8.3 Earth's Atmosphere

The Earth’s atmosphere can be thought of as an “ocean of air” enveloping the planet, exerting pressure at sea level defined as 1 bar (related to the term barometer).
At sea level, each square centimeter of Earth’s surface experiences the weight equivalent to 1.03 kilograms pressing down.
Humans have adapted to live at this pressure; significant deviations in pressure (either lower or higher) hinder human function effectively.
The total mass of the Earth's atmosphere is approximately 5 × 10^{18} kilograms, which is only about a millionth of the total mass of Earth.

Most of the atmosphere is concentrated near the Earth's surface, specifically within the bottom 10 kilometers where clouds form and commercial airplanes fly.
This region is known as the troposphere:
- It features a dynamic process where warm air, heated by the Earth’s surface, rises and is replaced by descending cooler air, thereby exemplifying convection.
- This circulation leads to the generation of clouds and wind.
- The temperature within the troposphere decreases rapidly with height, reaching nearly -50 °C at its upper boundary where the stratosphere begins.
The stratosphere extends from the upper troposphere up to about 50 kilometers above the surface:
- It is predominantly cold and cloud-free.
- Near the top of the stratosphere, a layer of ozone (O₃) exists, which absorbs harmful ultraviolet light from the sun, thereby protecting life on Earth.

Ozone depletion has been a significant concern since evidence emerged in the 1980s demonstrating that human activities were destroying atmospheric ozone.
Phasing out the production of industrial chemicals causing this depletion, primarily chlorofluorocarbons (CFCs), has seen a gradual recovery of the ozone layer, with the “ozone hole” over Antarctica diminishing as a result of international agreements.

At heights above 100 kilometers, the atmosphere thins significantly, allowing satellites to pass through with minimal friction.
This region, referred to as the ionosphere, is characterized by ionized atoms resulting from ultraviolet radiation stripping electrons from atoms.
There is a continuous, slow leak of the atmosphere into space, primarily affecting lighter gases like hydrogen and helium, which Earth cannot retain due to their lighter weight.

The primary components of Earth’s atmosphere at the surface include:
- 78% nitrogen (N₂)
- 21% oxygen (O₂)
- 1% argon (Ar)
- Traces of water vapor (H₂O), carbon dioxide (CO₂), and other gases.
Variable amounts of dust and water droplets are also present in the atmosphere.
Volatile materials are substances that evaporate at relatively low temperatures.
If the Earth's temperature increased past 100 °C, oceans would boil, dramatically altering atmospheric composition with water vapor.
The atmospheric pressure of the boiled water would remain around 300 bars, indicating a significantly water-dominated atmosphere.
Sedimentary carbonate rocks could release approximately 70 bars of CO₂ when heated, far surpassing current CO₂ levels of only 0.0005 bars.

There is substantial evidence demonstrating that Earth’s atmospheric composition has fluctuated throughout its history, influenced by factors such as volcanic activity and early life forms.
Possible sources of Earth’s original atmosphere and oceans may include:
1. Accumulation from debris left over from the solar system formation.
2. Release from the Earth’s interior via volcanic activity post-formation.
3. Impacts from comets and asteroids from outer space.
- Current evidence supports a combination of internal volcanic releases and cosmic impacts.

Weather refers to the short-term atmospheric conditions and their circulation patterns, heavily influenced by sunlight and planet rotation.
Climate describes long-term atmospheric effects, affecting conditions over decades and centuries,
- Changes in climate can be subtle yet impactful.
- A notable saying captures the distinction: “Climate is what you expect; weather is what you get.”
Variations in temperature and rainfall significantly impact agriculture, with minor temperature changes leading to drastic reductions in crop yields.
Historically, documented changes in Earth’s climate include periods of ice ages, with significant reductions in temperature, particularly affecting the Northern Hemisphere over the past half million years.
- The last ice age ended roughly 14,000 years ago, having lasted about 20,000 years, peaking with ice thickness nearly 2 kilometers over regions like Boston and extending to New York City.
- Ice ages result mainly from changes in Earth's axial tilt due to gravitational interactions.