Lesson video

This lesson is called Greenhouse gases 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 explain the effect of greenhouse gases on the temperature of the Earth's atmosphere.

So we're gonna come across a number of keywords today, and they're listed up here on the screen for you now.

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

Now in our lesson today, we're going to first of all look at the greenhouse gas carbon dioxide, and then we're gonna have a look at another greenhouse gas called methane.

Then we're gonna have a look at the greenhouse effect to which these gases contribute before we consider what global warming is.

So are you ready to go? I certainly am.

Let's get started.

When we burn fuel, we release carbon dioxide into the atmosphere.

So when I say fuel, I mean things that contain carbon, such as: coal, oil and gas, wood and paper, candle wax.

Those sorts of things all release carbon dioxide into the atmosphere when we burn them.

When we're burning them in oxygen, they are burning in a manner called complete combustion.

That means where all of the carbon inside those fuels is turned into carbon dioxide.

So examples of that include: the fuel in our cars, if we're using a gas hob at home, or if we are heating our house up using a gas boiler, or if we are generating electricity using a coal, oil, gas, biomass-fired power station, for instance.

All of these methods and many other ways besides use complete combustion to generate energy and release carbon dioxide in the process.

Now the levels of carbon dioxide in the atmosphere are now 50% higher than they were before the industrial revolution in the 18th century.

That's an enormous amount more carbon dioxide within the atmosphere.

So why is that important? Well, let's have a look at that in a bit more detail.

Now, carbon dioxide, along with methane and water vapour, are all greenhouse gases.

Carbon dioxide, CO2, methane, CH4, and water vapour, H2O.

Now all of these gases when present within the atmosphere absorb some types of infrared radiation that radiates back from the surface of the Earth.

And what they do is essentially trap that heat in the atmosphere, radiate it around the Earth and make the Earth's average temperature far warmer than it would otherwise be.

Now these gases, carbon dioxide, methane, and water vapour exist naturally within the atmosphere.

However, what we are doing on the planet, our human activities are adding significantly more of these greenhouse gases into the atmosphere.

So let's quickly check.

Which of the following are greenhouse gases? Water vapour, carbon dioxide, methane, and sulphur dioxide.

I'll give you five seconds to think about it.

Okay, so you should have chosen water vapour, carbon dioxide, and methane.

Well done if you got all three of those correct.

Carbon moves around the Earth from the atmosphere into organisms, into the soil, and back into the atmosphere through a number of different processes part of the carbon cycle.

And one of the parts of the carbon cycle is plants.

So plants act as carbon sinks because they absorb more carbon dioxide.

They absorb more carbon than they give out, than they release,.

And they absorb carbon through carbon dioxide during the process of photosynthesis.

Now photosynthesis is where a plant takes carbon dioxide and water and through a series of processes, combines it into glucose and releases oxygen.

And storing carbon in the form of glucose and its other molecules, such as starch and cellulose, allows the plant to build its body and it stores carbon, removes it out of the atmosphere.

And that is why we call plants carbon sinks.

However, some of the activities that we are undertaking on this planet, some of our human activities are reducing the ability for plants to act as carbon stores.

So for instance, in just six years, 800 million trees were felled, cut down, and mainly burnt from the Amazon rainforest alone to meet human need for meat.

So vast swathes of the Amazon rainforest were cut down, deforested and the land was cleared so that cattle could be grazed or food for cattle could be grown to be fed to the cattle and then we could farm those animals for meat.

Okay, let's check our understanding again.

So which of the following have led to an increase of carbon dioxide in the atmosphere? Deforestation, complete combustion of fuels, and photosynthesis.

I'll give you five seconds to decide.

So you should have said deforestation and the complete combustion of fuels.

Well done if you got both of those correct.

So what I'd like you to do is to summarise what we've covered so far by describing how deforestation leads to increased levels of carbon dioxide in the atmosphere.

And as part of your description, 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 how you got on.

So you should have said that trees remove carbon dioxide from the atmosphere by photosynthesis, and that is carbon dioxide plus water leading to glucose and oxygen.

And you may well have written that in just word equation, as I have on the screen here, or you might have included a symbol version of it instead, which is absolutely fine.

And then you could have gone on to elaborate to say that when trees are cut down, the carbon they contain may be released into the atmosphere, and with fewer trees there will be less photosynthesis occurring.

And this means less carbon dioxide is being removed from the atmosphere, which all concludes to lead to an increase in carbon dioxide levels in the atmosphere.

Well done if you've written all of that.

Add anything that you might have missed out and well done.

Okay, let's move on and have a look at methane.

So methane is another greenhouse gas.

However, methane is a much more potent greenhouse gas than carbon dioxide.

Now what that means is that one molecule of methane can absorb more than 28 times as much infrared radiation as one molecule of carbon dioxide.

So it can absorb much more energy and therefore radiate out much more energy.

However, there is a significantly smaller quantity of methane within the atmosphere than there is of carbon dioxide, which is an utter relief.

Also, methane has a turnaround in the atmosphere of about 12 years.

It lasts about 12 years in the atmosphere before it's broken down and disappears from the atmosphere.

However, carbon dioxide lasts substantially longer than that, 50 to 100 years.

So it means that if we can reduce methane emissions and do that quite significantly and quite rapidly, then we'll see quite a significant decrease in the amount of greenhouse gases in the atmosphere as a result, because of the significant impact that methane has on the greenhouse effect.

Now, methane is produced in a number of ways.

Quite a lot of the methane that ends up in the atmosphere is produced by the microbes that are sitting within the digestive system of cows, helping the cow to break down the grass that it is eating.

And that means that cows burp a lot of methane out of their bodies as part of this process, this digestion process.

Also, bacteria form part of the ecosystem found in paddy fields.

Now, paddy fields are where rice grows and they are waterlogged, they're waterlogged patches of land and microbes, bacteria within the soil, help the process of rice growing, and therefore, as part of their presence there within the paddy fields, they release a large amount of methane into the atmosphere.

And so rice is a huge contributor to methane within the atmosphere.

As well as cows and rice, humans contribute a lot of methane into the atmosphere via the landfill sites and the waste that we dump into those spaces, because just with the bacteria sitting within the gut of the cow and the bacteria sitting within the soil, the waterlogged soil of the paddy fields, bacteria also are present within landfill sites, digesting the organic matter and as part of that process, releasing carbon dioxide and methane into the atmosphere.

So landfill sites contribute quite a significant quantity of gases into the atmosphere.

So let's quickly check again.

Which of the following have led to increased levels of methane in the atmosphere? Landfill sites, cattle, deforestation, and rice paddy fields.

I'll give you five seconds to think about it.

Okay, so you should have chosen landfill sites, cattle and rice paddy fields.

Well done if you spotted all three.

Now what I'd like you to do is to have a look at what Sofia, Laura, Andeep and Lucas have been talking about.

All three of them have been discussing why the amount of greenhouse gases in the atmosphere is increasing, but firstly, who is correct? And secondly, what changes can you make to the other students to make their comments accurate again? So pause the video and come back to me when you are ready.

Okay, let's check our work.

So firstly, you should have said that Lucas is correct when he says that driving involves the combustion of fuels, which produces carbon dioxide.

And then to correct Sofia, Laura and Andeep, you should have said that, for Sofia, deforestation has led to an increase in carbon dioxide in the atmosphere.

For Laura, you should have said that landfill sites release carbon dioxide and methane as organic matter decomposes.

And for Andeep, you should have said that cattle farming and paddy fields increase methane in the atmosphere.

Just check your work over, make sure you've got all of those inaccuracies amended correctly, and well done indeed.

Okay, let's move on to have a look at what the greenhouse effect is.

Now, the greenhouse effect is a process that is going on on Earth, however, it starts with the Sun.

And that is because the Sun emits infrared radiation because it is extremely hot.

Now, that infrared radiation travels across the space between the Sun and the Earth and is felt by us, on our skin as warmth.

Now the Earth absorbs that infrared radiation and then reflects some of it out, and that is what is warming the surface of the Earth.

So that makes rocks feel warm, for instance, makes water evaporate, that kind of thing.

So we can feel that infrared radiation from the Sun beaming down onto us.

And we can also feel it being radiated off from the Earth as it's bounced off the surface of the Earth because of the warmth.

Now, the infrared radiation from the Sun feels much warmer on our skin than the reflected radiation does, say, coming from a rock or from a water pool or something like that.

And that's because there is much more energy contained within the infrared radiation from the Sun than there is being reflected from the surface of the Earth.

There's just a greater amount of energy in that infrared radiation and therefore it feels warmer.

So the Sun is emitting infrared radiation to Earth and it's passing through the atmosphere to the surface of the Earth.

Now the infrared radiation is absorbed by the surface of the Earth.

About 70% of it is absorbed by the surface of the Earth, more than two thirds of it.

And the rest of it is reflected back out into space, as you can see on the diagram there.

Now, infrared radiation, which is emitted or reflected by the Earth's surface, has much less energy than the energy that, than the infrared radiation did have when it came from the Sun.

And this is because the Earth's surface is much cooler than the Sun's surface, so that the Earth's surface is about, on average, 15 degrees centigrade, whereas the surface of the Sun is about 20,000 centigrade.

So there's a significant difference, which is why the radiation from the Sun feels so much warmer than the radiation from the Earth.

Now, greenhouse gases, which are present within the atmosphere, can absorb the radiation that is being reflected from the surface of the Earth and then reradiate it back out in all directions, which helps to warm up the Earth's atmosphere.

And we can see that in the diagram where the red reflected radiation arrows are then being bounced around the atmosphere in all directions.

And it is this re-reflection of energy of infrared radiation through the atmosphere and around and about, which is called the greenhouse effect.

So those red arrows where they're pointing in all sorts of different directions, that is the greenhouse effect.

But it's caused by all these previous processes that have happened and led up to it.

So let's just quickly check.

What do greenhouse gases in the atmosphere do? Do they A: Absorb infrared radiation from the Sun and radiate it in all directions, or B: Absorb infrared radiation from Earth and radiate it in all directions, or C: Absorb lower energy infrared radiation and radiate it in all directions, or D: Absorb higher energy infrared radiation and radiate it in all directions? I'll give you five seconds to think about it.

Okay, so you should have said that B and C are both correct, that greenhouse gases are absorbing infrared radiation from Earth at lower energy infrared radiation, and then radiating it in all directions.

Well done if you spotted both of those.

So to summarise the greenhouse effect, what I'd like you to do is to read each statement and decide if it is true or false.

And if it's false, correct the inaccuracies to make them true.

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

Okay, let's check our work.

So for the first statement, you should have said that infrared radiation is emitted by the Sun is true.

For the second statement, you should have said that it is false, and you should have corrected it by saying that greenhouse gases do not reduce the amount of higher energy infrared radiation from the Sun that reaches the Earth's surface.

For statement three, you should have said that that is also false and corrected it so that it says that sense cells in our skin detect infrared radiation as warmth.

For statement four, you should have said that this is true, on average, Earth emits lower energy infrared radiation than the Sun.

And for statement five, you should have said that that is also true, that greenhouse gases reduce the amount of lower energy infrared radiation that escapes from Earth's atmosphere.

Just review your work, make sure you've made all those corrections properly, and well done indeed.

Okay, let's move on to the last section of our lesson now, which is about global warming.

Now, greenhouse gases in the Earth's atmosphere means that the average temperature on Earth is about 15 degrees.

Obviously it's much, much colder at the poles and much, much warmer at the equator, but on average it's about 15 degrees.

Now that's really important to understand that that is not what it would really be if the greenhouse gases weren't present within the atmosphere, because if they weren't present within the atmosphere, then it would be much, much colder, about minus 20 on average, which would mean that the poles would be even colder than they are, and the equator would certainly not be anywhere near as warm as it is.

Now, minus 20 would mean that no life would exist on Earth.

It would be a barren, icy wasteland, very cold indeed.

So having greenhouse gases in the atmosphere, which is an entirely natural occurrence, is good for the planet because it allows life to flourish.

In fact, without greenhouse gases, life would not have even begun at all.

So let's quickly check that.

Why do we need greenhouse gases in the atmosphere? Is it A: To increase the temperature of Earth some more? Is it B: to maintain a suitable temperature for life on Earth? Or is it C: to prevent the temperature of Earth from increasing? I'll give you five seconds to think about it.

Okay, so you should have said that we need greenhouse gases in the atmosphere to maintain a suitable temperature for life on Earth.

Well done.

Now, greenhouse gases in the atmosphere are entirely natural to an extent.

However, if we add more greenhouse gases to the atmosphere, what it will do is raise the temperature of Earth even more.

Now, it doesn't stop the infrared radiation from the Sun penetrating the atmosphere, that continues to remain a relatively steady level.

But what it does mean is that the amount of infrared radiation at a lower energy level that is being radiated back from the Earth's surface into the atmosphere and then bounced around in all directions by the atmosphere, by the greenhouse gases in the atmosphere.

That re-radiation, that will increase.

And what that means is that it will heat the atmosphere further and lead to a higher average temperature.

And we are seeing the results of this happening on Earth right now.

So you can see that process also happening in the diagram where the red arrows, there are more red arrows being bounced around the atmosphere, and there are more red arrows because there are more greenhouse gases present within the atmosphere.

Now this increase in temperature is called global warming, and there is very strong evidence to show that human activity is driving the greenhouse effect causing global warming because we are releasing significant quantities of carbon dioxide and methane gases into the atmosphere which is accelerating this process.

So what causes global warming? Is it A: an increase in heating by the Sun? B: An increase in greenhouse gases in the atmosphere? Or C: An increase in the heating of Earth's atmosphere? I'll give you five seconds to decide.

So what causes global warming? Well, an increase in greenhouse gases in the atmosphere and an increase in the heating of the Earth's atmosphere both cause global warming.

Well done if you spotted both of them.

So in our last task of today's lesson, we have seven statements which explain how greenhouse gases can cause global warming, starting with "Higher-energy infrared radiation from the Sun passes through Earth's atmosphere." And what I'd like you to do is to order the statements A to F into the correct order to provide the complete explanation of how greenhouse gases can cause global warming.

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

Okay, let's check our work then.

So starting with "Higher-energy infrared radiation from the Sun passes through the Earth's atmosphere" should then have been followed by statement E: About 70% of this radiation is absorbed by Earth's surface.

Then statement F: Earth's surface emits lower energy infrared radiation.

Then C: This radiation can be absorbed by greenhouse gases.

Then A: Infrared radiation is emitted in all directions, warming the atmosphere.

Then B: More greenhouse gases cause more warming.

And finally, D: The increase in Earth's average temperature is called global warming.

Did you get all of those in the correct order? Just reorder them if you haven't.

And well done again for that work.

So we've come to the end of our lesson today.

Thank you very much for joining me and I hope you found it interesting.

In our lesson today, we've seen how high energy infrared radiation from the Sun can pass through the Earth's atmosphere and warm the surface of the Earth.

And in a lower energy form, the infrared radiation from Earth can then be absorbed by the greenhouse gases in the atmosphere as it is radiated from the surface of the Earth and bounced around the atmosphere.

Now, these greenhouse gases include carbon dioxide and methane, and after they have absorbed infrared radiation, they emit radiation in all directions which heats up the atmosphere.

And this heating is called the greenhouse effect.

Now, the greenhouse effect is mostly natural, but human activities can add extra greenhouse gases into the atmosphere.

And there is strong evidence that this is leading to an increase in the Earth's average temperature and is contributing to global warming.

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

Bye.