7.1 Greenhouse effect and anthropogenic climate change

Climate change is a hot topic in environmental science. The greenhouse effect, a natural process that keeps Earth warm, has been amplified by human activities. This has led to rising temperatures and various environmental impacts worldwide.

Anthropogenic greenhouse gases, mainly from burning fossil fuels and deforestation, are the primary culprits. CO2, methane, and nitrous oxide trap heat in the atmosphere, disrupting Earth's energy balance. Understanding these processes is key to addressing climate change.

The Natural Greenhouse Effect

Atmospheric Heat Trapping Mechanism

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• Greenhouse effect traps heat in Earth's atmosphere warming the surface and lower atmosphere
• Greenhouse gases (water vapor, carbon dioxide, methane, nitrous oxide) absorb and re-emit infrared radiation creating a blanket-like effect
• Solar radiation passes through the atmosphere absorbed by Earth's surface then emits infrared radiation partially trapped by greenhouse gases
• Balance between incoming solar radiation and outgoing infrared radiation determines Earth's energy balance and average temperature
• Without natural greenhouse effect Earth's average surface temperature would be approximately -18°C (0°F) instead of current 15°C (59°F)
• Greenhouse effect maintains habitable temperature range for life on Earth preventing excessive heat loss to space
  • Allows liquid water to exist on Earth's surface
  • Supports diverse ecosystems (tropical rainforests, coral reefs)

Historical Climate Variations

• Natural variations in greenhouse gas concentrations played significant role in past climate changes
  • Glacial-interglacial cycles over past 800,000 years
  • Rapid warming events (Paleocene-Eocene Thermal Maximum)
• Feedback mechanisms amplified or dampened climate changes
  • Ice-albedo feedback: more ice reflects more sunlight cooling Earth further
  • Carbon cycle feedback: warmer temperatures release more CO2 from oceans enhancing warming
• Proxy records provide evidence of past climate variations
  • Ice cores (trapped air bubbles)
  • Tree rings
  • Sediment cores

Anthropogenic Greenhouse Gases

Major Anthropogenic Greenhouse Gases

• Carbon dioxide (CO2) primary anthropogenic greenhouse gas
  • Produced by fossil fuel combustion (coal, oil, natural gas)
  • Deforestation reduces carbon sinks
  • Cement production releases CO2 during calcination process
• Methane (CH4) second most important anthropogenic greenhouse gas
  • Major sources include livestock (enteric fermentation)
  • Rice cultivation in flooded fields
  • Landfills (decomposition of organic waste)
  • Natural gas production and distribution (leaks)
• Nitrous oxide (N2O) produced by agricultural practices and industrial processes
  • Fertilizer use in agriculture
  • Soil management practices (tillage, crop rotation)
  • Industrial processes (nylon production, nitric acid manufacturing)
  • Fossil fuel combustion in vehicles and power plants

Synthetic Greenhouse Gases and Water Vapor

• Fluorinated gases synthetic compounds with high global warming potentials
  • Hydrofluorocarbons (HFCs) used in refrigeration and air conditioning
  • Perfluorocarbons (PFCs) used in electronics manufacturing and aluminum production
  • Sulfur hexafluoride (SF6) used in electrical insulation and magnesium production
• Water vapor most abundant greenhouse gas not considered direct anthropogenic greenhouse gas
  • Increases in response to warming caused by other greenhouse gases
  • Acts as a feedback mechanism amplifying warming

Atmospheric Lifetimes and Global Warming Potential

• Atmospheric lifetimes of greenhouse gases vary significantly
  • Methane approximately 12 years
  • Carbon dioxide variable ranging from decades to millennia
  • Some fluorinated gases persist for thousands of years
• Global warming potential (GWP) compares relative impact of greenhouse gases on climate change
  • CO2 reference gas with GWP of 1
  • Methane GWP of 28-36 over 100 years
  • Nitrous oxide GWP of 265-298 over 100 years
  • Some fluorinated gases have GWPs in the thousands or tens of thousands

Human Activities and Climate Change

Enhanced Greenhouse Effect

• Enhanced greenhouse effect amplifies natural greenhouse effect due to human-induced increases in atmospheric greenhouse gas concentrations
• Industrial activities since mid-18th century led to significant increase in atmospheric CO2 concentrations
  • Pre-industrial levels approximately 280 ppm
  • Current levels over 410 ppm (as of 2020)
  • Continuing to rise at rate of about 2-3 ppm per year
• Deforestation and land-use changes reduced Earth's natural carbon sinks
  • Tropical rainforest loss (Amazon, Congo Basin)
  • Conversion of grasslands to agriculture
  • Urban expansion reducing vegetated areas

Agricultural and Industrial Contributions

• Agricultural practices contribute to increased greenhouse gas emissions
  • Livestock farming major source of methane (cattle, sheep)
  • Rice cultivation in flooded fields produces methane
  • Fertilizer use boosts nitrous oxide levels in soil and atmosphere
• Industrial processes introduce new warming agents into atmosphere
  • Production and use of synthetic greenhouse gases (HFCs in refrigeration)
  • Cement production releases CO2 during clinker formation
  • Steel manufacturing produces CO2 during iron ore reduction

Urbanization and Land Surface Changes

• Urbanization affects local and regional climate patterns
  • Urban heat island effect increases temperatures in cities
  • Changes in land surface albedo alter reflectivity of Earth's surface
• Human-induced land surface changes impact climate
  • Deforestation alters local water cycles and reduces evaporative cooling
  • Agricultural expansion modifies surface energy balance
  • Draining of wetlands reduces natural carbon storage

Climate Feedback Mechanisms

• Feedback mechanisms can amplify enhanced greenhouse effect
  • Release of methane from thawing permafrost in Arctic regions
  • Reduced ice cover decreases albedo leading to more warming
  • Ocean acidification may reduce carbon uptake by marine ecosystems
• Potential for runaway warming scenarios if tipping points are reached
  • Collapse of West Antarctic Ice Sheet
  • Dieback of Amazon rainforest
  • Shutdown of Atlantic Meridional Overturning Circulation

Radiative Forcing and Earth's Energy Balance

Concept of Radiative Forcing

• Radiative forcing difference between incoming solar radiation absorbed by Earth and energy radiated back to space
• Measured in watts per square meter (W/m²)
• Positive radiative forcing leads to warming of Earth's surface
• Negative radiative forcing results in cooling of Earth's surface
• Greenhouse gases exert positive radiative forcing
  • Absorb and re-emit infrared radiation
  • Trap heat in lower atmosphere

Natural and Anthropogenic Forcing Agents

• Greenhouse gases primary anthropogenic forcing agents
  • CO2 largest contributor to positive radiative forcing
  • Methane and nitrous oxide also significant
• Aerosols can have both positive and negative radiative forcing effects
  • Sulfate aerosols reflect sunlight causing cooling
  • Black carbon absorbs sunlight causing warming
• Natural factors influence radiative forcing on various timescales
  • Changes in solar irradiance (11-year solar cycle)
  • Volcanic eruptions (release sulfur dioxide leading to temporary cooling)

Climate Sensitivity and Forcing Responses

• Effective radiative forcing accounts for rapid adjustments in atmosphere and land surface
  • Cloud formation and dissipation
  • Changes in atmospheric circulation patterns
• Climate sensitivity describes relationship between radiative forcing and resulting temperature change
  • Equilibrium climate sensitivity: long-term temperature response to doubling of CO2
  • Current estimates range from 1.5°C to 4.5°C for doubling of atmospheric CO2
• Transient climate response considers shorter-term temperature changes
  • Typically lower than equilibrium climate sensitivity
  • Accounts for ocean heat uptake delaying full warming effect