IAS-Climate-Natural Causes

​World at Risk – Lesson 2: Natural Causes of Climate Change

Open Ended

If climate has changed for millions of years, what natural factors could explain those changes?

​Lesson Objectives

​By the end of this lesson you will be able to: • Describe the main natural causes of climate change. • Explain how orbital changes, volcanic eruptions and solar variation affect global temperature. • Analyse evidence showing the impact of these processes. • Evaluate the relative importance of natural and human factors.

Open Ended

Which of the sources (tree rings, ice cores, ocean cores, pollen, historical) could also show natural causes of change?

​Key Question

​How do natural processes cause long-term climate fluctuations on Earth? Today we will examine three major mechanisms: 1 Orbital (Milankovitch) cycles 2 Volcanic activity 3 Solar variation

​Long-Term Climate Change

​Over geological time, the Earth’s climate has warmed and cooled many times. These long-term changes are mainly linked to the Earth’s orbit and how much solar energy reaches the planet’s surface.

​Milankovitch Cycles Overview

Milutin Milankovitch proposed that regular variations in the Earth’s orbit change how much solar energy reaches the planet.
These variations operate over tens of thousands of years and together explain the pattern of ice ages and interglacials seen in the geological record.

1 Eccentricity – The Shape of Earth’s Orbit

​The Earth’s orbit around the Sun slowly changes shape from nearly circular to more elliptical about every 100 000 years. When the orbit is more elliptical, the difference between the closest and farthest points from the Sun is greater. This means the Earth receives less solar energy at certain times, leading to colder glacial periods. When the orbit is more circular, solar energy is more evenly distributed and the climate is warmer and more stable.

Open Ended

Which would produce colder conditions, and why?

​2 Obliquity – The Tilt of the Axis

​The Earth’s axis is tilted at an angle that varies between 21.5° and 24.5° roughly every 41 000 years. A greater tilt increases the difference between summer and winter. A smaller tilt makes seasons milder and allows snow and ice to persist through cooler summers. When snow survives the summer, glaciers advance and global temperatures fall.

​Key Point:
Smaller tilt = weaker seasons = glaciers grow → global cooling. Larger tilt = stronger seasons = glaciers melt → global warming.

Open Ended

Explain in one sentence how a change in tilt could start a glacial period.

​3 Precession – The Wobble of Earth’s Axis

​As Earth spins, its axis slowly wobbles like a spinning top. This wobble completes one full cycle roughly every 23 000 years. Precession changes the timing of the seasons relative to Earth’s position around the Sun. For example, 11 000 years ago the Northern Hemisphere’s summer occurred when Earth was farthest from the Sun, making summers cooler and helping glaciers grow.

​Key Point:

​Precession changes when seasons occur and can strengthen or weaken existing climate trends caused by eccentricity and tilt.

Open Ended

Which of these has the greatest long-term influence on ice ages?

​How the Cycles Combine

​The three Milankovitch cycles overlap. When all three reduce the amount of solar energy reaching Earth’s surface, conditions become much colder and a glacial period begins. When they combine to increase solar energy, glaciers retreat and an interglacial begins.

​Evidence for Orbital Change

​Ice-core and ocean-sediment data show temperature changes that match Milankovitch cycles. Glacial periods occur roughly every 100 000 years, following the same rhythm as changes in Earth’s orbit. This strong correlation is evidence that orbital forcing is a major driver of natural climate variation.

​Volcanic Activity

​Volcanoes can influence the Earth’s climate by releasing gases and ash into the atmosphere. Large eruptions inject sulphur dioxide (SO₂) high into the stratosphere. This forms sulphate aerosols, which reflect sunlight and cause short-term cooling.

​Short-Term Cooling from Eruptions

​When sunlight is blocked by aerosols, global temperatures can drop by up to 0.5–1 °C for one to three years. However, volcanic effects are temporary because particles eventually fall back to Earth.

​Case Study – Mount Pinatubo (1991)

​Mount Pinatubo in the Philippines erupted in June 1991. It released about 20 million tonnes of sulphur dioxide into the stratosphere. Global temperatures fell by around 0.6 °C for nearly two years. After the aerosols cleared, temperatures returned to their previous levels.

​Long-Term Volcanic Effects

​Over millions of years, repeated eruptions release carbon dioxide into the atmosphere.

This gradual build-up of CO₂ can increase global temperatures and cause long-term warming.

Open Ended

What relationship do you notice?

​Solar Variation

​The Sun’s energy output changes slightly over time.
Sunspots — darker, cooler regions on the Sun’s surface — indicate periods of high magnetic activity.

When sunspots are numerous, the Sun emits slightly more radiation, increasing temperatures on Earth. The typical sunspot cycle lasts about 11 years.

​The Maunder Minimum (1645–1715)

​Between 1645 and 1715, sunspot activity was very low — a period known as the Maunder Minimum. It coincided with some of the coldest winters of the Little Ice Age in Europe. This suggests reduced solar output contributed to cooling at that time.

​Modern Solar Activity

​Modern satellite data show that total solar output has remained almost constant since 1978. However, global temperatures have continued to rise rapidly. This indicates that solar variation cannot explain recent warming.

​Other Natural Factors

​Other natural processes can influence climate, including: • Ocean circulation changes – shifts in major currents alter heat distribution. • Plate tectonics – movement of continents changes ocean and wind patterns over millions of years. • Albedo feedback – ice and snow reflect sunlight; when they melt, darker surfaces absorb more heat, increasing warming.

​Evaluating Natural Causes

​Natural processes explain past climate changes but not the rapid rise in temperature since 1850. Solar activity has remained steady. Volcanic effects cause short-term cooling, not warming. Orbital cycles act over tens of thousands of years. This means the current trend must have an additional, human cause.

Open Ended

Natural processes still influence the climate today, but their effects are small compared to human activities.
Write a short paragraph explaining why geographers must consider both natural and human factors when analysing climate change.

​Summary

​Today you learned that natural processes such as orbital cycles, volcanic eruptions, and solar variation have shaped past climates. However, none of these can explain the rapid warming since 1850. This means human activities are now the dominant influence on global climate.

​Lesson Objectives

​By the end of this lesson you will be able to: • Describe the main natural causes of climate change. • Explain how orbital changes, volcanic eruptions and solar variation affect global temperature. • Analyse evidence showing the impact of these processes. • Evaluate the relative importance of natural and human factors.

Open Ended

Open Ended