2.2 Atmospheric composition and greenhouse gases

Earth's atmosphere is a mixture of gases, each playing a distinct role in climate. Nitrogen and oxygen make up nearly all of it, but trace gases like carbon dioxide and methane punch far above their weight. These greenhouse gases trap heat, warming the planet and shaping climate patterns.

Greenhouse gases come from both natural and human sources. Burning fossil fuels releases $CO_2$, while agriculture produces methane. Some of these gases linger in the atmosphere for centuries; others break down in just years. Understanding these differences is central to figuring out how to address climate change.

Atmospheric Composition and Greenhouse Gases

Composition of Earth's atmosphere

The atmosphere is dominated by just two gases, with everything else making up roughly 1% or less. That remaining 1% matters enormously, though, because it includes the greenhouse gases that regulate Earth's temperature.

• Nitrogen ($N_2$): ~78% by volume. The most abundant atmospheric gas. It's relatively unreactive and doesn't play a direct role in the greenhouse effect.
• Oxygen ($O_2$): ~21% by volume. Essential for aerobic life, but also not a greenhouse gas.
• Argon ($Ar$): ~0.93% by volume. An inert noble gas with no climate impact.
• Water vapor ($H_2O$): Varies from about 0.1% to 4% by volume depending on location and weather. It's the most abundant greenhouse gas and a major driver of weather, but its concentration is controlled mainly by temperature rather than direct emissions (more on this below).
• Carbon dioxide ($CO_2$): ~0.042% by volume (about 420 ppm as of 2024). A small fraction, but rising steadily due to fossil fuel combustion and deforestation.
• Other trace gases: Neon, helium, methane ($CH_4$), krypton, and xenon each make up less than 0.002% by volume. Despite their tiny concentrations, gases like $CH_4$ have significant warming effects.

Role of greenhouse gases

Greenhouse gases are atmospheric gases that absorb and re-emit infrared (heat) radiation. The primary ones are water vapor ($H_2O$), carbon dioxide ($CO_2$), methane ($CH_4$), nitrous oxide ($N_2O$), and ozone ($O_3$).

Here's how the greenhouse effect works, step by step:

1. Sunlight (mostly visible light) passes through the atmosphere and warms Earth's surface.
2. The warmed surface emits energy back upward as infrared radiation.
3. Greenhouse gas molecules absorb some of that outgoing infrared radiation instead of letting it escape to space.
4. These molecules then re-emit the energy in all directions, including back down toward the surface.
5. This "trapping" of heat keeps the lower atmosphere and surface warmer than they would otherwise be.

Without any greenhouse gases, Earth's average surface temperature would be around $-18°C$ (roughly $0°F$) instead of the current $+15°C$ ($59°F$). So the natural greenhouse effect is essential for life.

The problem arises when human activities increase greenhouse gas concentrations beyond natural levels. This creates an enhanced greenhouse effect, which drives additional warming. That extra warming leads to rising sea levels, shifting weather patterns, and disrupted ecosystems.

Sources of greenhouse gases

Each major greenhouse gas has both natural and human (anthropogenic) sources.

Carbon dioxide ($CO_2$)

• Anthropogenic: Fossil fuel combustion (coal, oil, natural gas), deforestation, and cement production. Fossil fuels are by far the largest source.
• Natural: Respiration by organisms, volcanic eruptions, and decomposition of organic matter.

Methane ($CH_4$)

• Anthropogenic: Agriculture (especially livestock digestion and rice paddies), landfills, and leaks during fossil fuel extraction and transport (natural gas infrastructure is a major contributor).
• Natural: Wetlands (the largest natural source), termites, and wildfires.

Nitrous oxide ($N_2O$)

• Anthropogenic: Agricultural fertilizers (the dominant human source), industrial processes, and biomass burning.
• Natural: Microbial processes in soils and oceans.

Water vapor ($H_2O$)

• Anthropogenic: Irrigation and fossil fuel combustion contribute small amounts directly.
• Natural: Evaporation from oceans, lakes, and rivers, plus transpiration from plants.

Water vapor is a bit different from the other greenhouse gases. Its atmospheric concentration is primarily controlled by temperature: warmer air holds more moisture. This makes water vapor a powerful feedback rather than a direct driver. As $CO_2$ warms the planet, more water evaporates, which traps more heat, which causes further warming.

Ozone ($O_3$)

• Anthropogenic: Forms near the surface (ground-level ozone) through photochemical reactions involving nitrogen oxides and volatile organic compounds from vehicles and industry.
• Natural: Produced in the stratosphere by UV radiation interacting with $O_2$, and in small amounts by lightning.

Atmospheric lifetime of gases

Atmospheric lifetime is the average time a gas molecule stays in the atmosphere before being removed through chemical reactions, absorption by oceans or vegetation, photolysis (breakdown by sunlight), or deposition on Earth's surface.

Why does this matter? A gas with a long lifetime accumulates in the atmosphere even if emissions are relatively small each year. A gas with a short lifetime can still cause significant warming if it's emitted in large quantities continuously.

Some key lifetimes to know:

• $CO_2$: Centuries to millennia. There's no single removal process that clears it quickly. Some is absorbed by oceans and plants within decades, but a substantial fraction persists for thousands of years. This is why $CO_2$ reductions are so critical for long-term climate goals.
• $CH_4$: ~12 years. Much shorter-lived than $CO_2$, but molecule-for-molecule it traps roughly 80 times more heat over a 20-year period. Cutting methane emissions produces relatively fast climate benefits.
• $N_2O$: ~114 years. Long-lived and about 270 times more potent than $CO_2$ per molecule over 100 years.
• $H_2O$: ~9 days. Very short-lived, which is why its concentration is driven by temperature rather than by cumulative emissions.

The takeaway: reducing emissions of long-lived gases like $CO_2$ and $N_2O$ is essential for limiting warming over decades and centuries. Reducing short-lived but potent gases like $CH_4$ can slow warming more quickly in the near term. Effective climate strategy targets both.