15.1 Greenhouse gases and their sources

Greenhouse gases trap heat in Earth's atmosphere, causing global warming. Water vapor, carbon dioxide, methane, and nitrous oxide are the main culprits, with human activities like fossil fuel burning and agriculture significantly increasing their concentrations since pre-industrial times.

These gases come from various sources, including energy production, farming, and industrial processes. Understanding their global warming potential helps prioritize emission reduction efforts. Land use changes, like deforestation, also play a crucial role in greenhouse gas dynamics.

Greenhouse Gases and Their Sources

Primary greenhouse gases

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• Water vapor (H2O) most abundant greenhouse gas contributes approximately 50% of the greenhouse effect through absorption and re-emission of infrared radiation
• Carbon dioxide (CO2) second most significant greenhouse gas contributes approximately 20% of the greenhouse effect concentrations have increased by over 40% since pre-industrial times (late 18th century) due to human activities
• Methane (CH4) contributes approximately 4-9% of the greenhouse effect concentrations have more than doubled since pre-industrial times due to agriculture, livestock, and fossil fuel extraction
• Nitrous oxide (N2O) contributes approximately 6% of the greenhouse effect concentrations have increased by about 20% since pre-industrial times due to agricultural practices (fertilizer use) and industrial processes
• Ozone (O3) contributes approximately 3-7% of the greenhouse effect concentrations vary depending on altitude and location formed by chemical reactions in the atmosphere
• Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) synthetic compounds with high global warming potential concentrations have decreased due to the Montreal Protocol which phased out their production

Sources of greenhouse emissions

• Fossil fuel combustion burning of coal, oil, and natural gas for energy production primary source of CO2 emissions have increased significantly since the Industrial Revolution (late 18th century)
• Agriculture and livestock practices contribute to greenhouse gas emissions:
  1. Rice cultivation and livestock farming (cattle, sheep) emit CH4 through anaerobic decomposition
  2. Fertilizer use and manure management release N2O emissions
  3. Emissions have increased due to population growth and changes in agricultural practices (intensification, expansion)
• Industrial processes release greenhouse gases:
  1. Cement production, chemical manufacturing, and metal production emit CO2
  2. Emissions have increased with industrial development and economic growth
• Waste management practices contribute to greenhouse gas emissions:
  1. Landfills and wastewater treatment facilities emit CH4 through anaerobic decomposition
  2. Emissions have increased with population growth and urbanization
• Transportation sector burns fossil fuels in vehicles (cars, trucks, planes) releasing CO2 emissions have increased with the growth of the transportation sector and globalization

Land use and deforestation impacts

• Deforestation clearing of forests for agriculture (palm oil, soy), logging, or urban development reduces the capacity of ecosystems to absorb CO2 through photosynthesis and burning of cleared vegetation releases stored carbon as CO2
• Land degradation from overgrazing, soil erosion, and desertification reduces the ability of land to store carbon degraded land may become a net source of greenhouse gases
• Wetland drainage for agriculture (rice) or development releases stored carbon and CH4 reduces the capacity of wetlands to sequester carbon
• Urbanization conversion of natural landscapes to urban areas reduces vegetation cover urban heat island effect can increase local temperatures and energy consumption (air conditioning)

Global warming potential concept

• Global warming potential (GWP) measures a greenhouse gas's ability to trap heat in the atmosphere relative to CO2 calculated over a specific time horizon (typically 100 years) CO2 has a GWP of 1 by definition
• Methane (CH4) has a GWP of 28-36 over 100 years meaning one ton of CH4 has 28-36 times the warming effect of one ton of CO2 over a century
• Nitrous oxide (N2O) has a GWP of 265-298 over 100 years one ton of N2O has 265-298 times the warming effect of one ton of CO2 over a century
• Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) have GWPs ranging from hundreds to thousands over 100 years highly potent greenhouse gases regulated by the Montreal Protocol
• Application of GWP allows for comparison of the warming effects of different greenhouse gases helps in developing emission reduction strategies and policies targeting the most potent gases (CH4, N2O, HFCs)