9.3 - The Greenhouse Effect

Learning Objectives

I. Solar Radiation and Albedo

• Solar Radiation Fate: Not all solar radiation reaches Earth’s surface. About 25% of incoming radiation gets reflected right back out into space by the atmosphere and clouds. About 20% is absorbed by the atmosphere and clouds, then radiated back out to space or back down to Earth.

• Albedo: How much sunlight gets absorbed or reflected depends on a surface’s albedo.
• Low Albedo Surfaces (Dark): Things like dark, deep ocean waters, exposed soil, or blacktop absorb sunlight and then give off infrared radiation.
• High Albedo Surfaces (Light): Things like snow or polar ice caps reflect sunlight right back out into space.

II. The Greenhouse Effect Mechanism

• Definition: The greenhouse effect is the idea that gases in Earth’s atmosphere trap heat and radiate it back down to Earth.

• Importance: Without the natural greenhouse effect, there would be no life on Earth, because the planet would be too cold to support liquid water on much of its surface.
• Without greenhouse gases, Earth's average surface temperature would be about $-18^\circ \text{C}$ ($0^\circ \text{F}$). With them, it's about $15^\circ \text{C}$ ($59^\circ \text{F}$), so the natural greenhouse effect keeps the planet about $33^\circ \text{C}$ warmer.

• Energy Conversion:
1. Visible and UV light (solar radiation) comes in from the sun and strikes Earth’s surface.
2. That energy gets converted to infrared radiation, which humans experience as warmth or heat.
3. Greenhouse gases like $\text{CO}_2$ or methane absorb this infrared radiation and radiate a portion of it back down toward Earth.
4. The portion radiated back down to Earth is the greenhouse effect.

III. Major Greenhouse Gases and Sources

• Carbon Dioxide ($\text{CO}_2$): Probably the most critical, because it’s emitted in the greatest amount. Sources include fossil fuel combustion, decomposition of organic matter, and deforestation (since cut trees can’t sequester $\text{CO}_2$).

• Methane ($\text{CH}_4$): Sources include natural gas extraction and combustion, and anaerobic decomposition, meaning the breakdown of organic matter in low-oxygen conditions. That happens naturally in bogs and wetlands, and it’s a big contributor when permafrost thaws.
• Human sources: cattle and other grazing animals (microbes in their stomachs make methane, which they burp out), flooded rice paddies, landfills, leaks from natural gas wells and pipelines, and coal mines. Agriculture, fossil fuels, and waste are the three biggest human sources.

• Nitrous Oxide ($\text{N}_2\text{O}$): The biggest human source is agriculture. In the US, about three-quarters of $\text{N}_2\text{O}$ emissions come from managing agricultural soils (EPA). Specifically, adding nitrogen fertilizer gives soil bacteria extra nitrogen, and in over-fertilized, over-watered (anaerobic) soils, denitrification converts the excess nitrate into $\text{N}_2\text{O}$. Manure, burning fossil fuels, and some industrial processes add smaller amounts.

• CFCs (Chlorofluorocarbons): These also function as greenhouse gases.
• Sources: CFCs still leak out of old refrigerators, air conditioners, and foam insulation made before the phase-out. Their replacements, HCFCs and HFCs (9.2), are strong greenhouse gases too.

IV. Water Vapor ($\text{H}_2\text{O}$)

• Water is a greenhouse gas because it absorbs and emits infrared radiation.

• Water vapor is the most abundant greenhouse gas and causes about half of the natural greenhouse effect. But it doesn't drive global climate change, because it has a very short residence time: a water molecule stays in the air for only about 9 days before it condenses and falls as rain or snow.
• Water vapor is condensable, meaning the amount the air can hold is set by temperature (warmer air holds about 7% more water vapor per $1^\circ \text{C}$). Extra water vapor rains out within days, and as the atmosphere cools, water vapor condenses and falls back down to Earth. So humans can't change long-term water vapor levels directly.
• The other gases ($\text{CO}_2$, $\text{CH}_4$, $\text{N}_2\text{O}$) are non-condensable: they stay in the air for years to centuries whatever the temperature, so they set the temperature. Water vapor then follows the temperature, which makes it a positive feedback: warming from $\text{CO}_2$ lets the air hold more water vapor, which traps more heat.

V. Global Warming Potential (GWP)

• Definition: GWP measures how much heat a given mass (say 1 kg) of a gas traps over a 100-year period, compared with the same mass of $\text{CO}_2$.

• Baseline: GWP is always measured relative to $\text{CO}_2$, which is set equal to 1.

• Factors Determining GWP:
1. Residence Time: How long the gas stays in the atmosphere.
2. Infrared Radiation Absorption: How well the gas absorbs and re-emits infrared radiation back down to Earth.

1. Multiply the mass by the GWP: 2,000 t × 28 = 56,000 metric tons of CO2e.

2. Put it in context: a typical US passenger car emits about 4.6 metric tons of $\text{CO}_2$ a year (EPA), so 56,000 ÷ 4.6 ≈ 12,000 cars' worth of warming.

3. Why landfills capture and burn this gas: burning 2,000 t of methane makes only about 5,500 t of $\text{CO}_2$ (44 g of $\text{CO}_2$ per 16 g of $\text{CH}_4$), about a tenth of the warming of releasing it, and the energy can make electricity.