6.1 Greenhouse effect

The greenhouse effect is a natural process that warms Earth's surface by trapping heat. Greenhouse gases like carbon dioxide and methane absorb infrared radiation, causing the planet to be warmer than it would be without them.

Human activities have significantly increased greenhouse gas concentrations since the Industrial Revolution. This enhanced greenhouse effect is leading to global warming, with wide-ranging impacts on climate patterns, ecosystems, and human societies.

Greenhouse gases

• Greenhouse gases are atmospheric gases that absorb and emit infrared radiation, trapping heat in the Earth's atmosphere
• The primary greenhouse gases include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and water vapor (H2O)
• Increased concentrations of greenhouse gases in the atmosphere lead to a stronger greenhouse effect and global warming

Major greenhouse gases

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• Carbon dioxide is the most significant anthropogenic greenhouse gas, primarily due to fossil fuel combustion and deforestation
• Methane is a potent greenhouse gas with sources including livestock, rice cultivation, landfills, and natural gas production
• Nitrous oxide is produced by agricultural practices (fertilizers), industrial processes, and fossil fuel combustion
• Water vapor is the most abundant greenhouse gas, but its atmospheric concentration is not directly affected by human activities

Sources of greenhouse gases

• Anthropogenic sources of greenhouse gases include burning fossil fuels, deforestation, cement production, and various agricultural practices
• Natural sources include volcanic eruptions, wildfires, and microbial processes in soils and oceans
• Human activities have significantly increased atmospheric concentrations of greenhouse gases since the Industrial Revolution

Atmospheric lifetimes

• Greenhouse gases have varying atmospheric lifetimes, which affect their long-term impact on the climate
• Carbon dioxide can remain in the atmosphere for hundreds to thousands of years
• Methane has a shorter atmospheric lifetime of about 12 years but has a higher global warming potential than CO2
• Nitrous oxide has an atmospheric lifetime of approximately 114 years

Greenhouse effect mechanism

• The greenhouse effect is a natural process that warms the Earth's surface and lower atmosphere
• Greenhouse gases in the atmosphere absorb and re-emit infrared radiation, trapping heat and causing the Earth to be warmer than it would be without these gases
• The greenhouse effect is essential for maintaining Earth's habitable temperature range

Solar radiation

• The Sun emits shortwave radiation, primarily in the visible and ultraviolet wavelengths
• About 30% of incoming solar radiation is reflected back to space by clouds, aerosols, and the Earth's surface
• The remaining 70% is absorbed by the Earth's surface and atmosphere, warming the planet

Infrared radiation

• The Earth's surface emits longwave infrared radiation, which has a longer wavelength than solar radiation
• Greenhouse gases absorb this infrared radiation and re-emit it in all directions, including back towards the Earth's surface
• This process traps heat in the lower atmosphere, causing the greenhouse effect

Radiative forcing

• Radiative forcing is the change in energy flux caused by a change in greenhouse gas concentrations or other climate-altering factors
• Positive radiative forcing indicates a warming effect, while negative radiative forcing indicates a cooling effect
• Anthropogenic greenhouse gas emissions have increased radiative forcing, leading to global warming

Positive feedback loops

• Positive feedback loops amplify the initial warming caused by the greenhouse effect
• Examples include the ice-albedo feedback (melting ice reduces Earth's reflectivity, leading to more absorption of solar radiation) and the water vapor feedback (warmer air holds more water vapor, a potent greenhouse gas)
• These feedback loops can accelerate warming and lead to more severe climate change impacts

Impacts of greenhouse effect

• The enhanced greenhouse effect, caused by increased atmospheric concentrations of greenhouse gases, leads to various environmental and societal impacts
• These impacts are primarily driven by the resulting global temperature rise and associated climate change
• The severity of these impacts depends on the magnitude and rate of greenhouse gas emissions and the Earth's climate sensitivity

Global temperature rise

• The Earth's average surface temperature has risen by approximately 1.1°C since the pre-industrial era, largely due to anthropogenic greenhouse gas emissions
• Continued emissions are projected to cause further warming, with the magnitude depending on future emission scenarios
• Global temperature rise leads to changes in weather patterns, sea level rise, and more frequent and intense extreme weather events

Climate change consequences

• Climate change caused by the enhanced greenhouse effect has wide-ranging consequences for natural systems and human societies
• These consequences include changes in precipitation patterns, more frequent and severe droughts and floods, and shifts in the distribution of plant and animal species
• Sea level rise, caused by thermal expansion of the oceans and melting of land-based ice, threatens coastal communities and infrastructure

Ecosystem disruptions

• Climate change alters the timing of seasonal events (phenology), affecting the interactions between species and their environment
• Shifts in species' ranges and changes in community composition can disrupt ecosystem functioning and services
• Ocean acidification, caused by the absorption of atmospheric CO2, negatively impacts marine organisms and ecosystems

Human health implications

• Climate change affects human health through various pathways, including heat stress, air pollution, and the spread of vector-borne diseases
• Extreme weather events can cause injury, displacement, and mental health impacts
• Changes in food production and water availability can lead to undernutrition and water-borne diseases

Anthropogenic contributions

• Human activities have significantly increased atmospheric concentrations of greenhouse gases since the Industrial Revolution
• The primary anthropogenic sources of greenhouse gases are fossil fuel combustion, deforestation and land use change, agricultural practices, and industrial processes
• Addressing these sources is crucial for mitigating climate change and its impacts

Fossil fuel combustion

• Burning fossil fuels (coal, oil, and natural gas) for energy production and transportation releases CO2 into the atmosphere
• Fossil fuel combustion is the largest source of anthropogenic greenhouse gas emissions
• Transitioning to low-carbon energy sources is essential for reducing emissions from this sector

Deforestation and land use

• Deforestation and land use change release stored carbon into the atmosphere and reduce the capacity of ecosystems to absorb CO2
• Tropical deforestation, often driven by agricultural expansion and logging, is a significant contributor to global greenhouse gas emissions
• Sustainable land management practices and reforestation efforts can help mitigate emissions from this sector

Agricultural practices

• Agricultural activities, such as livestock production and rice cultivation, emit methane and nitrous oxide
• The use of nitrogen-based fertilizers and the decomposition of agricultural waste also contribute to greenhouse gas emissions
• Adopting sustainable agricultural practices, such as improved manure management and precision farming, can help reduce emissions

Industrial processes

• Various industrial processes, such as cement production and the use of refrigerants, release greenhouse gases
• Cement production involves the calcination of limestone, which releases CO2, while some refrigerants have high global warming potentials
• Improving energy efficiency, adopting alternative materials, and phasing out potent greenhouse gases can help mitigate industrial emissions

Mitigation strategies

• Mitigation strategies aim to reduce greenhouse gas emissions and enhance carbon sinks to limit the magnitude of climate change
• A combination of technological, behavioral, and policy changes is necessary to achieve significant emission reductions
• Effective mitigation requires action across multiple sectors, including energy, transportation, land use, and industry

Renewable energy transition

• Transitioning from fossil fuels to renewable energy sources, such as solar, wind, and hydropower, can significantly reduce greenhouse gas emissions
• Renewable energy technologies have become increasingly cost-competitive and are being deployed at a growing scale
• Policies supporting renewable energy deployment, such as feed-in tariffs and renewable portfolio standards, can accelerate the transition

Carbon capture and storage

• Carbon capture and storage (CCS) technologies aim to capture CO2 emissions from large point sources, such as power plants and industrial facilities, and store them in geological formations
• CCS can help reduce emissions from sectors that are difficult to decarbonize, such as heavy industry and fossil fuel-based power generation
• Challenges for CCS include high costs, limited storage capacity, and potential leakage risks

Energy efficiency improvements

• Improving energy efficiency in buildings, transportation, and industry can reduce energy demand and associated greenhouse gas emissions
• Energy efficiency measures include building retrofits, fuel-efficient vehicles, and industrial process optimization
• Policies promoting energy efficiency, such as building codes and appliance standards, can drive adoption of efficient technologies

Sustainable land management

• Sustainable land management practices, such as reduced deforestation, reforestation, and improved agricultural practices, can enhance carbon sinks and reduce emissions
• Protecting and restoring natural ecosystems, such as forests and wetlands, can help sequester carbon and provide co-benefits for biodiversity and ecosystem services
• Policies supporting sustainable land management, such as payments for ecosystem services and land use planning, can incentivize adoption of these practices

International agreements

• International agreements and cooperation are essential for addressing the global challenge of climate change
• These agreements aim to establish shared goals, commitments, and mechanisms for reducing greenhouse gas emissions and adapting to climate impacts
• The effectiveness of international agreements depends on the level of participation, ambition, and implementation by participating countries

United Nations Framework Convention

• The United Nations Framework Convention on Climate Change (UNFCCC) is an international treaty established in 1992 to address climate change
• The UNFCCC provides a framework for international cooperation on climate change mitigation and adaptation
• The Convention recognizes the principle of "common but differentiated responsibilities," acknowledging the different capabilities and responsibilities of developed and developing countries

Kyoto Protocol

• The Kyoto Protocol, adopted in 1997 under the UNFCCC, set legally binding emission reduction targets for developed countries
• The Protocol introduced market-based mechanisms, such as emissions trading and the Clean Development Mechanism, to facilitate emission reductions
• The Kyoto Protocol had limited effectiveness due to the non-participation of some major emitters and the lack of emission reduction commitments for developing countries

Paris Agreement

• The Paris Agreement, adopted in 2015 under the UNFCCC, aims to strengthen the global response to climate change
• The Agreement sets a goal of limiting global temperature rise to well below 2°C above pre-industrial levels, with efforts to limit the increase to 1.5°C
• Countries submit Nationally Determined Contributions (NDCs) outlining their emission reduction targets and actions
• The Paris Agreement also includes provisions for climate finance, technology transfer, and capacity building to support developing countries

Challenges and limitations

• International climate agreements face various challenges and limitations, including:
  • Ensuring adequate participation and ambition from all countries
  • Balancing the needs and responsibilities of developed and developing countries
  • Addressing potential economic impacts and ensuring a just transition
  • Mobilizing sufficient financial resources for mitigation and adaptation actions
  • Monitoring and enforcing compliance with commitments
• Overcoming these challenges requires sustained political will, global cooperation, and innovative solutions

Greenhouse effect vs global warming

• The greenhouse effect and global warming are related but distinct concepts
• Understanding the differences between these concepts is important for effectively communicating about climate change and its causes
• Confusion between the two terms can lead to misconceptions about the natural greenhouse effect and the human-induced enhancement of this effect

Definitions and distinctions

• The greenhouse effect is a natural process by which greenhouse gases in the atmosphere absorb and re-emit infrared radiation, warming the Earth's surface and lower atmosphere
• Global warming refers to the observed increase in the Earth's average surface temperature over the past century, primarily due to human activities that enhance the greenhouse effect
• The natural greenhouse effect is essential for maintaining Earth's habitable temperature range, while global warming is the result of human-induced changes to the atmospheric composition

Relationship between concepts

• The greenhouse effect is the mechanism by which global warming occurs
• Anthropogenic emissions of greenhouse gases enhance the natural greenhouse effect, leading to global warming
• The magnitude of global warming depends on the concentration of greenhouse gases in the atmosphere and the Earth's climate sensitivity

Common misconceptions

• A common misconception is that the greenhouse effect is harmful or unnatural, when in fact, it is a necessary process for maintaining Earth's habitable climate
• Another misconception is that global warming is caused by the ozone hole or changes in the Sun's activity, rather than by anthropogenic greenhouse gas emissions
• Accurately distinguishing between the greenhouse effect and global warming is crucial for understanding the causes and consequences of climate change

Paleoclimatology and greenhouse effect

• Paleoclimatology is the study of past climate conditions and changes using various proxy records, such as ice cores, tree rings, and sediment layers
• Understanding past climate variations and their relationship to greenhouse gas concentrations provides valuable context for the current anthropogenic climate change
• Paleoclimate evidence demonstrates the strong influence of greenhouse gases on the Earth's climate over long timescales

Ice core records

• Ice cores from polar regions and glaciers contain trapped air bubbles that provide a record of past atmospheric composition, including greenhouse gas concentrations
• Ice core records show a strong correlation between past changes in atmospheric CO2 and global temperature
• The current atmospheric CO2 concentration is higher than at any time in at least the past 800,000 years, based on ice core evidence

Past climate variations

• Earth's climate has varied naturally over its history, with alternating periods of glacial (cold) and interglacial (warm) conditions
• Past climate variations have been driven by factors such as changes in the Earth's orbit (Milankovitch cycles), volcanic activity, and variations in solar output
• However, the current rapid warming and increase in greenhouse gas concentrations are primarily attributable to human activities, rather than natural factors

Implications for current warming

• Paleoclimate evidence demonstrates the sensitivity of the Earth's climate to changes in greenhouse gas concentrations
• The current anthropogenic increase in greenhouse gas concentrations is occurring at a much faster rate than past natural changes, leading to unprecedented warming
• Studying past climate variations helps improve our understanding of the Earth's climate system and the potential consequences of current and future warming
• Paleoclimate data also inform the development and validation of climate models used for projecting future climate change