2.2 Greenhouse effect and atmospheric absorption
4 min read•august 16, 2024
The greenhouse effect is crucial for Earth's energy balance, trapping heat to maintain a habitable temperature. Without it, our planet would be about 33°C colder. Greenhouse gases like CO2 absorb and re-emit longwave radiation, reducing heat escape to space.
Human activities have significantly increased greenhouse gas concentrations since the Industrial Revolution. This enhanced greenhouse effect leads to and climate change. Mitigation strategies aim to reduce emissions and limit temperature increases through renewable energy and improved land management.
The Greenhouse Effect and Earth's Energy Balance
Natural Process and Temperature Regulation
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- Greenhouse effect traps heat in Earth's atmosphere through certain gases warming the planet's surface and lower atmosphere
- Earth's energy balance maintains equilibrium between incoming solar radiation and outgoing terrestrial radiation influenced by the greenhouse effect
- Greenhouse gases absorb and re-emit longwave radiation reducing heat escape into space
- Without greenhouse effect, Earth's average surface temperature would be approximately 33°C colder making the planet inhospitable for most life forms
- Crucial role in maintaining Earth's habitable temperature range supports global water cycle
- Atmospheric window allows some to escape directly to space (wavelengths between 8-12 micrometers)
- Greenhouse gases primarily absorb radiation in specific wavelength bands (CO2 absorbs strongly around 15 micrometers)
Energy Balance Components and Radiative Forcing
- Solar radiation input averages about 340 W/m² at top of atmosphere
- Approximately 30% of incoming solar radiation reflected back to space (Earth's albedo)
- Remaining 70% absorbed by Earth's surface and atmosphere
- Earth emits longwave radiation to balance incoming solar energy
- Greenhouse gases increase downward longwave radiation causing net warming effect
- measures change in Earth's energy balance due to greenhouse gas increase
- Positive radiative forcing leads to warming, negative radiative forcing leads to cooling
- Current total anthropogenic radiative forcing estimated at 2.3 W/m² (primarily due to CO2)
Primary Greenhouse Gases and Their Sources
Major Greenhouse Gases and Their Properties
- (CO2) most significant greenhouse gas produced by , , and industrial processes
- (CH4) potent greenhouse gas with major sources including livestock (enteric fermentation), rice paddies, landfills, and natural gas production
- Nitrous oxide (N2O) produced by agricultural practices (fertilizer use), industrial activities, and burning of fossil fuels and biomass
- Water vapor (H2O) most abundant greenhouse gas concentration primarily controlled by hydrological cycle and temperature
- Chlorofluorocarbons (CFCs) and replacements (HCFCs and HFCs) synthetic greenhouse gases used in refrigeration and aerosols
- Ozone (O3) acts as greenhouse gas in troposphere formed by photochemical reactions involving pollutants (NOx and VOCs)
Atmospheric Lifetimes and Global Warming Potential
- CO2 atmospheric lifetime ranges from decades to millennia due to complex carbon cycle
- CH4 has relatively short atmospheric lifetime of about 12 years but high global warming potential
- N2O atmospheric lifetime approximately 114 years with significant long-term impact
- CFCs and HCFCs can persist in atmosphere for decades to centuries
- Global Warming Potential (GWP) compares warming effect of different gases relative to CO2
- Methane has GWP of 28-36 over 100 years, meaning it's 28-36 times more potent than CO2
- Nitrous oxide has GWP of 265-298 over 100 years
- Some HFCs have extremely high GWPs (thousands times more potent than CO2)
Atmospheric Absorption and Emission of Radiation
Molecular Interactions and Radiation
- Atmospheric gases selectively absorb and emit radiation based on molecular structure and wavelengths of incoming and outgoing radiation
- of greenhouse gases primarily covers infrared region where Earth emits most thermal radiation
- Greenhouse gas molecules vibrate when absorbing infrared radiation increasing kinetic energy and warming atmosphere
- Excited molecules re-emit radiation in all directions including back towards Earth's surface contributing to warming effect
- Vibrational modes of molecules determine specific absorption bands (CO2 bends and stretches)
- Rotational transitions in molecules also contribute to absorption and emission spectra
Radiative Transfer and Atmospheric Properties
- Different greenhouse gases have varying absorption spectra and radiative forcing potentials influencing effectiveness in trapping heat
- Concept of optical depth explains how concentration and vertical distribution of greenhouse gases affect absorption and emission properties
- Beer-Lambert law describes attenuation of radiation as it passes through atmosphere
- Atmospheric window allows certain wavelengths of infrared radiation to escape directly to space
- Lapse rate (change in temperature with altitude) influences vertical distribution of greenhouse gas absorption
- Saturation effects occur when increasing gas concentration no longer leads to proportional increase in absorption
Anthropogenic Impacts on the Greenhouse Effect
Human Activities and Greenhouse Gas Emissions
- Human activities significantly increased atmospheric concentrations of greenhouse gases (CO2, CH4, N2O) since Industrial Revolution
- Enhanced greenhouse effect caused by anthropogenic emissions leads to global warming and climate change
- Deforestation and land-use changes reduce Earth's capacity to absorb CO2 exacerbating accumulation of greenhouse gases in atmosphere
- Industrial processes (cement production) and agricultural practices (rice cultivation) contribute to emission of non-CO2 greenhouse gases with high global warming potentials
- Long atmospheric lifetimes of some greenhouse gases (CO2) result in delayed response to emission reductions and prolonged climate impacts
- Keeling Curve shows steady increase in atmospheric CO2 concentration since 1958
- Current CO2 levels exceed 410 , highest in at least 800,000 years
Feedback Mechanisms and Mitigation Strategies
- Positive feedback mechanisms amplify anthropogenic greenhouse effect (melting permafrost releasing methane)
- Ice-albedo feedback: melting ice reduces reflectivity, leading to more absorption and further warming
- Water vapor feedback: warmer atmosphere holds more water vapor, enhancing greenhouse effect
- Mitigation strategies aim to reduce greenhouse gas emissions and limit enhanced greenhouse effect
- Renewable energy adoption (solar, wind) reduces fossil fuel dependence
- Improved land management practices (reforestation, sustainable agriculture) enhance carbon sinks
- Carbon capture and storage technologies aim to remove CO2 from atmosphere or point sources
- International agreements (Paris Agreement) set targets for limiting global temperature increase
Key Terms to Review (18)
Absorption spectrum: An absorption spectrum is a spectrum that shows the wavelengths of light absorbed by a substance, typically observed as dark lines or bands against a continuous spectrum. This phenomenon occurs when specific wavelengths of light are absorbed by atoms or molecules in a medium, which can be directly linked to the greenhouse effect and atmospheric absorption as certain gases selectively absorb infrared radiation, influencing the Earth's energy balance.
Albedo effect: The albedo effect refers to the measure of reflectivity of a surface, specifically how much sunlight is reflected back into space rather than absorbed. This concept plays a crucial role in regulating temperatures across different climates, as surfaces with high albedo, like ice and snow, reflect more sunlight, while darker surfaces, such as forests or oceans, absorb more heat. Understanding this effect is key to grasping climate dynamics and the varying impacts of global warming in tropical, temperate, and polar regions.
Back radiation: Back radiation refers to the infrared radiation emitted by the Earth's surface and atmosphere that is absorbed and re-radiated back towards the surface. This process plays a crucial role in maintaining the Earth's temperature by trapping heat, contributing to the greenhouse effect and atmospheric absorption.
Carbon dioxide: Carbon dioxide (CO₂) is a colorless, odorless gas that is naturally present in the Earth's atmosphere as a trace gas and is a key component of the carbon cycle. It plays a significant role in the greenhouse effect by trapping heat in the atmosphere, contributing to global warming and climate change. CO₂ emissions result from various human activities, such as burning fossil fuels and deforestation, and they influence climate models and projections about future climate scenarios.
Climate feedback loops: Climate feedback loops are processes that can amplify or dampen the effects of climate change, creating a cycle where the initial change can lead to further changes in the climate system. These loops are crucial in understanding how different components of the Earth's climate interact, particularly through mechanisms like the greenhouse effect and atmospheric absorption, which contribute to the warming of the planet and influence future climate dynamics.
Climate Sensitivity: Climate sensitivity refers to the measure of how much the Earth’s average temperature is expected to increase in response to a doubling of carbon dioxide concentrations in the atmosphere. It is a critical concept that helps us understand the potential impacts of greenhouse gas emissions on global temperatures and climate systems, as well as the resulting radiative forcing effects and feedback mechanisms involved in climate change.
Deforestation: Deforestation is the large-scale removal of trees from forested areas, often resulting in damage to the quality of the land. This process affects not only local ecosystems but also has significant implications for global climate, biodiversity, and carbon storage. The removal of trees disrupts biomes and vegetation distribution, contributes to greenhouse gas emissions, influences ecosystem structure and function, alters albedo effects, and impacts the greenhouse effect and atmospheric absorption.
Energy flux: Energy flux refers to the rate at which energy is transferred through a surface per unit area, typically expressed in watts per square meter (W/m²). This concept is crucial in understanding how energy is absorbed, reflected, and emitted by the Earth's surface and atmosphere, which directly influences temperature and climate. Energy flux plays a significant role in processes like the albedo effect and the greenhouse effect, affecting how much solar energy is retained or lost in the environment.
Fossil fuel combustion: Fossil fuel combustion is the process of burning fossil fuels, such as coal, oil, and natural gas, to produce energy. This process releases carbon dioxide (CO2) and other greenhouse gases into the atmosphere, contributing to global warming and climate change by enhancing the greenhouse effect. The emissions from fossil fuel combustion are significant drivers of radiative forcing, which affects the balance of energy in the Earth's atmosphere.
Global warming: Global warming refers to the long-term increase in Earth's average surface temperature due to human activities, particularly the release of greenhouse gases into the atmosphere. This rise in temperature influences various climate phenomena and is linked to changes in weather patterns, sea level rise, and impacts on ecosystems and biodiversity.
Infrared radiation: Infrared radiation is a type of electromagnetic radiation with wavelengths longer than visible light, typically ranging from about 700 nanometers to 1 millimeter. It plays a crucial role in the Earth's energy balance by being emitted from the Earth's surface and absorbed by greenhouse gases in the atmosphere, contributing to the warming of the planet.
James Hansen: James Hansen is a prominent American climatologist and former director of NASA's Goddard Institute for Space Studies, widely known for his research on climate change and its impacts. His work has significantly advanced the understanding of global warming, making connections between human activities, greenhouse gas emissions, and the climate system's response.
Methane: Methane is a potent greenhouse gas with the chemical formula CH₄, consisting of one carbon atom bonded to four hydrogen atoms. As a major contributor to climate change, methane has a significantly higher warming potential than carbon dioxide over a short timescale, making it critical in discussions about greenhouse gas emissions and their impact on radiative forcing. Its sources include natural processes and human activities, leading to its consideration in climate models and the understanding of atmospheric absorption dynamics.
Ppm: ppm stands for 'parts per million,' a unit of measurement commonly used to describe the concentration of substances in a mixture or solution. This term is particularly relevant in discussions around greenhouse gases, as it quantifies the amount of these gases present in the atmosphere, influencing radiative forcing and the overall greenhouse effect. Understanding ppm helps to gauge the impact of emissions on climate change and environmental health.
Radiative forcing: Radiative forcing is a measure of the influence that factors such as greenhouse gases, aerosols, and changes in land use have on the energy balance of the Earth’s atmosphere. It reflects the change in energy fluxes at the top of the atmosphere, which can either warm or cool the planet, ultimately affecting climate patterns. This concept connects to atmospheric moisture, temperature trends, climate models, energy balance, and the interactions among various components of the climate system.
Scenario modeling: Scenario modeling is a technique used to predict and analyze the potential impacts of different variables on future outcomes, particularly in relation to climate change and environmental conditions. By creating various scenarios based on different assumptions about greenhouse gas emissions, technological advancements, and societal responses, this method helps researchers and policymakers understand how changes in atmospheric absorption and the greenhouse effect could influence global warming and climate patterns.
Svante Arrhenius: Svante Arrhenius was a Swedish scientist who is best known for his pioneering work in physical chemistry and his early contributions to climate science, particularly the greenhouse effect. He formulated a mathematical equation to describe how increased carbon dioxide levels in the atmosphere could lead to global warming, connecting the processes of solar radiation and Earth's energy budget with the mechanisms of atmospheric absorption and the greenhouse effect.
Temperature anomaly: A temperature anomaly refers to the difference between the observed temperature and a long-term average temperature for a specific region or time period. It is a crucial indicator used to understand how temperatures are changing over time, highlighting variations that may indicate broader climatic trends. By assessing temperature anomalies, scientists can analyze patterns of warming or cooling in relation to natural and anthropogenic factors affecting the climate system.