Earth as a System: Energy, Matter, and Life · Lesson 5 of 13
Role of the Atmosphere
“The atmosphere is part sunscreen, part blanket, and entirely more organised than it looks.”
• Describe the main composition and layers of the atmosphere. • Explain how the atmosphere filters incoming radiation. • Explain the natural greenhouse effect. • Distinguish natural warming from enhanced warming. • Explain why the ozone layer is important. • Distinguish stratospheric ozone from ground-level ozone.
Atmosphere
The atmosphere is the layer of gases that surrounds the Earth and is held close to it by gravity.It is made mainly of nitrogen and oxygen, along with smaller amounts of gases such as argon, carbon dioxide and water vapor. The atmosphere provides the air we breathe, protects Earth from some harmful radiation, helps control temperature, and plays an important role in weather and the water cycle.
A planet without a useful atmosphere would be a much harsher place to live. During the day, its surface could be exposed to intense radiation from the Sun, while at night it could lose heat very quickly and become extremely cold. Earth’s atmosphere helps reduce these extreme changes and makes conditions more suitable for life.
The gases surrounding Earth do much more than simply provide oxygen for breathing. The atmosphere acts like a protective layer around the planet. It filters some harmful incoming radiation, helps distribute heat from warmer regions to cooler regions, and carries water vapour through the water cycle. It also slows the escape of heat from Earth’s surface by absorbing and re-emitting some of the infrared radiation that would otherwise be lost to space.
Because of these functions, the atmosphere plays an important role in keeping Earth’s temperature within a range that supports liquid water and life. It connects processes involving sunlight, heat, air and water, making it an essential part of Earth’s environment.
| Major gas | Approximate proportion in dry air | Importance |
|---|---|---|
| Nitrogen | 78 per cent | Reservoir for the nitrogen cycle |
| Oxygen | 21 per cent | Supports respiration and combustion |
| Other gases | About 1 per cent | Include argon, carbon dioxide and variable water vapour |
The troposphere extends from the surface to roughly 12 km, although its thickness varies. Most clouds and weather occur here, and temperature generally decreases with height at an average rate of about 6.5 °C per kilometre. Above it, the stratosphere extends to about 50 km. Ozone absorbs ultraviolet radiation there, warming part of this layer and protecting life below.
Above the stratosphere lie the mesosphere, thermosphere and exosphere. They have smaller direct roles in controlling surface climate, but together form the transition toward outer space. The commonly used boundary for the beginning of outer space is near 100 km above the surface.
Meet a Scientist
K. R. Ramanathan climbed to an altitude of about 18,000 feet in the Himalayas in 1934 to measure ozone levels and found them lower than expected. His observations helped build understanding of how ultraviolet absorption varies with altitude and pollution. He later led early work on seasonal-rain forecasting. Such atmospheric studies require calibrated instruments, repeated measurements and comparison across places and seasons.
The greenhouse effect is the warming produced when certain atmospheric gases absorb part of the outgoing infrared radiation and reradiate energy in different directions.
Visible sunlight passes through the atmosphere and warms the surface. The warm surface emits infrared radiation. Carbon dioxide, methane and water vapour absorb some of this outgoing energy and reradiate it, including back toward the surface. This natural process keeps the average surface warm enough for liquid water and life.
Problem
Why is the natural greenhouse effect necessary?
- 1.The surface receives short-wavelength solar radiation.
- 2.It emits part of that energy as infrared radiation.
- 3.Greenhouse gases delay the escape of some outgoing energy.
- 4.The lower atmosphere and surface remain warmer than they would be otherwise.
- 5.Without the natural effect, present forms of life would face a much colder planet.
Problem
What happens when greenhouse-gas concentration increases?
- 1.More outgoing infrared radiation can be absorbed.
- 2.The rate of energy escaping to space is altered until a new balance is reached.
- 3.The lower atmosphere and surface warm.
- 4.Warmer air and oceans influence rainfall, ice, sea level and ecosystems.
- 5.The natural process has been strengthened, rather than newly created.
Pause and Ponder
A nearby planet with a dense carbon-dioxide atmosphere is extremely hot. This comparison shows that greenhouse warming depends on atmospheric composition as well as distance from the Sun. Planetary temperature results from incoming energy, reflection and the ability of the atmosphere to slow outgoing heat.
Why is the Ozone Layer So Important?
The ozone layer is the ozone-rich region of the stratosphere that absorbs much of the Sun’s harmful ultraviolet radiation.
Chlorofluorocarbons, commonly called CFCs, were once widely used in refrigerators, air conditioners, aerosol sprays, and some industrial products. CFCs are very stable near Earth’s surface, but when they slowly reach the stratosphere, strong ultraviolet radiation can break them apart and release chlorine-containing substances.
These chlorine substances react with ozone and can destroy ozone molecules again and again. Because a single chlorine atom can take part in many such reactions, even a relatively small amount of CFCs can cause significant damage to the ozone layer over time. The effect becomes especially severe over the polar regions during certain seasons, where conditions favour rapid ozone destruction. This seasonal thinning is often referred to as the ozone hole.
The ozone layer is important because it absorbs much of the Sun’s harmful ultraviolet radiation before it reaches Earth’s surface. Recognising the danger, countries around the world agreed to reduce and gradually stop the use of major ozone-depleting chemicals. As a result of this international cooperation, the amount of these harmful substances in the atmosphere has been decreasing, and the ozone layer is showing signs of gradual recovery.
| Location | Role of ozone |
|---|---|
| Stratosphere | Absorbs harmful ultraviolet radiation and protects life |
| Near the ground | Acts as an air pollutant that can irritate lungs and damage plants |
Ozone depletion allows more ultraviolet radiation to reach the surface. Enhanced greenhouse warming concerns increased retention of outgoing infrared energy. The processes can interact with climate, but they are not the same problem.
Quiz
Which description best matches Greenhouse Effect?
Which description best matches Ozone Layer?
Which term matches this description: The greenhouse effect is the warming produced when certain atmospheric gases absorb part of the outgoing infrared radiation and reradiate energy in different directions.
Which term matches this description: The ozone layer is the ozone-rich region of the stratosphere that absorbs much of the Sun’s harmful ultraviolet radiation.
Which statement is a key takeaway from this lesson?
Practice Problems
- Explain why the troposphere is closely linked with weather.
- Trace solar energy through the natural greenhouse effect.
- Distinguish natural and enhanced greenhouse warming.
- Compare helpful stratospheric ozone with harmful ground-level ozone.
Key Takeaways
• The atmosphere filters incoming radiation and slows outgoing heat loss. • Weather occurs mainly in the troposphere. • Stratospheric ozone absorbs much ultraviolet radiation. • Greenhouse gases absorb outgoing infrared radiation. • Ozone depletion and enhanced greenhouse warming must not be confused.