Climate, Weather and Climate Change

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1. Key words

2. Weather and climate are not the same

Weather is what the air is like at one place at one time: how hot or cold it is, whether it is raining, how strong the wind is, how cloudy it is. Weather changes from hour to hour and from day to day. A sandstorm in Cairo this afternoon is weather.

Climate is the average weather of a region over a long time — scientists usually use at least 30 years of measurements. Climate tells you what to expect. "Egypt has hot, dry summers and mild winters with very little rain" describes Egypt's climate.

Scientists show a climate with a climate graph: bars for the average rainfall in each month and a line for the average temperature.

Reading this graph: the hottest month is August (28 °C); the coldest is January (14 °C), so the temperature range is 28 − 14 = 14 °C. The wettest month is December (55 mm). June, July and August have no rain. So the climate has hot, dry summers and mild, wet winters.

3. The Earth's climate moves in a cycle

The Earth's climate has never stayed the same. For hundreds of thousands of years it has moved in a cycle: long, cold ice ages, when huge ice sheets covered much of North America and northern Europe, and shorter warm periods between them. We are living in a warm period now; it began about 11 700 years ago.

• One full cycle (from one warm period to the next) takes about 100 000 years — far longer than human history.
• The temperature usually falls slowly into an ice age over tens of thousands of years, then rises quite quickly into a warm period.
• At the coldest point of the last ice age, about 20 000 years ago, so much water was frozen in ice sheets that the sea level was about 120 m lower than today.
• Scientists think the cycle is started by slow changes in the Earth's path around the Sun and in the tilt of its axis. These change how much sunlight reaches different parts of the Earth.

4. The evidence for ice ages and warm periods

Nobody was measuring temperatures 100 000 years ago, so how do we know? Scientists use several kinds of evidence, and they all agree with each other.

5. Climate can change when the atmosphere changes

The atmosphere is the layer of gases around the Earth. Some of these gases — carbon dioxide, methane and water vapour — are greenhouse gases. This is how they warm the Earth:

1. Energy from the Sun (sunlight) passes through the atmosphere and warms the Earth's surface.
2. The warm surface gives off energy as heat (infrared).
3. Some heat escapes to space, but greenhouse gases absorb some of it and send it back towards the Earth.
4. So the atmosphere traps heat, like the glass of a greenhouse — this is the greenhouse effect.

The greenhouse effect is natural and we need it: without it the Earth's average temperature would be about −18 °C, too cold for most life. The problem is when the amount of greenhouse gas increases — more heat is trapped and the climate warms.

The evidence today: careful measurements since 1958 show that the carbon dioxide in the air has risen every year, from about 317 parts per million in 1960 to more than 410 in 2020. Ice cores show it was never above about 300 parts per million in the 800 000 years before the Industrial Revolution. Over the same time the Earth's average temperature has risen by more than 1 °C. Most scientists agree that this change is caused mainly by humans burning fossil fuels.

Possible effects of climate change: sea levels rise (ice on land melts and sea water expands when it warms) — a danger for low coasts such as the Nile Delta; more heatwaves, droughts and floods; habitats change and some species cannot survive (for example coral reefs in the Red Sea can be damaged by hot water). We can reduce the change by using renewable energy, saving energy, and planting trees.

6. Worked examples

Worked example 1 — Tick (✓) to show whether each sentence describes weather or climate. [3]

1. Look for time words: "this morning", "tomorrow", "yesterday" → weather (a, c, e).
2. "Has hot summers", "in a typical year" describe what is usual over many years → climate (b, d).
3. Marks: 5 correct = 3; 4 correct = 2; 3 correct = 1.
4. Common mistake: (d) is about rain, so students tick weather. Rain can be part of climate — the key words are typical year.

Worked example 2 — Use the climate graph in section 2. (a) Which is the wettest month? [1] (b) Calculate the range of average temperature. [1] (c) Describe the climate of the city. [2] (d) Explain why this is a climate graph and not a weather record. [1]

1. (a) December (55 mm, the tallest bar).
2. (b) Highest 28 °C (August) − lowest 14 °C (January) = 14 °C. Include the unit.
3. (c) Summers are hot and dry (1); winters are mild/cool and wetter (1). Quote numbers if you can: about 28 °C and no rain in summer.
4. (d) The values are averages over 30 years, not measurements from one day or one year (1).

Worked example 3 — Use the ice-age graph in section 3. (a) Is the Earth in a warm period or an ice age now? [1] (b) About how long is one cycle from one warm period to the next? [1] (c) Describe how ice cores give evidence for this cycle. [2]

1. (a) A warm period (the line is at the warm level, 0 °C compared with today, at the right-hand end).
2. (b) The warm peaks are about 400, 335, 245 and 128 thousand years ago, roughly 100 thousand years apart → about 100 000 years.
3. (c) Snow builds up in yearly layers, so deeper ice is older (1); the ice and the trapped air bubbles show the temperature and the carbon dioxide at that time, and they go up and down in a repeating pattern (1).
4. Common mistake: writing 100 years or 1000 years. The cycle is very long — about a hundred thousand years.

Worked example 4 — Explain how burning fossil fuels can change the Earth's climate. [3]

1. Burning fossil fuels releases carbon dioxide into the atmosphere (1).
2. Carbon dioxide is a greenhouse gas; it traps heat / stops some heat escaping to space (1).
3. More trapped heat makes the Earth's average temperature rise (global warming / climate change) (1).
4. Exam tip: 3 marks = 3 linked steps: gas released → traps heat → temperature rises. Do not say the gas "makes a hole" — that is a different topic (the ozone layer).

Worked example 5 — In 1991 a very large volcano erupted in the Philippines. For about the next two years, the Earth's average temperature was slightly lower. Suggest why. [2]

1. The eruption threw ash and gases high into the atmosphere (1).
2. These blocked / reflected some sunlight, so less energy reached the Earth's surface and it cooled (1).
3. This shows that any change in the atmosphere can change the climate — warming or cooling.

Practice

All pages in this course

Start here

• Stage 8 Science: The Tests and How to Prepare
• Thinking and Working Scientifically: Models, Investigations and Analysing Results

Biology

• Joints and Antagonistic Muscles
• Blood and Its Components
• Breathing, Gas Exchange and Diffusion in the Lungs
• A Balanced Diet, Energy Stores and Lifestyle
• Aerobic Respiration
• Ecosystems, Bioaccumulation and Invasive Species

Chemistry

• The Structure of the Atom
• Purity, Concentration and Chromatography
• Word Equations, Exothermic and Endothermic Reactions
• The Reactivity of Metals
• Impure Products and Solubility of Salts

Physics

• Speed and Distance–Time Graphs
• Balanced and Unbalanced Forces, and Moments
• Pressure, Particles and Diffusion
• Reflection and Refraction
• Dispersion and Colour
• Magnetic Fields and Electromagnets

Earth and space

• The Earth's Magnetic Field and Earth's Resources
• Climate, Weather and Climate Change
• Galaxies and Asteroids

Practice tests

• Stage 8 Science — Practice Test 1
• Stage 8 Science — Practice Test 2
• Stage 8 Science — Practice Test 3