Lesson video

This lesson is called Evidence for global warming, 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 describe some of the evidence to show that global warming is caused by human activities.

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

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, look at evidence of rising temperatures before we look at evidence for the causes of global warming.

So are you ready to go? I certainly am.

So let's get started.

The Earth's surface temperature is measured in thousands of places every day, including on the oceans.

And it's measured using a wide range of different equipment, from local weather stations on land to weather buoys positioned out in the seas and oceans.

And all of these have high-quality thermometers, which take very accurate measurements of the temperature in their location.

Now, these measurements are taken automatically, usually once every hour, and then they're collated back at a central data point.

We also collect data about the temperature of the Earth's surface from space using weather satellites.

Now these are good, but they are not as accurate as recording the temperature on the Earth's surface, actually at the Earth's surface.

But they do provide us with a good measurement of the temperature across the whole planet.

Now, these temperature measurements across the whole world have been recorded for nearly 200 years, so going back a very long time.

And the automatic measurements allow average temperature calculations to be made for each day and every year.

Now, if we look at the average temperature of a particular location such as York, so let's look at the average temperature in York on the 28th of February, 2025.

Now, if we plot that temperature change on a graph, we can see how it was cold at nighttime, it got cooler, and at six o'clock in the morning, it was at its coldest for the whole of the day.

Then, as the sun came up, the temperature increased, reaching a peak of nine degrees at just after midday.

So about two o'clock in the afternoon before it started to cool back down again as the sun set.

So if we then average that temperature out across the whole of the day, we would see that with a low of zero degrees and a high of nine degrees, we would get an average temperature for the 28th of February, 2025 in York of 3.

7 degrees centigrade.

That would be the average temperature for that day.

So which measures temperature most accurately? A school science thermometer, a weather station thermometer, or a weather satellite? I'll give you five seconds to decide.

Okay, so the most accurate thermometer is the weather station thermometer.

Well done if you chose that.

So the global surface temperature is the mean or the average temperature of all of the places on the Earth's surface.

Now this is a really difficult measurement to calculate because there are, firstly, an enormous number of measurements involved in creating that calculation.

And also because a lot of the measurements are clustered around where there are higher populations and fewer measurements on the ocean, more on land, because, of course, the land is much more easily accessible.

Now this is a problem because 70% of the Earth's surface is covered in ocean.

So an unevenly distributed collection of data makes it very difficult to calculate the average global surface temperature at any one time, but it is doable with some clever mathematics and data models.

Now, this collection and number crunching of data is done by four large research centres across the globe, and they collect the worldwide temperature data and then they use a different method to calculate the global surface temperature.

So these locations, these research centres, are the National Oceanic and Atmospheric Administration in the United States of America, NASA, also in America, the Met Office here in the UK, and the Japanese Meteorological Agency.

So these are the four large research centres who are collecting all of that temperature data from across the globe and number crunching it into a general global average surface temperature.

Now, these four groups of scientists using the same data, but different models to interpret it, get very similar results.

And this is a good thing because it means that the data is reliable and can be trusted.

So if we look at the data here on the graph, we can see those four different agencies represented as different lines on the graph.

Their average global surface temperature results plotted as different colours on the graph, and they are all within a very small margin of error difference from each other.

And in some places they are absolutely identical to each other.

You can't see all four different lines at all in some places.

So we know that these results are reliable and that we can trust them.

So why can we trust the calculation of global surface temperature? Is it because it is done by scientists? Is it because the same calculation has been checked four times, or is it because four different calculations get the same result? I'll give you five seconds to choose.

Okay, so you should have said that we can trust the calculation of global surface temperature because four different calculations give the same result.

Well done if you chose that.

Now this data goes back to about 1880, but before that, there were not enough measurements to calculate the global surface temperature.

However, we can use other methods to help us estimate what the global surface temperature was in these times before this more robust data was being collected.

And we can use tree rings and ice cores to help us work that out.

Now, the colour and the width of tree's rings shows the conditions that the trees were growing in at the time that that bit of growth occurred.

And so it changes during the course of the tree's life, and we can see that reflected in the tree rings.

Whereas in ice cores, they contain layers and layers of snow, and each layer is the annual ice and snowfall that occurred at that time.

They're layered one on top of each other.

So the further down the ice core you go, the further back in time you were looking, and the conditions of the environment affect the amount of snow that was recorded, so that was layered down in the ice layer.

And also the properties that that ice and the trapped substances within it have.

So let's have a look at tree rings in a bit more detail.

So a new tree ring forms each year as the tree grows.

And when the weather is warm and wet, the tree will have more moisture and more energy, and, therefore, the tree ring will be wider, whereas in cooler and drier conditions, the tree ring will be thinner.

So having a look at the width of the tree ring will give us an indication as to whether the weather was wet and warm or whether it was cool and dry.

And so we can compare older tree rings with tree rings growing in the years of the measurements that we've got, and that will help us to calculate the annual average temperatures in years that we don't actually have those direct measurements.

So that's one way that we can calculate the average temperatures on Earth when we don't have the actual data to go by.

Now, ice cores allow us to do a similar thing.

So an ice core is extracted from ice as a long cylinder.

It's drilled out of an iceberg or a glacier or an ice sheet, and it's got layers upon layers upon layers of old ice.

And each new season has new ice laid on top of it.

Now, darker ice forms in winter and lighter layers are put down in summertime.

So we can use the evidence that is trapped in the layers of ice to see what the average temperature would've been for each year recorded in the ice layer.

Because there are differences in the atoms that are stored, which indicate what the average temperature was like.

Then, we can match that data up to the data that we have calculated using the tree rings.

Now some ice cores go back about a million years.

So some of them have ice layers hundreds of thousands, if not a million, years old, so go back a very, very long time.

Now, trees don't go back as far as that, but we are able to compare tree rings with ice core records.

And if we combine all of those different ways of calculating global surface temperature, we can calculate what the average temperature was for the last 2000 years.

We can see that plotted on the graph there.

So the average temperature for Earth over the last 2000 years was 13 degrees plus or minus about 0.

3 degrees.

So it was a little bit warmer than 13 degrees, a little bit cooler than 13 degrees at certain times over the last 2000 years.

But more or less the same around about that mark for the whole of that period of time, except in the last 150 years or so.

In the last 150 years from about 1850, we can see this rapid and unexpected increase in temperature.

And this is called global warming.

Global warming is this rapid and significant increase in temperature, average surface temperature on Earth.

So the temperature fluctuations before 1850, they're normal, whereas the temperature changes from 1850 onwards are not normal, and that is what we call global warming.

So how could scientists work out the average temperature of a thousand years ago? Could they use sets of old measurements? Could they examine and compare tree rings, or could they examine and analyse ice cores? I'll give you five seconds to decide.

Okay, so scientists could work out the average temperature of a thousand years ago by examining and comparing tree rings and ice cores.

Well done if you chose both of those options.

So what I'd like you to do is to firstly describe what the graph shows.

So have a look at the graph, its axes, and the line on the graph, and describe, say what you see using numbers from the graph as well if you can.

And then I would like you to say why almost every climate scientist is certain that the temperatures on the graph are correct.

So use the information that we've covered in the lesson to explain that.

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

Okay, let's check our work.

So firstly, I asked you to describe what the graph shows, and you should have said that the graph shows that the Earth's global surface temperature fluctuated slightly each year until around 1850.

And between approximately 1500 and 1850, the average temperature was slightly lower than the global average, but after 1850, the temperature increased rapidly to over 14 degrees centigrade.

So just check your work.

Have you got all of those main points there? Do add to your work if you need to, and well done.

Then I asked you to say why almost every climate scientist is certain that the temperatures on the graph are correct.

And you might have said that since about 1880, huge numbers of temperature measurements have been taken from around the world, and global surface temperatures have been worked out from measurements by four different groups of scientists who all get very similar results.

But before 1880, tree ring evidence and ice core evidence can be used, and this evidence gives the same results.

So well done if you've mentioned all of those points.

Do add to your work if you need to, and well done.

Okay, let's move on to have a look at the evidence of the causes of global warming.

So the amount of carbon dioxide in the atmosphere is now measured in many places around the world as well.

So we know that temperature is recorded in many different places, well, carbon dioxide is too.

Now, the first measurements were taken in Hawaii in 1958, and we have an accurate set of results since then.

Now you can see in the graph on the screen the carbon dioxide concentration in the atmosphere from 1958 up to the present day.

And you can see how the levels of carbon dioxide have increased over that time.

The thin line shows the annual fluctuations, but the thicker line shows how that averages out, and it increases during the whole of that period of time.

Now, tiny amounts of gas from the atmosphere are trapped in those ice core layers each year, and that means that we can go back to about a million years ago to see what the atmosphere was like way back then.

So these ice cores are really important records of the atmosphere over this very long period of time.

And if we look at that data, we can see that atmospheric carbon dioxide levels have fluctuated.

They have gone up and down over the course of the last 800,000 years as shown on the graph.

But at all of those points, they have remained below 300 parts per million of carbon dioxide in the atmosphere until 1950.

And from 1950 onwards, that level was increasing and broke the 300 parts per million mark and has continued to increase rapidly ever since.

So we can see this data and these changes in the atmosphere going back 800,000 years or so from the data collected from ice cores and the atmosphere that was trapped in those ice sheets.

So how can the increase in carbon dioxide levels in recent years be described? A, part of a regular pattern of increases and decreases, B, increasing faster than normal, or C, increasing higher than normal? I'll give you five seconds to decide.

Okay, so you should have said that the increase in carbon dioxide levels in recent years can be described as increasing faster than normal and increasing higher than normal.

Well done if you chose both of those options.

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

The levels of carbon dioxide in the atmosphere have increased with human activities that produce carbon dioxide.

So the graph on the left shows the amount of carbon dioxide that has been added into the atmosphere by human activity, and we can see that, from about 1850 onwards, the amount of carbon dioxide being added into the atmosphere by human activity has increased and increased significantly.

And that is reflected in the graph on the right, which is the amount of carbon dioxide in the atmosphere, which has increased pretty quickly as well.

So if we look at that same period of time in both of these graphs, we can see that they reflect each other.

The amount of carbon dioxide added into the atmosphere by human activities on the left is similar to the amount of carbon dioxide levels in the atmosphere in the graph on the right.

Now, we've talked about carbon dioxide levels, but the levels of methane gas in the atmosphere have also increased in the same way as the levels of carbon dioxide.

Now, both carbon dioxide and methane are greenhouse gases, and carbon dioxide and methane are released by a number of different human activities, including burning fossil fuels for electricity and heating, and to transport us around, to industry farming and the decomposition, the breakdown of waste materials in landfill sites.

Now, the greenhouse effect explains how increased amounts of greenhouse gases cause global warming.

So we can see that when we undertake a range of different activities, we add carbon dioxide into the atmosphere.

Now, this increases the levels of carbon dioxide into the atmosphere which enhances the greenhouse effect.

And enhancing the greenhouse effect leads to global warming.

And we can see all of these trends reflected in the graphs of data that we have collected over the last several hundred years or more.

So global surface temperature depends on the amounts of greenhouse gases in the atmosphere.

True or false? So you should have said that that is true, and that is true because when we add extra greenhouse gases, we enhance the greenhouse effect and increase the global surface temperature.

But some greenhouse gases will have more effect than others.

So carbon dioxide and methane are being put into the atmosphere by the activities that we are undertaking.

However, they do not stay in the atmosphere forever.

There is a cycle of activity that is going on.

So greenhouse gases are emitted from activities that we are undertaking and enter the atmosphere.

And after a period of time, those greenhouse gases transfer out from the atmosphere into other things.

Now, if we add in more gases into the atmosphere than we take out, then the amount of greenhouse gases in the atmosphere will increase.

If we reduce the amount of greenhouse gas emissions, then the levels of greenhouse gases will increase more slowly.

And you can see that in the pink line on the graph projecting potential future emissions and carbon dioxide levels.

If we reduce greenhouse gas emissions enough, then the level of greenhouse gases will eventually start to fall.

And again, that's shown in the graph in the pink lines projecting into the future of what might happen if we manage to reduce greenhouse gases enough to make them start to fall.

Now, if more greenhouse gases are transferred out of the atmosphere than are moved into it, we know that the greenhouse gas levels will fall.

So which of these statements about greenhouse gas emissions are correct? A, reducing emissions will slow down global warming, B, reducing emissions will stop global warming, or C, reducing emissions could reverse global warming.

I'll give you five seconds to decide.

Okay, so you should have said that reducing emissions will slow down global warming and could reverse global warming.

Well done if you said both of those.

So what I'd like you to do now is to consider this, that some pupils are discussing a model for the amount of greenhouse gases in the atmosphere.

So they are filling up a bath which has no plug.

The water represents the greenhouse gases, and the bath represents the atmosphere.

So have a think about how this model is working and then I'd like you to comment on how this is a good model for representing greenhouse gases in the atmosphere and how is it not a good model for representing greenhouse gases in the atmosphere? So have a think about this model and how it works and how it relates to greenhouse gases in the atmosphere.

Before you consider those questions, pause the video and come back to me when you are ready.

Okay, let's review your work.

So how is this a good model? Well, you might have said that the tap can change how quickly or slowly greenhouse gases, the water, are added into the atmosphere, the bath.

And the water falling down the plug hole shows that greenhouse gases do not stay in the atmosphere forever.

They do leave it again.

And that the total amount of greenhouse gases, so the total amount of water in the bath, in the atmosphere, depends mostly on how quickly they are added rather than how quickly they are taken away, which is fairly constant.

So that is how it is a good model.

So how is this not a good model? Well, you might have said that all of the water is at the bottom of the bath, whereas the greenhouse gases are mixed throughout the atmosphere in with the air, and that water fills most of the bath, whereas actually greenhouse gases are just a very tiny part of the atmosphere, because most of the atmosphere is made of nitrogen and oxygen.

You might also have said that some types of greenhouse gases leave the atmosphere more quickly than others, whereas in this model, all of the water leaves at the same speed.

So we don't have any of that differentiation.

So just review your work and see what you have written, add anything that you might have missed into your own arguments as well, and well done.

That was quite a challenging task.

Okay, we've come to the end of our lesson and what we've seen today is that measurements of the Earth's temperature are used to calculate the global surface temperature, which is the average temperature of all of the places on the Earth's surface across the whole year.

Now that evidence comes from lots of different places, but we can also collect evidence from ice cores and we can compare that to evidence from tree rings.

And by doing so, we can calculate the global surface temperature of times in the past when we don't have temperature measurements to go by.

All of this data then contributes to the picture of evidence of global warming.

We've also seen how the gases trapped in the ice cores gives us evidence of changing levels of gases in the atmosphere.

And this evidence shows temperature changes, temperature increases, and how the temperature increases as greenhouse gases increase.

So we've been able to develop an understanding of the greenhouse effect and explain how increasing the amount of greenhouse gases in the atmosphere causes global warming.

So I hope you found today interesting.

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

Bye.