-largest contributor to radiative forcing:
  • Higher global warming potential than CO2 on a per-molecule basis (28 times more potent over 100 years)
  • Shorter atmospheric lifetime (~12 years) limiting long-term impact
• Nitrous oxide emerges as the third most important anthropogenic greenhouse gas:
  • Lower concentration but high global warming potential (265 times more potent than CO2 over 100 years)
  • Long atmospheric lifetime (~114 years)

Comparing Greenhouse Gas Impacts

• Fluorinated gases, while present in small quantities, exert significant warming effects:
  • Extremely high global warming potentials (thousands to tens of thousands times more potent than CO2)
  • Atmospheric lifetimes ranging from decades to millennia
• Scientists use the concept of CO2-equivalent emissions to compare radiative forcing of different greenhouse gases:
  • Takes into account global warming potentials over specific time horizons (20, 100, 500 years)
  • Allows for standardized comparison and policy-making (carbon trading schemes)
• The relative importance of each greenhouse gas varies depending on the timescale considered:
  • Short-term: Methane plays a more significant role
  • Long-term: CO2 dominates due to its persistence in the atmosphere