S4.14.7 Chapter 7

Climate change

Joseph’s grandmother says the seasons have shifted and the sea comes further up the shore than it did. How do scientists know that the Earth is warming?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A From a few hot summers that people remember.
You picked A. Read on. At the end, the chapter comes back to your guess.
B From one thermometer at one place, read for many years.
You picked B. Read on. At the end, the chapter comes back to your guess.
C From computer models alone, with no measurements.
You picked C. Read on. At the end, the chapter comes back to your guess.
D From records at thousands of places on land and sea, and from satellites, which agree with each other.
You picked D. Read on. At the end, the chapter comes back to your guess.

By the end you can

Use data to describe how global temperature has changed
Explain effects such as rising seas, droughts and floods

Climate change is a lasting change in the pattern of weather in a place, or in the whole Earth. The climate has changed before. In the past, ice ages came and went over thousands of years. The causes were slow changes in the Earth’s orbit. Changes in ice and in the carbon dioxide of the air made the swings larger. What is new is the speed of today’s change, and its cause.

Start with the data. The air temperature near the Earth’s surface is measured at thousands of places. They are on land, on ships and on buoys at sea. Several groups of scientists, in different countries, work out the average for the whole Earth in different ways. They include space agencies, weather offices and universities. Their results agree to within about 0.1 °C.

Scientists give the answer as how far the global average is above the average of 1850 to 1900. That is the late 1800s, before much fossil fuel was burned. Because it is an average of many readings, it hardly moves in a single hot or cold season.

Worked example

Over 2001 to 2010 the global average was 0.9 °C above 1850 to 1900. Over 2011 to 2020 it was 1.1 °C above. By how many °C did it rise from one decade to the next?

The rise is the later value minus the earlier value.
The later value is 1.1 °C and the earlier is 0.9 °C.
1.1 − 0.9 = 0.2 °C.

Answer: 0.2 °C

The global average temperature, as a ten year average, above 1850 to 1900. The values are rounded to 0.1 °C. The major records differ by about 0.1 °C.

The ten years from 2015 to 2024 averaged about 1.2 °C above the late 1800s level. The eleven years from 2015 to 2025 were the eleven warmest in the records. The year 2024 was the warmest, at about 1.55 °C above. 2023 and 2025 came next, at about 1.4 °C. With the ups and downs of single years smoothed out, the long-term warming is now about 1.3 °C. One year above 1.5 °C is not the same as a long-term warming of 1.5 °C. Each of the last four decades was warmer than every decade before it.

The air is not the only thing that changed. Many signs point the same way.

Five signs of a warming world. Each is measured on its own, and all point the same way.

What is the cause? Scientists studied the Sun, volcanoes, the Earth’s orbit and the gases in the air. The result is clear. Recent warming is caused mainly by human activity, mostly by burning coal, oil and gas. This is the conclusion of the Intergovernmental Panel on Climate Change. It collects the work of thousands of scientists. The national science academies of countries all over the world agree.

There are several separate lines of evidence. Carbon dioxide takes in infrared. Laboratories showed this in the 1800s, and calculations soon showed that more of it would warm the Earth. The extra carbon dioxide in the air matches the amount we burn. About half of what we put into the air stays there. The sea and plants take the rest. Carbon from fossil fuels has no carbon 14 left, and the extra carbon dioxide has the same mark.

The pattern of the warming also fits. The lowest layer of air is warming while the stratosphere above it is cooling, which is what extra greenhouse gas does. If the Sun were the cause, every layer would warm. The Sun’s output has not risen since the 1980s, while temperatures have kept rising. Volcanoes make the air cooler for a while, not warmer.

Possible causes of the recent warming, and what the evidence says of each.

Now the effects. The sea is rising. The global average sea level has risen about 20 cm since 1900. Roughly two fifths of this is because water expands when it warms. This is thermal expansion. Most of the rest is melting ice on land. So far it has come mostly from glaciers, and more and more from the great ice sheets of Greenland and Antarctica. Since about 1970, expansion has been about half. The melt water runs into the sea.

The rate of sea level rise, in mm each year. The rise is speeding up. Over 2015 to 2024 it was about 4.7 mm a year (WMO).

Worked example

From 2006 to 2018 the sea level rose by about 3.7 mm each year. If this rate held, how many mm would it rise in 25 years?

Multiply the rate by the number of years.
Split 3.7 into 3 and 0.7. Then 25 × 3 = 75, and 25 × 0.7 = 17.5.
So the rise is 75 + 17.5 = 92.5 mm.

Answer: 92.5 mm

Not all melting ice raises the sea. Ice that already floats on the sea, such as Arctic sea ice, takes up the space of its own melt water. When it melts, the level hardly changes. Ice on land is different. Its melt water is new water for the sea.

When floating ice melts, the level stays the same. When ice on land melts, the level rises.

Rising seas flood low coasts, river deltas and small islands. They also push salt water into farmland and drinking water, and they make storm surges reach further. People living on low ground have the most to lose.

Next, rain. Warmer air can hold more water vapour. For each °C of warming it can hold about 6 to 7 % more. When the vapour condenses, more of it can fall at once. So the heaviest downpours become heavier, and floods are worse.

The most water vapour that 1 m³ of air can hold. Warmer air holds a good deal more.

Worked example

Air at 20 °C can hold up to 17.3 g of water vapour in each m³. Air at 30 °C can hold up to 30.4 g. By what percent does the amount rise? Give a whole number.

The rise is 30.4 − 17.3 = 13.1 g.
Divide by the starting value: 13.1 ÷ 17.3 is about 0.76.
As a percent, 0.76 × 100 = 76 %.
So the amount of vapour rises by about 76 %.

Answer: 76 %

That is for each 10 °C. For each 1 °C it is about 6 to 7 %. Warmer air also dries the land faster, because water evaporates more quickly from soil, lakes and plants. So the same warming that makes the heaviest rain heavier can also dry soils between rains. Some places get wetter and others drier. A drought is a long spell with too little water. Droughts and floods can both become worse, in different places and at different times.

There are other effects too. Heat waves are hotter and more frequent. The sea takes in carbon dioxide and becomes a little more acid. So far the pH has fallen by about 0.1. This harms shell making animals and corals. Corals bleach when the sea is too warm. The effects differ from place to place, and the poorest people, who have least to protect themselves, often face the most risk.

Scientists are careful about single events. A single storm, flood or drought has many causes. But the warming makes some of them more likely, or stronger. It loads the dice, and over many events the change in the odds can be measured.

Try it

Ice on water and ice on land

You need: two glasses of the same size, water, two ice cubes, if you have ice, a small stone that sticks out of the water, a marker pen or a strip of tape.

Be safe: If you do not have ice, read the result and picture instead. Wipe up any spilled water so no one slips. Do not drink the water.

Put the stone in one glass. Pour water into both glasses to the same height, so that the stone sticks out above the water in the first glass.
In the second glass, float an ice cube on the water. In the first glass, rest the second ice cube on the top of the stone.
Mark the water level on the outside of each glass with the pen or tape. Be careful not to move the glasses.
Wait until the ice has melted. Look at the marks. In which glass has the level changed?

What you will see: In the glass with the floating ice the level has not changed. In the glass with the ice on the stone the level has gone up. Floating ice takes up the room of its own melt water. Ice that sits on land adds new water.

If you cannot try it, this is what happens.

A wrong idea many people have

A very cold winter shows that the Earth is not warming.

Why it feels right: If the Earth were warming, it seems every day and every season would be warmer.

The test that breaks it: The average of the whole Earth over a whole year, with thousands of readings, is different from the weather outside one window. Cold spells and hot spells come and go, and a warming climate has both. The 10 year average keeps rising.

What is true: Weather varies from day to day and from place to place. Climate change shows in the long average of the whole Earth.

A wrong idea many people have

The melting of the Arctic sea ice is what will raise the sea level most.

Why it feels right: Melting ice sounds like it must add water to the sea.

The test that breaks it: Sea ice floats, like the ice cube in the glass. It already pushes aside its own weight of water, so when it melts the level hardly changes. Ice on land, in glaciers and the ice sheets, adds new water.

What is true: Sea level rises mainly because of ice melting on land, and because warming water expands.

A wrong idea many people have

Climate has always changed naturally, so people cannot be the cause of this change.

Why it feels right: It is true that the climate changed before people burned fuel, so the argument sounds fair.

The test that breaks it: Forests have always burned by lightning too. But if a fire is lit by a match, we can still tell that the match lit it. In the same way, scientists have measured the natural causes. They are the Sun, volcanoes and the orbit. They are too small, or too slow, to explain the warming. The carbon in the extra carbon dioxide carries the mark of fossil fuel.

What is true: Natural causes changed the climate in the past. This time, the evidence shows that the cause is human activity, mainly burning fossil fuels.

Key words

climate change: a lasting change in the pattern of weather, in a place or in the whole Earth.
thermal expansion: the growth in volume of a substance when it is warmed. Warm sea water takes up more room.
sea level: the height of the surface of the sea. The average of the whole Earth has been rising.
drought: a long spell with too little water, when the soil and rivers run dry.

Back to your guess

The answer is D: From records at thousands of places on land and sea, and from satellites, which agree with each other.

Temperatures are measured at thousands of places on land, on ships and buoys, and from satellites. Several groups of scientists work out the average in different ways, and they agree. Memory and a single thermometer can mislead. Many records together are a strong proof.

Keep it

Copy this into your exercise book.

The Earth is about 1.3 °C warmer than in the late 1800s. People cause most of it. Seas rise: warm water expands, land ice melts. Warm air holds more vapour: heavier rain, drier soil.

Draw: Two glasses with ice: floating ice leaves the level the same, ice on a rock raises it.