✅ Strengths
Direct measurements. Many stations around the world. Accurate and easy to compare year by year.
Causes of Climate Change » Evidence for Climate Change
What you'll learn this session
Study time: 30 minutes
Cambridge spec: 5.1.1
- The difference between weather, climate and climate change
- Four kinds of evidence that the climate has changed
- How each source works, and its strengths and limitations
- How to judge how reliable a piece of evidence is
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Unlock This CourseWeather, climate and climate change
Weather is what the atmosphere is doing at a place right now or over a few days: rain this morning, 24°C this afternoon. Climate is the average weather of a place over a long time, usually 30 years or more. Climate change is a long-term change in the average climate, for a place or for the whole planet.
One hot summer does not prove the climate is changing. Scientists look for a trend that lasts for decades. To find one, they need evidence from the past. No single source is perfect, so geographers use several and compare them.
Key terms:
- Weather: the state of the atmosphere at a place over a short time.
- Climate: the average weather of a place over a long period, usually 30 years or more.
- Climate change: a long-term change in average climate patterns.
- Evidence: information that can be used to show that something is true.
- Trend: the general direction in which something changes over time.
Source 1: global temperature data
From about 1850, weather stations on land and thermometers on ships began to record temperatures. Scientists combine millions of these readings to work out the average temperature of the whole Earth. The Met Office and other science agencies do this and publish the results.
The trend is clear: the world has warmed. The IPCC (the Intergovernmental Panel on Climate Change) reports that global surface temperature in 2011–2020 was about 1.1°C higher than in 1850–1900. Most of this warming has happened since about 1900, and the last few decades have been the warmest on record.
❌ Limitations
Only goes back about 170 years. Early records came mostly from Europe and North America, with patchy coverage of the southern oceans, poles and deserts. Cities can read warmer than the countryside, so scientists have to correct for this.
Source 2: ice cores
In Antarctica and Greenland, snow falls year after year and never melts. Each layer is squashed into ice by the weight of the next. Scientists drill long tubes of ice, called ice cores, out of the ice sheet. The deeper the ice, the older it is.
- Tiny air bubbles are trapped in the ice. They hold samples of the ancient atmosphere, so scientists can measure how much carbon dioxide was in the air.
- The water in the ice contains different forms (isotopes) of oxygen. The mix of these isotopes shows how cold it was when the snow fell.
- Counting back down the layers shows the age of the ice.
Ice cores from Antarctica go back about 800,000 years. They show that temperature and carbon dioxide have gone up and down together through many cold periods and warm periods.
🏔 Strengths
Reaches back hundreds of thousands of years. Gives both the gases and the temperature in one sample.
❌ Limitations
Only available from the polar ice sheets, so it is a record of those places. Drilling is slow and costly. The oldest layers are squashed thin, so year-by-year detail is lost.
Source 3: sea ice positions
Sea ice is frozen sea water. Since 1979, satellites have photographed the Arctic Ocean from space and measured sea ice extent, which is the total area covered by ice. It is smallest each September at the end of the summer melt. Satellite records show the September ice has been shrinking, by roughly 12% per decade compared with the 1981–2010 average.
🛰 Strengths
Covers the whole Arctic, every day, with the same method. Easy to see with a map or graph.
❌ Limitations
Only about 45 years long, so it cannot show change before 1979. Sea ice also varies a lot from year to year, so you need the long trend.
Source 4: historic writing and paintings
Before instruments, people left written and painted records. Diaries and ship logs describe harsh winters and cold summers. Records of harvest dates show how warm each growing season was. Paintings show glaciers, which in many places were bigger than they are today.
A famous example is the Thames frost fairs in London. In some very cold winters the river froze solid and Londoners held fairs on the ice. The last one was in 1814. The Thames has not frozen like this since, partly because winters warmed, and partly because the river was later made deeper and faster-flowing and the old London Bridge was rebuilt.
📜 Strengths
Gives a picture of life and weather before instruments, and of places where nothing was measured.
❌ Limitations
Not exact numbers. The writer or painter may exaggerate, be biased or only record unusual events. Often covers one place only, and may be hard to date.
Putting the evidence together
| Source | Goes back | Shows |
|---|---|---|
| Thermometer records | about 1850 | Global temperature trend |
| Ice cores | about 800,000 years | Past gases and temperature |
| Sea ice satellites | 1979 | Arctic ice extent |
| Writing and paintings | hundreds of years | Local weather and glaciers |
Each source covers different times and places. When they all point the same way, scientists can be much more confident than if they relied on one.
Worked example
A student reads that Arctic sea ice extent was 7.0 million km2 in September 1980 and 4.5 million km2 in September 2020 (made-up figures). Change = 7.0 − 4.5 = 2.5 million km2. Percentage change = 2.5 ÷ 7.0 × 100 = about 36% decrease. Always give the units and say whether it is an increase or a decrease.
Common mistakes
Mixing up weather and climate: "It snowed today, so there is no climate change" confuses one day with a 30 year average. Another slip is saying a source is "wrong" when you mean it is limited. Say what it cannot show, such as "only covers 45 years".
Exam-style question
Fig. 1.1 is made-up data for the area of Arctic sea ice each September.
| Year | 1980 | 1990 | 2000 | 2010 | 2020 |
|---|---|---|---|---|---|
| Extent (million km2) | 7.0 | 6.2 | 6.0 | 4.9 | 4.5 |
Fig. 1.1
(a) Define the term climate. [2 marks]
(b) Describe the trend shown in Fig. 1.1. [3 marks]
(c) Explain one limitation of using ice cores as evidence of climate change. [2 marks]
(d) Assess how useful historic paintings and writing are as evidence of climate change. [6 marks]
Model answer
(a) The average weather of a place (1) over a long period, usually 30 years or more (1).
(b) Sea ice extent fell from 7.0 to 4.5 million km2 between 1980 and 2020 (1), a fall of 2.5 million km2 (1). The extent fell in every decade, with the biggest fall between 2000 and 2010 (1.1 million km2) and the smallest between 1990 and 2000 (0.2 million km2) (1).
(c) Ice cores only come from polar regions such as Antarctica (1), so they cannot show what the climate was like in other places such as the tropics (1).
(d) Top level answer: Historic writing and paintings are useful because they go back hundreds of years, before thermometers, for example diaries of harsh winters, harvest dates and the Thames frost fairs. Paintings of glaciers show where the ice reached in the past. They give a picture of real life in places where nothing was measured. However, they have limits. They give no exact numbers, they may be biased or exaggerated or record only unusual events, and they often cover one place only. Overall they are useful as supporting evidence, but they are less reliable than thermometer records, ice cores or satellites, so they are best used alongside those sources (up to 6 marks for developed strengths, limitations and a judgement).
Exam tip
For "assess" questions about evidence, give strengths and limitations, then finish with a judgement about how reliable the source is. Use data from the figure with units when you describe a trend.