Fragile Environments Under Threat » Natural Causes of Climate Change
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 7.1 Fragile environments are under threat from desertification, deforestation and global climate change (c)
- Why the climate has always changed naturally, long before people had any effect
- The three natural causes in the spec: Milankovitch cycles, solar variation and volcanism
- How to link each cause to a warmer or cooler climate and to its timescale
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Unlock This CourseThe climate has always changed
When people talk about climate change today, they usually mean warming caused by human activity. But Earth's climate was changing for millions of years before there were any people. Over the last two million years it has swung between cold periods, when ice covered much of northern Europe and North America, and warmer periods in between. Those swings had natural causes. The spec names three: Milankovitch cycles, solar variation and volcanism.
This is the first of two lessons on the causes of climate change. This one is about natural causes. The next lesson, The Enhanced Greenhouse Effect, covers how human activities add to them.
Key terms:
- Natural climate change: a change in climate with natural causes, not caused by people.
- Milankovitch cycles: long, regular changes in Earth's orbit and tilt that alter how much sunlight reaches different parts of the Earth.
- Solar variation: changes in the amount of energy given out by the Sun.
- Sunspot: a darker, cooler patch on the surface of the Sun.
- Volcanism: volcanic activity, including eruptions that throw ash and gases into the air.
Cause 1: Milankovitch cycles
Earth goes round the Sun, and it spins on a tilted axis. Neither of these stays exactly the same forever. Small changes in them mean that different amounts of sunlight reach different parts of the Earth at different times. These changes are called Milankovitch cycles, after the scientist who worked them out. There are three of them.
🌍 Orbit shape
Earth's path round the Sun changes slowly from nearly circular to slightly oval and back again. This takes roughly 100,000 years. When the orbit is more oval, the Earth is sometimes further from the Sun, so it receives less heat.
🌐 Tilt of the axis
The tilt of Earth's axis nods between about 22 and 24.5 degrees. This takes roughly 41,000 years. A bigger tilt gives stronger seasons, with hotter summers and colder winters.
🌀 Wobble
Like a spinning top, the Earth wobbles as it spins, so the axis points to different places in space. This takes roughly 26,000 years. It changes which season happens when the Earth is closest to the Sun.
Each cycle is slow, so the effect on climate builds up over tens of thousands of years. When the cycles combine in the right way, summers in the northern hemisphere stay cool. Snow from the winter does not all melt, ice builds up, and an ice age begins. When they combine the other way, ice melts and the climate warms. Milankovitch cycles are the main reason why the Earth has moved in and out of ice ages.
Worked example
Question: A scientist says the tilt of the Earth's axis changes every 41,000 years. How many complete tilt cycles fit into 205,000 years?
Divide the time by the length of one cycle: 205,000 ÷ 41,000 = 5. So five complete cycles happen in 205,000 years. This shows why Milankovitch changes are far too slow to explain warming over a few decades.
Cause 2: Solar variation
The Sun is not perfectly steady. The amount of energy it gives out goes up and down a little. Scientists notice this by counting sunspots. These are darker, cooler patches on the Sun's surface, and they come and go in a cycle of about 11 years. More sunspots go with a slightly more active Sun that gives out a little more energy. Fewer sunspots go with a slightly less active Sun.
The changes over a single cycle are small, so they do not cause big shifts in the climate. Longer quiet spells could have a bigger effect. Between about 1645 and 1715 there were very few sunspots, a time called the Maunder Minimum. It happened during the Little Ice Age, when winters in Europe were often very cold, and many scientists think the quiet Sun played a part, together with other causes.
The link is simple to remember: a more active Sun can warm the Earth slightly, and a less active Sun can cool it slightly.
Cause 3: Volcanism
A big volcanic eruption can throw huge amounts of ash and gases high into the atmosphere. One of these gases is sulphur dioxide. High up, it turns into tiny droplets that spread round the planet. The droplets reflect some of the Sun's energy back into space, so less heat reaches the surface and the Earth cools. Remember from the lesson on longer-term volcanic impacts that this cooling is called a volcanic winter. Here we are interested in it as a cause of climate change.
- In 1815 Mount Tambora in Indonesia erupted. The following year, 1816, became known as the "year without a summer" in Europe and North America, with cold weather and failed harvests.
- In 1991 Mount Pinatubo in the Philippines erupted. Global temperatures fell by about 0.5°C for around two years afterwards.
Volcanoes also give out carbon dioxide, which warms the climate. But the amount is far smaller than the amount people release, and the cooling from ash and sulphur is bigger in the short term. So the main effect of a single eruption is a short spell of cooling.
Comparing the three causes
🌍 Milankovitch cycles
Effect: warm and cold periods, including ice ages. Timescale: tens of thousands of years. Very regular.
☀️ Solar variation
Effect: slight warming or cooling. Timescale: about 11 years for the sunspot cycle, with longer quiet spells.
🌋 Volcanism
Effect: short-term cooling after big eruptions. Timescale: a few years. Cannot be predicted well.
Common mistakes
- Saying Milankovitch cycles are caused by people. They are natural changes in the Earth's orbit and tilt.
- Saying volcanoes make the Earth warmer. The ash and sulphur droplets from a big eruption usually cool it for a short time.
- Forgetting the timescale. Always say how long each cause takes, for example about 41,000 years for tilt.
- Mixing up sunspots and the Sun's output. More sunspots go with a slightly more active Sun.
Exam-style question
(a) Name the three natural causes of climate change given in the specification. [3 marks]
(b) Explain how a large volcanic eruption can change the climate. [3 marks]
Model answer
(a) Milankovitch cycles (1), solar variation (1), volcanism (1).
(b) The eruption throws ash and sulphur dioxide high into the atmosphere (1). These form tiny droplets that reflect some of the Sun's energy back into space (1). Less heat reaches the surface, so the Earth cools for a few years (1).
Exam tip
For an "explain" question, write a chain: what is released, what it does, and the result. Each link in the chain is a mark, so do not jump straight to "it gets cooler".