Causes of Climate Change » Natural Causes of Climate Change

Natural causes: the climate has always changed

The evidence in the last lesson, Evidence for Climate Change, shows that the climate has gone up and down for hundreds of thousands of years, long before there were many people. So something natural must be able to change the climate. Geographers study three main natural causes: changes in Earth's orbit, changes in the Sun, and volcanic eruptions.

Key terms:

• Milankovitch cycles: slow, regular changes in Earth's orbit and tilt that change how much sunlight reaches different parts of the planet.
• Glacial period (ice age): a long cold spell when ice sheets spread across large areas of land.
• Interglacial: a warmer spell between two glacial periods.
• Sunspot: a dark, cooler patch on the surface of the Sun.
• Solar output: the amount of energy the Sun gives out.

Cause 1: orbital changes (Milankovitch cycles)

Earth goes round the Sun in an orbit and spins on a tilted axis. The scientist Milutin Milankovitch worked out that three small changes repeat over very long periods. Together they change how much sunlight reaches each part of the Earth in each season.

When these cycles line up so that northern summers are cooler, snow that falls in winter may not melt completely. Year after year it builds up into huge ice sheets and a glacial period begins. When the cycles line up the other way, the ice melts back and an interglacial begins. Over the last 800,000 years the ice cores show glacial periods and interglacials alternating, about every 100,000 years. We are in an interglacial now.

Remember the limitation: these cycles are very slow. Their effect on climate builds up over thousands of years.

Cause 2: sunspots and solar output

The Sun is the source of almost all of Earth's heat, so any change in the Sun's energy can change the climate. Sunspots are dark patches on the Sun. They look dark because they are cooler than the area around them, but the Sun gives out slightly more energy when there are many sunspots. The number of sunspots rises and falls in a cycle of about 11 years.

Between about 1645 and 1715 there were very few sunspots. This period is called the Maunder Minimum. It happened during the Little Ice Age, a cold period in Europe, and many scientists think the lower solar output was one of the reasons for the cold. Other causes, such as volcanic eruptions, probably played a part too.

Cause 3: volcanic activity

A large explosive eruption blasts ash and gases high into the atmosphere. The ash falls out within days or weeks. The gas sulphur dioxide is more important for climate. High up, it turns into tiny droplets that spread around the world and reflect sunlight back into space. Less sunlight reaches the surface, so the Earth cools.

1. A huge eruption sends ash and sulphur dioxide high into the atmosphere.
2. The sulphur dioxide forms tiny droplets that spread around the globe.
3. The droplets and ash reflect sunlight back into space.
4. Less energy reaches the surface, so global temperatures fall.
5. After a year or two the droplets fall out and temperatures recover.

Example: Mount Pinatubo, 1991. The eruption in the Philippines put about 20 million tonnes of sulphur dioxide into the atmosphere. The average temperature of the Earth fell by about 0.5°C over the next year or two, then climbed back.

So volcanoes cause short-term cooling, not long-term warming. Volcanoes do give out carbon dioxide, but the amount is far smaller than the amount people produce.

Natural cause	How it works	Timescale
Orbital changes	Changes the sunlight reaching the Earth in each season	26,000 to 100,000 years per cycle
Sunspots	Changes the energy given out by the Sun	About 11 years per cycle, with longer quiet spells
Volcanic eruptions	Sulphur dioxide and ash reflect sunlight	One to three years after one eruption

Why natural causes cannot explain recent warming

The global temperature has risen by about 1.1°C since 1850–1900, and most of that rise has happened since the mid 20th century. Natural causes do not fit this:

• Orbital changes are far too slow. They work over thousands of years, but this warming has happened in decades.
• Solar output has not risen. Satellites have measured the Sun since the 1970s. Its output has not gone up since the 1980s, but temperatures have kept climbing.
• Volcanoes cool the Earth. Their effect is short, and big eruptions would have brought temperatures down, not up.

Scientists therefore look for another cause. The next lesson, Humans and the Enhanced Greenhouse Effect, explains it.

Exam-style question

Fig. 1.1 is a table of made-up data. It shows the average number of sunspots seen each year and the average winter temperature in Region X for five periods.

Period	1	2	3	4	5
Sunspots per year	55	50	5	3	48
Winter temperature (°C)	3.1	3.0	1.6	1.5	2.9

Fig. 1.1

(a) Define the term sunspot. [2 marks]

(b) Describe the pattern shown in Fig. 1.1. [3 marks]

(c) Explain how a large volcanic eruption can cool the climate. [4 marks]

(d) To what extent can natural causes explain the warming of the Earth since the mid 20th century? [6 marks]