What you'll learn

• What climate change means and why it is measured at a global scale.
• The three natural causes OCR expects: changes in solar energy, Earth’s orbit, and volcanic activity.
• How human activities increase heat-trapping gases and cause recent global warming.
• How to compare natural and human causes clearly in exam answers.

Start point: climate, weather, and warming

Before you learn the causes, make sure the key words are secure. Climate change is not about one hot day or one wet winter. It is about long-term patterns.

Climate change is studied at a global scale because the atmosphere and oceans move heat and gases around the planet. For example, carbon dioxide released in one country mixes through the atmosphere and can affect temperatures worldwide.

The energy balance: the prerequisite idea

Earth’s temperature depends on the balance between energy coming in and energy going out.

Solar radiation is energy from the Sun. Most arrives as short-wave radiation, which can pass through the atmosphere and warm the land and oceans. Earth then gives off long-wave infrared radiation, which is heat energy travelling back towards space.

Some incoming radiation is reflected by clouds, ice and bright surfaces. Albedo means how reflective a surface is: high-albedo surfaces, such as ice sheets, reflect more sunlight.

Natural causes of climate change

A natural cause is a process not caused by human activity. A scientific theory is an explanation supported by evidence, not just a guess.

OCR expects you to know three natural causes: variations in energy from the Sun, changes in Earth’s orbit, and volcanic activity. Orbital changes include eccentricity meaning orbit shape, axial tilt meaning the angle of Earth’s axis, and precession meaning the wobble in the direction Earth’s axis points. For volcanic activity, an aerosol is a tiny particle or droplet suspended in air, and the stratosphere is the atmospheric layer above most day-to-day weather.

1. Variations in energy from the Sun

The Sun’s energy output is not perfectly constant. Solar output means the amount of energy the Sun emits. If solar output increases, more incoming solar radiation reaches Earth, which can contribute to warming. If it decreases, less energy reaches Earth, which can contribute to cooling.

One linked idea is the sunspot cycle. Sunspots are darker, cooler-looking areas on the Sun’s surface linked to changes in solar activity. Sunspot numbers tend to rise and fall over roughly 11-year cycles.

However, recent global warming cannot be explained well by solar variation alone. Satellite measurements do not show a long-term increase in solar energy large enough to match the rapid warming seen since the mid-20th century.

2. Changes in Earth’s orbit

Earth’s path around the Sun and the angle of Earth’s axis change slowly over long timescales. These are often called Milankovitch cycles, which are natural cycles affecting how solar energy is distributed across the planet.

The three main orbital changes are:

• Eccentricity: the shape of Earth’s orbit changes from more circular to more oval. This affects Earth’s distance from the Sun during the year.
• Axial tilt: the angle of Earth’s axis changes slightly. More tilt can make seasons more extreme; less tilt can make seasons milder.
• Precession: Earth’s axis wobbles, changing the timing of seasons in relation to Earth’s position in its orbit.

These cycles are important for explaining past glacial periods and interglacial periods. A glacial period is a colder time when ice sheets expand; an interglacial period is a warmer time between glacials.

3. Volcanic activity

Large explosive volcanic eruptions can affect climate by sending ash and sulphur dioxide high into the atmosphere. Sulphur dioxide can form sulphate aerosols in the stratosphere.

These aerosols reflect some incoming solar radiation back to space. This means less energy reaches Earth’s surface, so global temperatures can fall for a short time.

A classic located example is Mount Pinatubo in the Philippines, which erupted in 1991. It released large amounts of material into the atmosphere and was linked with short-term global cooling of roughly 0.5°C for around 1-2 years, though exact figures vary by source.

Volcanoes can also release carbon dioxide, but volcanic carbon dioxide emissions are far smaller than human emissions globally each year. So volcanoes are not a strong explanation for the long-term warming trend today.

Human causes: the enhanced greenhouse effect

The greenhouse effect is a natural process that keeps Earth warm enough for life. Without it, Earth would be much colder. The problem is the enhanced greenhouse effect, where human activities add extra greenhouse gases to the atmosphere, trapping more heat.

The diagram below compares the natural greenhouse effect with the enhanced greenhouse effect.

How the enhanced greenhouse effect works

The process is a chain:

1. Short-wave radiation from the Sun passes through the atmosphere and warms Earth’s surface.
2. Earth emits long-wave infrared radiation back towards space.
3. Greenhouse gases absorb some of this outgoing heat and re-radiate it in different directions, including back towards Earth.
4. Human activities increase the concentration of greenhouse gases, so more heat is retained.
5. Earth’s average surface temperature rises, causing global warming and wider climate change.

Main human activities increasing greenhouse gases

The Industrial Revolution was the period from the late 1700s when many economies began using factories, steam power and large amounts of coal. Since then, fossil fuel use has grown massively.

Fossil fuels are coal, oil and natural gas formed from ancient plants and animals over millions of years. Burning them releases carbon dioxide that had been stored underground.

A carbon sink is a store that absorbs more carbon than it releases. Forests and oceans are major carbon sinks, so damaging them can increase the amount of carbon dioxide left in the atmosphere.

Named examples help your answer feel geographical. You could refer to deforestation in the Amazon Basin in Brazil, forest loss and peatland burning in Indonesia, or fossil fuel energy use in major economies such as China, India, the USA and the EU. Use examples carefully: total emissions, emissions per person and historical emissions can show different patterns.

Comparing natural and human causes

Natural causes have affected climate in the past, but they work on different timescales and often have different effects.

• Solar variation can affect incoming energy, but recent solar output does not match the strength and pattern of recent warming.
• Orbital changes are very slow and are linked to ice age cycles over thousands of years.
• Volcanic eruptions usually cause short-term cooling because aerosols reflect sunlight.
• Human activity has rapidly increased greenhouse gases since industrialisation, especially through fossil fuel burning, deforestation, farming and industry.