The Atmosphere on Earth

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  • 0:01 Earth
  • 0:44 Earth's Ancient Atmosphere
  • 3:05 The Ozone Layer
  • 4:22 Humans & the Ozone Layer
  • 5:15 Humans & Carbon Dioxide
  • 7:31 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson describes the evolution of the Earth's atmosphere, from its ancient past to the present day to how it's being changed by humans today. You'll learn about how outgassing, photosynthesis, and oceans all played a role in its formation, what air is composed of, and the real effect of human pollution on our changing atmosphere.


Our beautiful and homey planet, Earth, is very different today compared to its ancient past. Four billion years ago, when the Earth was forming, you wouldn't have been able to breathe, you would have fried from the dangerous radiation coming from the sun, and you would have ducked for cover left and right as furious volcanoes erupted all over the place.

That was the nature of Earth back then: inhospitable and deadly. It is much different from the beautiful, life-giving planet we call home today. How did we get from one to the other? How do we as humans impact the evolution of Earth's atmosphere today? This lesson will teach you that and more.

Earth's Ancient Atmosphere

Billions of years ago, Earth's atmosphere was likely filled with rich amounts of carbon dioxide, nitrogen, and water vapor that was outgassed, or released, from the Earth's interior as a result of high levels of volcanic activity. Over time, things, thankfully, changed.

Like the bubbles in a soft drink prove, carbon dioxide can easily dissolve in water. When carbon dioxide combined with things like calcium in our oceans to form limestone, it helped remove its dangerous and excessive levels from the ancient atmosphere on Earth.

Meanwhile, about 2-2.5 billion years ago, photosynthetic plants had appeared in numbers large enough to produce oxygen at a rate faster than it was being removed by chemical reactions in the ancient Earth. As a point of clarity, photosynthesis is a process that produces energy for plants by absorbing carbon dioxide and releasing oxygen.

What I just described should make it evident to you that life on Earth doesn't exist because of oxygen. Most of life on Earth doesn't need oxygen. Our current levels of oxygen came to be mainly because of life itself.

Nowadays, air, a mixture of gases making up Earth's atmosphere, consists of 78% nitrogen, 21% oxygen, and 1% of gases, like argon and carbon dioxide. Of course, other things, like water vapor, dust, industrial pollutants, and even microbes are found in the air as well.

With every step you take and every move you make, you need to breathe in air to power your body, but with every breath you take, you might be breathing in a deadly virus. It's a tradeoff; one that's well worth the risk though.

Half of the air I described is found in the first four miles above the Earth's surface, and the total mass of the Earth's atmosphere is 5,000 trillion tons. That sounds like a lot, but it's actually only about one-millionth the mass of Earth itself.

It is Earth's force of gravity that keeps this atmosphere from floating away into space, although some atoms do escape into space at the top of our atmosphere.

The Ozone Layer

Prior to enough oxygen (O2) accumulation in the Earth's atmosphere, an ozone layer could not form. The ozone layer is a protective layer of oxygen ions (O3) lying 15-30 km, which is about 9-19 miles above the Earth's surface. Before the ozone layer, the sun's ultraviolet radiation - the stuff that gives you skin cancer and harms plants we need - could easily pass through the Earth's atmosphere.

But as the Earth's atmosphere evolved and oxygen (O2) levels increased within it, these oxygen molecules were split by the sun's UV rays, like a sword slicing through a rope connected to a ball at either end. This process formed single oxygen atoms. When one oxygen atom combined with the remaining oxygen molecules, simple math dictates you get O3, which is ozone. These ozone molecules are really good at absorbing the dangerous UV rays coming in from space. Basically, the pale blue ozone layer is a force shield that blocks bullets - that is to say UV rays - coming from space that are trying to hurt us and life on Earth in general.

Humans and the Ozone Layer

Some of this protective ozone, which also protects plants vital to our survival, is being destroyed by things like chlorofluorocarbons released by aerosol sprays, air conditioners, and refrigerators. This, and other chemicals, revert the ozone back into ordinary oxygen (O2) molecules, which doesn't block UV radiation. This means UV radiation increases at the Earth's surface and harms life, including yourself, as a result.

But don't get the ozone protecting us way up high confused with the ozone produced as a result of human industrial and automotive pollution! Yes, breathing in ozone produced by these methods is harmful to our health. It can cause irreversible lung damage! We do need ozone though; except, we need it higher up, where it helps us, not hurts us.

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