This article is about the Earth’s atmosphere. We will start by defining atmosphere and exploring why it is important. We will then find out about the Earth’s atmosphere millions of years ago before looking at how and why it has evolved over time. After that, we willl look at the Earth’s atmosphere today.

• This article is about the Earth's atmosphere.
• By the end of this article, you should be able to:
• Define the atmosphere.

• Explain the role of the atmosphere.

• Compare and contrast the atmosphere today with the atmosphere millions of years ago.

• Explain how and why the Earth’s atmosphere has changed.

What is the Earth’s atmosphere?

When we talk about air, we are actually just talking about the atmosphere. It starts at our feet and extends up to 10,000 kilometres above the Earth’s surface, where it merges with the solar winds.

Layers of the atmosphere

We can split the atmosphere up into different layers, each with a different height and composition of gases.This information is summarised in the table below:

Why is the atmosphere important?

Although the atmosphere doesn’t look like much, it plays a vital role in supporting life. In fact, without the atmosphere, we would not exist at all! Outer space is essentially one big vacuum, and this is what would surround us if our atmosphere vanished. There would be no noise - sound can’t travel in a vacuum - and the sky would look black, not blue. We wouldn’t be around to appreciate it, though. If we didn’t die rapidly from the differences in pressure outside and inside of our bodies, we would soon be killed by the lack of oxygen. Even if we did survive, we would have to deal with extreme sunburn from the Sun’s unfiltered radiation, plus extreme changes in temperature. Just take a look at the Moon, for example. It has almost no atmosphere. Because of this fact, the temperature fluctuates massively: it is a scorching 120℃ on the light side of the Moon but can reach below a chilly -200℃ in the dark!

In summary, the atmosphere:

• Protects us from solar radiation.
• Provides us with essential gases we need to survive.
• Regulates the temperature.
• It also provides us with weather and shields us from meteors and other bits of space junk en route to collide with our planet.

How did the Earth’s atmosphere form?

We now know that our atmosphere is essential to life. However, it hasn’t always been there and did not look the same as it does now. Let’s take some time to explore our atmosphere’s history.

The Earth's early atmosphere

The Earth formed about 4.6 billion years ago. At the beginning it had almost no atmosphere. But then volcanoes started to erupt, spewing out gases like carbon dioxide, water vapour and nitrogen. These formed the Earth’s early atmosphere and helped regulate our planet’s temperature.

This early atmosphere was stable for a long period of time. It contained mostly carbon dioxide, with small amounts of water vapour, nitrogen, ammonia, methane and sulfur dioxide. There was little or no oxygen. In fact, it was very similar to the atmospheres of planets like Venus and Mars.

The arrival of oxygen

As the Earth gradually cooled, water condensed to form the first oceans.These oceans were home to the first forms of life. Although there are many different theories about how life appeared on Earth, most agree that the first life forms were simple organisms found deep within the sea. They broke down chemicals like amino acids for energy and released methane and carbon dioxide.

But around 2.7 billion years ago, a different type of organism evolved deep in the oceans. These were a type of bacteria called cyanobacteria, or blue-green algae. They received their energy in a different way - by using sunlight to react water and carbon dioxide together. We now know this process as photosynthesis.

Fig. 1: A cyanobacterium. StudySmarter originals.

Photosynthesis releases oxygen. As oxygen levels in the atmosphere gradually increased, ozone started forming in the stratosphere. This ozone blocked out some of the harmful UV radiation from the Sun and made it possible for life to move out of the oceans and onto land. Plants colonised the Earth, increasing the proportion of oxygen in the atmosphere and decreasing the proportion of carbon dioxide through photosynthesis. Eventually, oxygen levels were high enough for animals to evolve. These animals converted the energy obtained from photosynthesis into a different form of energy that they could use. This involved a process called respiration, which returned carbon dioxide and water back into the atmosphere.

Composition of the Earth's atmosphere

What does our present day atmosphere look like? Well, the Earth’s atmosphere has been stable for the past 200 million years or so. It has reached an equilibrium. This means that the amount of carbon dioxide taken in from the atmosphere by plants for photosynthesis is equal to the amount released by animals in respiration. Likewise, the amount of oxygen released in photosynthesis is equal to the amount taken in for respiration. But how much of each gas do we actually have?

The most important gas in Earth's atmosphere

Well, first and foremost, the atmosphere contains a lot of nitrogen. Nitrogen molecules are fairly unreactive and so tend to hang around without reacting with anything else. In fact, our atmosphere is around 78% nitrogen.

Most importantly for life, the atmosphere contains around 21% oxygen. It also contains 0.9% argon and trace amounts of other gases, such as water vapour. Just 0.04% of our atmosphere is carbon dioxide. This is a stark contrast to the Earth’s early atmosphere!

Fig. 2: The composition of our atmosphere. StudySmarter Originals.

Where did all the carbon go?

You might remember from earlier in the article that the Earth’s early atmosphere contained a lot of carbon dioxide. In contrast, our atmosphere today only contains 0.04% carbon dioxide! Where has all this carbon dioxide disappeared to?

Dissolved in oceans

One way that levels of carbon dioxide in the atmosphere decreased was by dissolving in oceans. Once dissolved, it then reacted with metal oxides to create carbonate rocks.

Stored through photosynthesis

Another way the carbon dioxide levels in the atmosphere decreased was through photosynthesis. During photosynthesis, plants take in carbon dioxide and use the carbon to make new plant material, such as roots, shoots and leaves. When animals eat these plants, the carbon is transferred to them and some is used to make new animal tissues.

Normally when plants and animals die, their remains are decomposed and the carbon from their tissues returns to the atmosphere. However, if the conditions are anaerobic, meaning that there is no oxygen present, the dead plant and animal remains do not decompose. The remains are instead gradually covered by layers and layers of sediment. This puts lots of pressure on the organic remains. Over millions of years, the pressure and heat build up so much that the remains eventually turn into carbon-containing fossil fuels such as gas, coal and oil:

• Coal is formed when tree matter doesn't decompose, for example in boggy swamps.

• Oil and gas are formed when plants, animals and microscopic organisms in oceans don’t decompose.

In this way, lots of carbon is locked away.

Carbon-containing fuels are great sources of heat and energy. The problem is that humans are burning them extremely quickly, releasing all the stored carbon back into the atmosphere. This is increasing the amount of carbon dioxide in the atmosphere again. You’ll discover why this is a problem in the next article, “Greenhouse gases”.