The Atmosphere Story

One change, seen twice

The air you are breathing is roughly four fifths nitrogen and one fifth oxygen, with small proportions of other gases including carbon dioxide, water vapour and the noble gases. It has not always been like that, and the story of how it changed is what this topic is about. The early atmosphere is thought to have been very different: mostly carbon dioxide, released by intense volcanic activity, with almost no oxygen at all. Getting from there to here took billions of years and a small number of changes, and your specification wants you to be able to sequence them and, more importantly, to explain the mechanism of each rather than just name it. Here is the idea that makes the whole thing simpler than it looks. The two biggest changes are the same event seen twice. When algae and later plants appeared, they photosynthesised, and photosynthesis takes in carbon dioxide and gives out oxygen. So the oxygen going up and the carbon dioxide coming down are not two separate stories: they are one process described from two ends. There is a second route for the carbon dioxide as well, which ran alongside: it dissolved into the newly formed oceans, and from there ended up locked away in rocks and in the remains of living things. Hold on to that and four bullet points become one explanation.

Words for the air and the rocks

Six terms. Each one names a mechanism rather than a thing, which is how this topic is examined.

Put the stages in order

Put the stages in the evolution of the Earth's atmosphere into the order they happened.

• Intense volcanic activity releases gases, thought to be mostly carbon dioxide, along with water vapour
• The Earth cools, the water vapour condenses, and the oceans form
• Carbon dioxide begins dissolving into the newly formed oceans
• Algae appear and photosynthesise, and later plants do the same
• Over billions of years oxygen rises and carbon dioxide falls
• The atmosphere reaches roughly its present composition of about 80% nitrogen and 20% oxygen

What the first air was mostly

What is the Earth's early atmosphere thought to have consisted of mostly, and where did it come from?

• Mostly carbon dioxide, released by intense volcanic activity, with very little oxygen
• Mostly oxygen, released from the rocks as the Earth cooled
• Mostly nitrogen, in roughly the proportion found today
• There was no atmosphere at all until plants appeared

How each stage worked

Naming the stages earns little; explaining the mechanism of each is what the marks are for. Here is each one with its mechanism attached. The early atmosphere came from volcanoes. Intense volcanic activity released gases from inside the Earth, thought to be mostly carbon dioxide together with water vapour, so the atmosphere was formed from below rather than being there from the start. The oceans formed by condensation. As the Earth cooled, the water vapour in the atmosphere condensed into liquid water, which collected to form the oceans. That is a straightforward change of state, and saying so is the mechanism. The oceans then began removing carbon dioxide, because carbon dioxide dissolves in water. This is the first of the two CO2 routes, and it started before anything was alive. Photosynthesis did the rest, in both directions at once. Algae appeared first, and later plants, and photosynthesis takes in carbon dioxide and releases oxygen. Over billions of years this raised the oxygen and lowered the carbon dioxide together, which is why they are one process rather than two. And the carbon was locked away permanently. Carbon dioxide that had dissolved was taken up by sea creatures to build shells and skeletons, which formed sedimentary rocks; other carbon was locked into fossil fuels formed from the remains of living things. That is what "locked into rocks" actually means, and it is worth writing out, because an answer that says carbon dioxide "went into rocks" without saying how has named a stage rather than explained one.

Plot what the air is now

The grid shows two gases along the bottom: 1 is nitrogen and 2 is oxygen. Plot two points showing the approximate percentage of the atmosphere each one makes up today.

Which two removed the carbon dioxide

Select the TWO processes that reduced the amount of carbon dioxide in the atmosphere.

• Photosynthesis by algae and plants, which takes carbon dioxide in and gives oxygen out
• Carbon dioxide dissolving into the oceans, and later being locked into sedimentary rocks and fossil fuels
• Volcanic activity, which absorbed carbon dioxide back into the Earth
• Nitrogen replacing it directly in a chemical reaction

The story from the evidence

Go back far enough and the air came from _____ activity, venting gases from deep inside the planet. Once it cooled, the _____ vapour among those gases condensed and the oceans collected. Those oceans then started stripping carbon dioxide out of the air, because it can _____ in sea water. Chalk cliffs made of shells, and the coal we burn, are both places that carbon ended up. Oxygen only arrived once algae began _____. The air today is about 80% _____.

The first air and the air now

The two ends of the story, side by side. Everything in the middle is the mechanism that got from one column to the other.

Naming a stage is not explaining it

A question asks you to explain how the amount of oxygen in the atmosphere increased. Four marks. Here is the difference between an answer that names and one that explains. The naming version: "Plants appeared and made oxygen, so the amount of oxygen went up." Everything in that is true, and it would score very little, because it repeats the question with one extra word. The explaining version: "Algae appeared in the oceans first, and later plants on land. Both carry out photosynthesis, which takes in carbon dioxide and releases oxygen as a product. Because this happened continuously over billions of years, and because far more oxygen was released than was used up, the proportion of oxygen in the atmosphere gradually rose while the proportion of carbon dioxide gradually fell. Today oxygen makes up about a fifth of the atmosphere." Look at what the second version adds. It names WHAT appeared and in what order. It names the PROCESS and says what goes in and what comes out. It says WHY the change was gradual rather than sudden. And it connects the rise in oxygen to the fall in carbon dioxide, showing they are the same process. That last move is worth doing whenever this topic comes up, because a question about one of them can always be answered more fully by mentioning the other. The test to apply: does every sentence add a mechanism, or does it just restate the outcome in different words?

Match each stage to its mechanism

The answer that stops at the label

Four sentences from answers on this topic. Select the ONE that names a stage without explaining any mechanism.

• Water vapour condensed as the Earth cooled, and the liquid water collected to form the oceans.
• The carbon dioxide went into the rocks, so the amount in the air went down.
• Photosynthesis takes in carbon dioxide and releases oxygen, so it changed the atmosphere in both directions.
• Sea creatures used dissolved carbon to build shells and skeletons, which formed sedimentary rocks.

Atmosphere drill

Answer from memory. Each question asks for a mechanism or a proportion.

Explain how the atmosphere changed

Describe the Earth's atmosphere today, and explain how it changed from the early atmosphere to its present composition. Explain the mechanism of each stage rather than naming it.

• Give the approximate composition of the atmosphere today
• Describe what the early atmosphere is thought to have contained, and where it came from
• Explain how the oceans formed, and say what kind of change that was
• Explain how the oceans began to reduce the carbon dioxide
• Explain how photosynthesis increased the oxygen, naming what appeared first
• Explain why the same process also decreased the carbon dioxide
• Explain how carbon became locked into sedimentary rocks and fossil fuels
• Finish by putting the whole sequence in order