Topic 9.3 and 9.4: The Greenhouse Effect and Greenhouse Gas Increases

Essential Knowledge: The Greenhouse Effect is essential for maintaining the surface temperature necessary for life to exist on Earth.
Principal Greenhouse Gases (GHG): * Carbon Dioxide ( $CO_2$ ) * Methane ( $CH_4$ ) * Water Vapor ( $H_2O$ ) * Nitrous Oxide ( $N_2O$ ) * Chlorofluorocarbons (CFCs)
The Role of Water Vapor: While $H_2O$ vapor is a greenhouse gas, it does not contribute significantly to anthropogenically driven climate change because it has a very short residence time in the atmosphere.
Learning Intentions: To identify greenhouse gases, their specific sources, and their relative potency.

Solar Radiation Pathways: * Approximately $25\%$ is reflected back to space by clouds. * Approximately $20\%$ is absorbed by symbols in the atmosphere (clouds) and radiated back to space or toward Earth. * Approximately $50\%$ of solar radiation is absorbed by the Earth’s surface, warming it.
The Albedo Effect: * Low Albedo: Radiation is absorbed by the surface and converted to infrared radiation (heat). * High Albedo: Radiation is reflected back to space by snow and polar ice caps.
Three-Step Process of the Greenhouse Effect: 1. Solar rays (UV radiation and visible light) strike the Earth's surface and change into infrared radiation (heat). 2. The Earth's surface gives off this infrared radiation. 3. Greenhouse Gases (GHG) in the atmosphere absorb the infrared radiation and reflect it back toward Earth and into space, trapping heat.

Definition of GWP: The "potency" or measurement of how much a single molecule of a gas can contribute to the warming of the atmosphere over a $100$ -year period, relative to $CO_2$ .
GWP Comparisons: * Carbon Dioxide ( $CO_2$ ): Serves as the reference point/benchmark with a $GWP = 1$ . * Methane ( $CH_4$ ): Higher potency than $CO_2$ . * Nitrous Oxide ( $N_2O$ ): Higher potency than $CH_4$ . * CFCs: Possess the highest GWP of the major gases.
Factors Impacting GWP Impact: * Residence Time: How long the gas remains in the atmosphere. * Infrared Absorption: How effectively the gas absorbs and re-emits infrared radiation.

Carbon Dioxide ( $CO_2$ ): * GWP: $1$ . * Main Sources: Fossil fuel (FF) combustion, decomposition, and deforestation (which removes potential carbon sequestration).
Nitrous Oxide ( $N_2O$ ): * GWP: Approximately $300$ . * Principal Source: Management of agricultural soils, specifically denitrification of nitrate in overwatered and over-fertilized soils. (Note: It is distinct from $NO_x$ ).
CFCs / HCFCs / HFCs: * GWP: Ranges from $1,600$ to $13,000$ . * Sources: Refrigerants, coolants, and aerosols. * Special Note: Although many are being phased out or substituted, they have an exceptionally long residence time of $50$ to $500$ years.
Water Vapor ( $H_2O$ ): * Characteristics: It is condensable in the atmosphere. * Concentration: Determinant is temperature (the atmosphere holds more water at higher temperatures). * Anthropogenic Context: Not considered a major driver of the enhanced (human-caused) greenhouse effect because its concentration is determined by natural temperature cycles rather than direct human activity.

GWP: Ranges from $23$ to $84$ .
Anthropogenic Sources ( $60\%$ of total): * Fossil fuels: $85-105\,Tg/yr$ . * Domestic ruminants (cattle, sheep, goats): $85-95\,Tg/yr$ via enteric fermentation in the rumen and burping. * Waste decomposition/Landfills: $65-90\,Tg/yr$ . * Biomass Burning & Biofuels: $30-40\,Tg/yr$ . * Rice cultivation: $30-40\,Tg/yr$ .
Natural Sources ( $40\%$ of total): * Wetlands: $140-280\,Tg/yr$ (via anaerobic decomposition). * Termites: $30-75\,Tg/yr$ . * Freshwaters: $10-70\,Tg/yr$ . * Geological (including Oceans): $1-10\,Tg/yr$ . * Wild animals: $15\,Tg/yr$ . * Wildfires: $1-5\,Tg/yr$ . * Hydrates: $2-20\,Tg/yr$ . * Permafrost Thawing: Currently estimated at $0-1\,Tg/yr$ but represents a significant future threat.

Learning Intentions: To identify threats to human health and the environment posed by climate change.
Essential Knowledge: Global climate change leads to rising sea levels due to melting ice sheets and ocean water expansion, as well as the spread of disease vectors from the tropics toward the poles.
Media Coverage: Headlines from "The New York Times," "The Washington Post," and "The Guardian" highlight concerns regarding global warming, droughts, food prices, and extreme heatwaves.

Primary Causes: * Thermal Expansion: As water temperature increases, the space between $H_2O$ molecules increases, raising levels. * Melting Polar and Glacier Ice: Increased GHGs lead to warmer temperatures, melting ice sheets and adding liquid water to the oceans.
Environmental Impacts: * Coastal Ecosystem Flooding: Results in habitat loss and the destruction of storm buffers. This impacts commercial food sources and tourism/recreation economics. * Tundra Ecosystem Loss: Species like Polar bears, penguins, and Reindeer depend on frozen ecosystems. Adélie penguins, for instance, nest on ice and snow-free terrain; snowmelt and flooding drowning their eggs/chicks threaten survival. * Loss of Freeze-Thaw Cycle: Saltwater intrusion (salinization) affects freezing. Saline water freezes at a lower temperature, leading to longer periods of unfrozen conditions and altering the pace of ground thawing.

Infrastructure and Economy: * Property Damage: Every centimeter of rise increases damage, insurance fees, and costs for protective infrastructure. * Relocation: Coastal populations may be forced to relocate temporarily or permanently.
Saltwater Intrusion: Increased pressure forces saltwater into groundwater and wells, contaminating drinking water and negatively affecting agriculture, leading to inland refugee movement.
Disease Vectors: * Definition: Organisms that transmit disease from animal to human or human to human. * Examples: Malaria, Zika Virus, Dengue Fever, and Cholera. * Mechanism: Rising temperatures expand the range of these vectors into areas previously too cold for them, introducing high risks to new populations.

Definition: A legally binding international treaty on climate change signed by nearly every country.
Main Goal: To limit global warming to well below $2^\circ\text{C}$ above pre-industrial levels, with a secondary goal of limiting it to $1.5^\circ\text{C}$ .
Timeline: Adopted in December 2015; entered into force in November 2016.