Lesson video

This lesson is called Evolution of Earth's Atmosphere, and is from the unit Atmosphere and Changing Climate.

Hi there.

My name's Mrs. McCready, and I'm here to guide you through today's lesson.

So thank you very much for joining me today.

In our lesson today, we're going to describe how Earth's atmosphere developed over billions of years.

Now, we're gonna come across a number of keywords in our lesson today, and they're listed up here on the screen for you.

Now, you may wish to pause the video and make a note of them, but I will introduce them to you as we come across them.

So in our lesson today, we're going to first of all consider the Earth's early atmosphere before we consider how that has changed over time.

So are you ready to go? I certainly am.

Let's get started.

The Earth formed approximately 4.

6 billion years ago, and one scientific theory is that for the first billion years or so, the earth was very hot with intense volcanic activity.

Volcanoes released vast quantities of gases, and over the course of that billion years formed the Earth's atmosphere.

So it went from being just a very hot, rocky ball to being an atmosphere-covered hot, rocky ball.

Now, the atmosphere at that time would have been very different from the atmosphere that we know today.

It is likely that the atmosphere contained large amounts of carbon dioxide plus nitrogen, ammonia, methane and water vapour.

Now, there's no ammonia in the atmosphere nowadays, and the quantities of the other substances have changed dramatically over the course of the last 4 billion years.

It's also very likely that there was little or no oxygen present in the atmosphere, unlike today.

So how old approximately is the earth? 100 million years, 4.

6 billion years or 2.

5 billion years? I'll give you five seconds to think about it.

So how old is the earth? 4.

6 billion years old.

Well done if you got that correct.

And which gas was the most abundant in the Earth's early atmosphere? Oxygen, methane, or carbon dioxide? What do you think? So you should have said carbon dioxide.

Well done.

Now, the Earth's early atmosphere is thought to have been very, very different to Earth's current atmosphere, but much more similar to the atmospheres that we see on Mars and Venus today.

So if we look at the composition of the Martian and the Venusian atmospheres, we can see that by a significant margin, carbon dioxide makes up the vast majority of the atmosphere.

So carbon dioxide is present in about 95, 96% on Mars and Venus.

Then everything else is in much smaller quantities obviously.

Nitrogen, 2.

7 or 4%, argon, 1.

6 or a trace, a trace of oxygen and a trace of other gases as well.

And it is thought that this is much more like what the Earth's early atmosphere was when it was initially formed by that volcanic activity right at the beginning of the solar system.

Now, obviously that atmosphere is extremely different from how it is now on Earth.

So could humans survive on Mars and Venus in that atmosphere with all else being the same? What do you think? Well, if all else was the same, humans would require an enormous amount of support to be able to survive in atmospheres like that on Mars and Venus.

Now, we don't know for certain what the Earth's atmosphere was like all those billions of years ago.

We can collect evidence to help us come to those conclusions, such as gases trapped in meteorites or released during volcanic eruptions or those gases contained within the Earth's oceans or rocks from other planets or other parts of Earth.

And if we can find very, very old rocks that were formed in the first few million years of Earth's formation, then it gives us an indication as to what the atmosphere may well have been like.

So rocks from 4.

3 billion years ago have been found, and they have helped us to suggest what the atmosphere was like because of the gases which were trapped within the rock or because of the composition of the rock itself.

But even with all of those potential evidence sources, we cannot be 100% sure of what the atmosphere was like 4.

6 billion years ago because the rock cycle does a great job of recycling rock.

So that turns sedimentary, which is rock which has been formed over settling periods, over very long periods of time through extreme heat and pressure into metamorphic rock and then into igneous rock as part of the volcanic process.

And so that process continues around in a cycle.

And this is a very efficient way of recycling rock and it's happened over the entire course of the earth's life.

And so many of the rocks that were there present at the beginning of the Earth's formation are simply not there anymore because they've been recycled through the rock cycle.

Also, 4.

6 billion years is a very, very long timescale.

And so there is only very limited evidence available to us.

Plus of course, there are no records dating back to that time because there was no life on earth to make those records in whatever form that might be.

So we only have very limited sources for our evidence, but we can still draw some fairly robust conclusions that we could be confident in from these various sources.

So true or false? Scientists have analysed the air trapped in ancient rocks to gather evidence on what the Earth's atmosphere was like billions of years ago.

True or false? Okay, that was true.

But can you justify your answer? Is it because air samples reveal that the atmosphere contained large amounts of carbon dioxide, or is it because records taken 4 billion years ago reveal that the atmosphere contained low levels of oxygen? What do you think? So you should have said that A is correct.

The air samples reveal the atmosphere composition.

Well done.

And why can scientists not say for certain what the composition of the earth early atmosphere was? Is it because data records have been lost or because some ancient rocks have been destroyed by the rock cycle or because scientists from this period have died or it is too long ago and there are no records? I'll give you five seconds to consider.

Okay.

So you should have said that it is because some of the ancient rocks have been destroyed by the rock cycle and it is too long ago and there are no records.

Well done.

So what I'd like you to do is to consider the table.

It shows the percentage composition of gases in the atmosphere on Mars and Earth today.

And I'd like you to use that information to describe how the composition of the Earth's atmosphere has changed over time, bearing in mind that the early earth atmosphere is likely to be very similar to the current Martian atmosphere.

So pause the video and come back to me when you are ready.

Okay, let's see what you have concluded.

So I asked you to use the information in the table to describe how the composition of the Earth's atmosphere has changed over time.

So you should have said that the early Earth's atmosphere was very similar to the atmosphere as of Mars today.

And the amount of carbon dioxide in the atmosphere has decreased from more than 90% to only naught point naught 4%.

The amount of nitrogen in the atmosphere has increased.

The amount of argon in the atmosphere is similar to the levels 4 billion years ago, and there is only naught 0.

13% oxygen in the Martian atmosphere, but the level of oxygen in Earth's atmosphere has increased from almost nothing to about 21% today.

Did you make all of those points? Well done if you did.

Okay, now let's look at how the atmosphere has changed over that time period.

So scientists can go and get a lot of information from volcanic eruptions.

They gather evidence from the eruptions as to how the earth may well have functioned many billions of years ago and how that therefore might have impacted the early atmosphere.

So when the Hunga Tonga-Hunga Ha'apai volcano erupted in 2022, it sent out a massive plume of water vapour into the atmosphere.

And the amount of water in that plume is estimated to be more than enough to fill 58,000 Olympic-sized swimming pools.

Really, an incredible amount of water was ejected into the atmosphere by that volcanic eruption.

Now, water vapour released during the volcanic eruptions in the early earth will have introduced water into the early earth atmosphere.

And over time, over about 1 billion years, the earth gradually cooled.

And as the earth cooled and the surface temperature reduced, the water in the atmosphere will have been able to condense from water vapour into liquid water.

So once the temperature reduced, decreased to below 100 degrees centigrade, that state change from gaseous water to liquid water would've been able to happen.

And as water vapour converted into water, it will have rained down onto the surface of the earth and over time built up to form oceans.

So let's consider this planet, Planet Z.

The surface temperature on Planet Z is 285 degrees centigrade.

So why does Planet Z contain no oceans? Is it because Planet Z does not contain enough water vapour to form an ocean? Is it because Planet Z has no volcanic activity to produce water vapour? Or is it because Planet Z is too hot because water boils at 100 degrees centigrade? I'll give you five seconds to decide.

Okay, so you should have concluded that the reason Planet Z has no oceans is because it is too hot.

Well done.

Now, over several billion years, the amount of oxygen gas in the atmosphere has increased from just a trace to 21%.

So about 3 billion years ago, organisms called algae evolved and started to introduce oxygen into the atmosphere.

And over the next billion years or so, plants evolved and collectively they increased the levels of oxygen in the atmosphere to levels where animals could evolve and survive on earth.

Now, algae are a really diverse range of organisms that can photosynthesize, and most of them are aquatic, so found in watery ecosystems. Now, these organisms photosynthesize, and photosynthesis involves the combination of carbon dioxide and water to form glucose and oxygen.

Now, algae aren't the only organisms on earth which can photosynthesize.

Of course all plants can photosynthesize as well, And also a subsection of algae called phytoplankton.

Now, phytoplankton are found in the oceans, and they are responsible for about half of all of the photosynthesis that takes place on modern earth.

So phytoplankton along with other forms of algae and all the plants on the earth are producing oxygen into the atmosphere.

And specifically, the phytoplankton are estimated to have produced about 50% of the oxygen that is present in the atmosphere now.

So phytoplankton are extremely important organisms within the Earth's ecosystem.

Not only are they important because of the amount of oxygen that they produce, but they're also an essential source of food in for many animals because they form the base of many aquatic food webs.

So in this food web, for instance, we can see that algae are the food source for shrimp which are eaten by cod, which are eaten by seals, which are eaten by polar bears.

So we can see how if it weren't for the allee at the start of that food chain, the polar bears, seals, cod, and shrimp would all find it much harder to survive.

So photosynthetic organisms are essential for life on earth, not only because they are producers and they form the beginning of all food webs, but also because of the vast amount of oxygen that they provide, which is essential for all living organisms to survive.

So true or false? Algae evolved and increased the level of carbon dioxide through photosynthesis.

So you should have said that that is false, but can you justify why? Is it because algae evolved and increased the level of carbon dioxide through respiration? Or is it because algae evolved and decreased the level of carbon dioxide through photosynthesis? So you should have said that answer B is correct because the levels of carbon dioxide decreased due to photosynthesis.

Well done.

And which option correctly shows the equation for photosynthesis? I'll give you five seconds to consider.

Okay, so you should have said that photosynthesis is best represented by A, carbon dioxide plus water leads to glucose plus oxygen.

Well done.

Now, the level of carbon dioxide in the atmosphere has massively decreased over time.

It's gone from more than 90% in the early earth atmosphere to just about naught point naught 4% in Earth's atmosphere today.

And this is almost entirely to do with the process of photosynthesis.

Photosynthesis in algae and plants removes carbon dioxide and releases oxygen into the atmosphere.

So not only is photosynthesis responsible for increasing the amount of oxygen within the atmosphere, it is also responsible for capturing carbon out of the atmosphere, removing that carbon dioxide and binding it into the organisms themselves.

And this process, this removal of carbon from the atmosphere is why organisms are carbon sinks.

So the percentage of carbon dioxide in the atmosphere today is higher than in Earth's early atmosphere, lower than in Earth's early atmosphere, or the same as in Earth's early atmosphere? I'll give you five seconds to decide.

So you should have said that it is lower than in Earth's early atmosphere.

Well done.

Now, a carbon sink is either a natural or an artificial system that takes out more carbon from the atmosphere than it releases, which is why it is called a sink.

So carbon is absorbed by the carbon sink from the atmosphere, but less carbon is released into the atmosphere, and therefore, the quantity of carbon dioxide in the atmosphere reduces over time.

So plants are carbon sinks because they absorb more carbon from the atmosphere during the process of photosynthesis than they release through the process of respiration.

And we can see that because plants use the carbon in carbon dioxide to build their bodies.

They require carbon for biomass, and therefore are storing carbon within their body by constructing it in the form of carbohydrates, which can also be converted into other compounds such as proteins and lipids, fats.

So by building biomass, plants are storing carbon, removing it out of the atmosphere and storing it within their body.

There are other carbon sinks on the planet.

However, oceans are an incredible carbon sink.

They dissolve carbon dioxide out of the atmosphere and have absorbed nearly a quarter of all the carbon dioxide produced by human activities.

Rocks also act as a carbon sink because the carbon dioxide in the oceans can be reacted to form insoluble compounds such as carbonates.

And these are then used by organisms living within the ocean to form shells and bones.

And then when these organisms die, they are then, they then, those organisms settle to the bottom of the sea onto the sea floor, and over the course of millions of years, they are converted into sedimentary rocks and store the carbon that they did originally contain as a living organism.

So sedimentary rock is an excellent example of a carbon sink because those, the animals that made the sedimentary rock in the first place had removed carbon from the atmosphere and then it is now trapped within the rock structure.

There are other examples of carbon sinks such as fossil fuels, and these are carbon sinks because they are formed from plants and marine animals that died many millions of years ago.

And just like the process that formed sedimentary rock, so coal, oil and gas are carbon sinks for the same reasons that animals and plants which took carbon out of the atmosphere and stored it within their bodies have then been converted into coal, oil and gas in the process of doing so, and therefore store carbon within the rocks.

The problem with fossil fuels is that as they are burnt as a fuel, the carbon that they contain is then released back into the atmosphere.

Now, we can't burn sedimentary rock, so we can't convert the carbon that is stored within the rock back into carbon dioxide into the atmosphere, but we can burn coal, oil and natural gas, and therefore we can undo the carbon sink and release the carbon stored within these organisms back into the atmosphere.

Now, it took hundreds of millions of years for those fossil fuels to form, but it's taken 150 years, maybe a little bit longer than that for us to undo a vast amount of that process.

And it is estimated that about 44% of the carbon produced by human activities is not absorbed by carbon sinks.

It simply adds to the existing carbon store within the atmosphere.

And this is what is driving climate change.

So which of these is not a carbon sink? The atmosphere, fossil fuels, oceans or rocks? I'll give you five seconds to decide.

So you should have said that the atmosphere is not a carbon sink.

Well done.

So what I'd like you to do is just to conclude this conversation by explaining why the levels of oxygen and carbon dioxide in earth's atmosphere are different today than they were 4 billion years ago.

And in your answer, I would like you to include the equation for photosynthesis.

So pause the video and come back to me when you are ready.

Okay, let's see what you've written.

So to explain changes in oxygen levels, you may have included that oxygen levels have increased as algae and plants evolved, and that these organisms, the algae in the plants, they photosynthesize and this adds oxygen into the atmosphere.

And the equation for photosynthesis is carbon dioxide plus water leads to glucose and oxygen.

And to explain changes in the carbon dioxide levels, you might have included that when algae and plants photosynthesize, they reduce the amount of carbon dioxide in the atmosphere by taking carbon out of the atmosphere.

And as more algae and plants evolved, they further reduced the amount of carbon dioxide in the atmosphere.

Also, carbon dioxide is removed from the atmosphere by being dissolved in the oceans.

And when the carbon was stored in the oceans, it was then converted into carbonates, which were then used by organisms to form shells and bones.

And this became sedimentary rock as those organisms died and settled to the bottom of the ocean floor.

So sedimentary rock is a carbon store, therefore reduces carbon dioxide levels within the atmosphere.

And you also might have included that some plants and marine animals that died were trapping the carbon in their bodies, which were then converted into fossil fuels, again, built up over many millions of years, and therefore, fossil fuels are a carbon sink all the while they are not being burnt.

So just review your answer, see how comprehensive it was, and anything that you've missed, and well done indeed.

Okay, we've come to the end of our lesson today, and I hope you've enjoyed it.

And in our lesson today, we've seen that on earth, the early atmosphere is thought to have contained large amounts of carbon dioxide and nitrogen, which was released through volcanic eruptions.

And as the earth cooled, the water vapour, which was also rejected by volcanic eruptions, was able to condense to form the oceans.

And this in part dissolved some of the carbon dioxide out of the early atmosphere.

Then evolution enabled algae and plants to evolve, and these were able to extract carbon dioxide out of the atmosphere through the process of photosynthesis.

And this process also added oxygen to the atmosphere.

So carbon dioxide concentrations reduced and oxygen concentrations increased within the atmosphere over a long period of time.

Now, specific forms of photosynthetic algae called phytoplankton, which are found in oceans, are responsible for about half of all of the photosynthesis that takes place on earth today and were in part responsible for the creation of carbon sinks.

Carbon sinks are places which store more carbon than they release, take out of the atmosphere more carbon than they put back in.

And carbon sinks include plants and algae, oceans and rocks, and they are responsible for removing carbon dioxide from the atmosphere and storing it within those bodies.

So I hope you found this lesson very interesting and you've learned a lot today.

Thank you very much for joining me today, and I hope to see you again soon.

Bye.