9.3.1  Natural causes of climate change

The amount of energy reaching the Earthas heat from the sun has varied over millennia due to changes in the intensity of activity in the sun’s magnetic core. The average temperature of the Earth’s surface increases when solar activity is high and decreases when solar activity is reduced. These changes in the intensity of solar activity take place in cycles lasting hundreds of years, with profound effects on the Earth’s climate. For example, between the 17th and 19th centuries, winters in the northern hemisphere were unusually long and cold, leading to reduced growing seasons, widespread crop failure and famine. This period has become known as the ‘Little Ice Age’ and is thought to have been caused partly by low solar activity.

The global climate can also be influenced by volcanic eruptions. The explosion of dust particles, ash, water vapour and gases such as sulphur dioxide into the atmosphere is dispersed around the globe and reflects sunlight back into space.

  • What effect do you think volcanic eruptions could have on the Earth’s surface temperature?

  • If some of the energy from sunlight is reflected back into space by the dust in the atmosphere, the average surface temperature of the Earth is likely to be reduced.

Even though the volcanic eruption may last only a few days, the effect on the global climate may last for several years. For example, the average surface temperature in the northern hemispherewas reduced by about 0.5 °C for a few years after the eruption of Mount Pinatubo in the Philippines in June 1991 (Figure 9.4).

Figure 9.4  The eruption of Mount Pinatubo, Philippines, released huge amounts of dust particles and gas into the atmosphere.

Solar activity and volcanic eruptions cannot be modified by human interventions. However, human activity is increasingly seen as a direct cause of climate change from the 20th century onwards.

9.3  What are the causes of climate change?

9.3.2  How do greenhouse gases contribute to climate change?