Lesson video

This lesson is called "Impact of Climate Change on Sea Levels," and it's from the unit "Climate Change and Greenhouse Gases." 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 how global warming can cause sea levels to rise.

And we're gonna come across a number of keywords in our lesson today.

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.

So, in our lesson today, we're going to first of all have a look at what ice sheets and glaciers are before we consider how these are different from sea ice.

And then we'll have a look at how the expansion of sea water contributes to sea level change as well.

So, are you ready to go? I certainly am.

So, let's get started.

Now, we have been measuring the level of the sea for more than a hundred years using a piece of equipment called a tidal gauge.

And a tidal gauge is this piece of measuring equipment, this instrument, which is used to measure the height of the surface of the sea and compare it to the land next to it.

So, we're always getting this comparison data from a tidal gauge.

And we've been doing this, as I say, for more than a hundred years.

From 1992, we've also been measuring sea levels by satellite as well using radar.

And radar has been able to give us some very accurate measurements of sea levels since then.

So, for the last a hundred years or so, we've been gathering quite substantial quantities of data, and very accurate data as well, about sea levels and how they have changed over time.

Now, if we look at that data, we can see that the average sea level rise has risen by 103 millimetres, plus or minus four millimetres, since 1992.

And this data comes from the satellite data that we have.

If we look at the data changes, the sea level changes over the last 100 years or so, we can see that in the first 90 years of that, from 1900 to 1992, the sea levels rose by about the same amount as well.

So, in a period of 90 years, at the start of the previous century, so from 1900 to 1992, the sea levels rose by about 103 millimetres.

And then, in the 30 years that have happened since 1992, the sea levels have risen by about 103 millimetres again.

So, it took 90 years for the sea levels to rise by 103 millimetres.

First off, and then, only 30 years for the same level of rise to occur again.

And therefore the sea level rise is happening much, much faster.

And we know we can be very confident in those conclusions because both the sea tide gauge data and the satellite data both agree with each other.

So, you can see them printed on top of each other on the graph.

The orange satellite data is overlaying the tidal gauge data and they are both saying the same changes.

They are both recording the same changes in sea levels over the same period of time, the last 30 years or so.

So, why are sea levels rising? Well, sea levels are rising because of global warming.

And they are rising because melting ice sheets and glaciers are pouring vast quantities of water into the sea.

Also, as part of global warming, sea water is expanding.

And as sea water expands, sea levels rise.

And sea water is expanding because the Earth is warming in this global warming process.

So, these two significant things are happening because of global warming, melting ice sheets and glaciers, and sea water expansion.

And let's have a look at both of those in a bit.

So, what are two causes of sea level rise? Expanding sea water, increasing rainfall, melting ice sheets, and more frequent tides? I'll give you five seconds to decide.

Okay, so you should have said that the two causes of sea level rises are expanding sea water and melting ice sheets.

Well done if you chose both of those.

So, what is an ice sheet? Well, if we look at Greenland, which is far north of us, we can see that it is an island and it's covered by an ice sheet.

Now, this ice sheet has an average thickness of 2,300 metres, which is twice the height of England's tallest mountain, Scafell Pike.

So, that mountain on top of itself is still not as thick as the average thickness of the ice sheet over Greenland.

Isn't that amazing? Now, that enormous ice sheet is not just really, really thick, but it also covers a vast area of 1.

7 million kilometres squared.

And it contains about 3000 billion tonnes of ice.

That's three with 15 zeros after it, kilogrammes.

Three quadrillion kilogrammes of ice.

That is an enormous amount.

Now, there's another ice sheet over Antarctica.

So, Antarctica is in the Southern Hemisphere.

And this one has an average thickness of 2,160 metres, so, it's not quite as thick as the Greenland sheet.

And it contains about 30,000 billion tonnes of ice.

So, it's a little bit smaller than the Greenland ice sheet, but it's still pretty massive nevertheless.

So, if we look at the globe and we can see the tip of South America right at the very bottom there, and then we go zooming down to the South Pole, we can still see where the tip of South America is, and there is the Antarctic land mass in white on the picture.

And that is where the ice sheet is, this one at least is.

So, that's about ice sheets.

What about glaciers? Well, a glacier is not an ice sheet, just to be clear.

A glacier is a large mass of ice that is moving very, very slowly downhill because it's being pulled by gravity.

So, it's sliding really slowly downhill because of gravitational forces pulling it down.

Now, it's moving very slowly.

And you can see in the picture there the Franz Joseph Glacier in New Zealand.

And it is in its thickest parts about 300 metres thick, so substantially thinner than an ice sheet.

Now, glaciers are much, much smaller than ice sheets.

And they're found in many, many places around the Earth where it is very cold.

So, in mountain ranges, such as in the Alps, and in the Himalayas, there are plenty of glaciers in and amongst the mountain ranges.

But these are much, much smaller than the ice sheets that we've been talking about at Greenland and in Antarctica.

And because they are moving, they are melting slightly.

And that means that rivers run off glaciers, providing fresh water to the local environment, which is really important.

Lots of humans require glaciers for drinking water for instance.

So, glaciers are these smaller bases of ice, which are moving very, very slowly, and have rivers running off them usually.

So, a glacier is another name for an ice sheet, true or false? So, you should have said that that is false, and that is because glaciers are not the same as an ice sheet.

They are different from ice sheets because they are much smaller and they flow very slowly downhill.

Well done if you got that distinction correct.

So, what is global warming doing to ice sheets? Well, it is causing ice sheets to melt.

And if we have a look at the picture there, we can see the Antarctic ice sheet changing substantially in size over the course of a 60-year period.

And the satellite measurements that have been taken between 2002 and 2017 of ice sheets have found that on average Greenland was losing 253 billion tonnes of ice every year, and Antarctica was losing 134 billion tonnes of ice each year.

Remembering that the Antarctic ice sheet is smaller than the Greenland ice sheet.

And you can see how much smaller it has become from the picture there.

Now, the problem with global warming is that the ice in the ice sheets and the glaciers is, overall, melting faster than it is being replaced by fresh snow.

And that is why they are losing mass.

So, the water from melting ice is moving via rivers into the sea, and therefore adding water into the sea that was previously trapped on land.

Ice is also breaking off glaciers and ending up in the sea to form icebergs.

So, in both instances, ice and water are being added to the sea from land-based snow and ice.

So, what conditions are needed for glacier to increase in size? Is it A, the mass of ice melting being greater than the mass of snow falling? Or B, the mass of ice melting being equal to the mass of snow falling? Or C, the mass of ice melting being less than the mass of snow falling? So, what conditions are needed for a glacier to increase in size? I'll give you five seconds to decide.

Okay, so, you should have chosen C, that the mass of ice melting is less than the mass of snow falling.

So, less water leaves the glacier than is added to it.

Well done if you chose that option.

Right, let's pause and make sure that we understand what ice sheets and glaciers are.

So, the first thing I'd like you to do is to label the diagram of a glacier.

And then I would like you to explain what is happening if the mass of the glacier is not changing.

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

Okay, let's check your work.

So firstly, I asked you to label the diagram of the glacier.

So, you should have seen that on the right hand side there is falling snow, and that is settling on top of the ice, which is the glacier.

And the glacier is sat on top of rock.

Now, that glacier is moving very slowly towards the left side of this diagram, and that's the flow of ice indicated by the grey arrow.

And along the way it is melting, and that melting ice runs off into a river or the sea.

So, make sure you've got all of those labels added and in the correct locations.

And then I asked you to explain what was happening if the glacier is not changing in size, how is that happening? And you should have said that "If the mass of the glacier is not changing, then the mass of snowfall must be equal to the mass of the amount of ice that is melting," because the "mass gained from the snow must equal the mass which is being lost by melting ice." Well done if you explained that properly.

Okay, let's carry on and have a look at sea ice.

So, if we go all the way up to the top of the Earth, to the North Pole, we won't come across land, rock-based land, like we would if we went to the South Pole because there is no land at the North Pole.

But, in winter, the sea around the North Pole freezes.

And therefore when we go to the very top of the world, it is possible to stand on the top of the world at the North pole on sea ice.

And you can see the polar bear doing pretty much that, standing on sea ice and walking along.

Now, sea ice and icebergs float on the surface of the sea.

And because they are already in the ocean or the sea, they don't add water to the sea when they melt.

So, melting icebergs and melting sea ice does not cause sea levels to rise.

So, let's look at that in a little bit more detail.

Now, when ice melts, the water it forms takes up less space than the ice did.

So, if you've ever seen how water freezes, you'll see that water in its frozen state takes up more space than the liquid did originally.

And therefore when it melts, it's going to take up less space.

So, if we had a beaker of ice and we allowed it to melt, we would see that the liquid level would reduce as the ice melts because liquid water takes up less space than ice does.

And therefore the water contained within an iceberg will not increase the level of sea water because it's taking up less space, not more.

So, if we take this iceberg, which is floating in sea water and melt it, we will see that the amount of space it took up in the water when it was frozen will be exactly the same amount of space it takes up in total as fully liquid, fully melted water.

So, when the sea ice melts, it isn't taking up any more space, it is taking up less.

And the whole of the iceberg, including the bit which is sticking above the surface of the water, compacts into the same space that all of the iceberg took up below the water.

So, which of the following is correct? One kilogramme of ice has a larger volume than one kilogramme of water? One kilogramme of water has a larger volume than one kilogramme of ice? Or, one kilogramme of ice and one kilogramme of water have the same volume? I'll give you five seconds to think about it.

Okay, so you should have said that one kilogramme of ice has a larger volume than one kilogramme of water.

Remember, when water expands, it takes up more space.

So, let's have a look at that in an experiment.

What I'd like you to consider is if you had a plastic bottle which was filled with water and ice, and then you placed it in a bowl of water, you would see that it floats, as you can see in the diagram.

So, what I'd like you to do is to predict what will happen when the ice in the bottle melts.

And then have a go at explaining why that is the case.

Then see if you can observe this happening and then improve your answer to your explanation.

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

Okay, let's see what you've written and perhaps what you've observed as well.

So, as the ice melts inside the plastic bottle, the bottle will sink lower in the water.

But, why is that? Well, that is because as the ice melts, it takes up less space in the bottle.

And the water in the bottle is then no different from the water in the bowl, so there's no reason for it to float because it's of the same density.

However, the plastic bottle itself is less dense than the water, and so that will float on water.

And so as it's wrapping around the water inside the plastic bottle, the whole thing will float a little and keep it near the surface, but not floating like it was when the water bottle was filled with ice.

Have you been able to observe that happen? Because if you have, you will have seen this occur.

So, can you now use that observation to improve your work? And your improved work should be closer to the answer in part two on the screen here than it was before.

Did you need to make many changes? Well done if you didn't.

Okay, let's move on now to have a look at the expansion of seawater and how this affects sea levels.

So, let's consider a thermometer because you may well be familiar with this.

You'll have seen that when you've used a thermometer, the coloured liquid in the thermometer expands as the thermometer gets warmer.

And that is how we read the temperature of the thermometer.

It's not just magically guessing, it's telling us the temperature of the liquid that it's in by the liquid in a thermometer expanding to show the temperature.

And this principle of liquids expanding as they get warmer, we can apply to sea water as well.

So, if you warm some sea water, its particles are going to have more energy.

And so they can move faster and collide around with each other more often.

And you can see that in the two diagrams there, that the particles in the water at 20 degrees centigrade are moving fairly slowly.

Whereas the particles in the 50 degree beaker are moving much faster and colliding much more frequently with each other.

And that is because the water at 50 degrees has more energy than the water has at 20 degrees.

Now, the higher the temperature, the larger the space is between the particles.

So, if we look again at still pictures of those particles, we can see that at 20 degrees the water particles, they were moving relatively slowly, and you can see that they're quite close together.

But, at 50 degrees, they're moving much faster and the gaps between them are much larger.

And that's because they're moving around faster, bumping into each other more often, and therefore taking up more space.

So, which representation of seawater shows it at its highest temperature, A, B, or C? I'll give you five seconds to decide.

Okay, so you should have chosen A as a representation of seawater at its highest temperature.

Well done if you did.

So, what's this got to do with global warming? Well, global warming is causing the average global surface temperature of Earth to increase.

And, because we are talking about the surface temperature of Earth, we are talking about all the water on the surface of the Earth, as well as all the land.

So, global warming is increasing the temperature of the water in the sea and oceans and this is causing it to expand.

And this is causing sea levels to rise.

So, the expansion of water in the oceans is causing about a third of the observed sea level rise.

And the other two thirds is caused by new water being added into the seas and oceans from melting ice sheets and glaciers.

And these are melting also because of global warming.

So, global warming is having a two-pronged attack on oceans and seas by melting land-based water, which is running off into the sea and the oceans, and causing the level to rise by about two thirds of the amount that we've seen.

And then also by warming the oceans and seas up itself, and that's causing the additional one third rise that we've observed over the last a hundred years or so.

So, what proportion of sea level rise is caused by melting ice sheets and glaciers? One third, two thirds, or all of it? So, you should have said that about two thirds of the sea level rises that we have observed has come from melting ice sheets and glaciers.

Well done if you did.

So, what I'd like you to do now is to summarise that part of the lesson by considering the example of the thermometer.

So, the thermometer contains a coloured liquid.

What happens to the liquid in a thermometer when it is heated? Can you explain your answer using ideas about particles, just like we did when we were talking about the sea water? So, pause the video and come back to me when you are ready.

Okay, let's check your work.

So, what would you say first, how would you describe what you are seeing? Well, as the temperature increases, the coloured liquid moves up the thermometer and along the scale.

And why is that happening? Well, your explanation may include the fact that the particles in the liquid move quickly and collide more rapidly and push each other apart.

And that causes the liquid to expand and rise up the thermometer.

And that gives us an indication of what the current temperature is.

So, well done if you've got all of those points, but do add to your work if you need to.

Okay, we've come to the end of our lesson now, and I hope you found it really interesting.

There's certainly lots of different pieces of science in this and things that you might not necessarily have considered before.

So, we've seen how measurements made using tide gauges and satellites are showing sea levels and how they are rising at about three millimetres every year.

We've also got measurements to show how ice is being lost from ice sheets and glaciers.

And this is caused by global warming, melting the ice sheets and glaciers faster than the snow is replenishing them and replacing the ice that they are losing.

Now, melting sea ice and icebergs do not cause a rise in sea levels because the amount of water that the melting iceberg takes up is exactly the same space as part of the iceberg that was floating underwater originally.

But, melting ice sheets and glaciers, which is essentially water trapped on land and that is moving into the sea, is causing sea levels to rise.

In addition to that, global warming is increasing the temperature of the water in the oceans and this is causing it to expand.

And the expansion of water is contributing to about a third of the rise in sea levels, whereas melting ice from ice sheets and glaciers is contributing to about two thirds of the sea level rises that we are observing.

So, I hope you found that really interesting.

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

Bye.