Natural causes of climate change

Climate change is not a new phenomenon and has occurred long before human influence. Several natural factors contribute to fluctuations in the Earth's climate over time, though they cannot fully explain the rapid temperature rises observed in recent decades.

Variations in solar energy

The Sun's energy output fluctuates due to sunspots, which are dark patches on its surface linked to magnetic activity. These occur in cycles roughly every 11 years, with more sunspots potentially increasing solar heat. However, solar output has remained nearly constant over the past five decades, so it does not account for recent climate shifts.

Changes in Earth's orbit (Milankovitch cycles)

The Earth's orbit around the Sun undergoes cyclical changes affecting how solar energy is distributed.

These cycles include:

• Axial tilt - The Earth's axis is tilted, and this angle shifts every 41,000 years due to the Moon's gravitational pull. A larger tilt angle is linked to higher average temperatures.
• Precession - The Earth wobbles on its axis in a cycle lasting about 26,000 years, altering the distribution of solar energy.
• Eccentricity - The shape of the Earth's orbit changes from nearly circular to slightly elliptical over a 100,000-year cycle. Warmer periods occur with a more elliptical orbit, while cooler periods align with a more circular orbit.

Volcanic activity

Eruptions release ash, gases, and liquids into the atmosphere. Sulphur dioxide can form volcanic aerosols when mixed with water vapour, reflecting sunlight away and causing a cooling effect. Winds spread these materials globally, lowering temperatures far beyond the eruption site.

The role of human activity in global warming

While natural factors have historically driven climate change, human actions have significantly accelerated temperature increases since the mid-20th century. These activities intensify the warming of the planet through the release of additional greenhouse gases.

Human contributions to rising temperatures

• Industrialisation - Since the industrial revolutions in Western Europe and the USA, human activities have dramatically increased greenhouse gas emissions, particularly through burning fossil fuels and industrial processes.
• Deforestation - Clearing forests and burning wood releases stored carbon into the atmosphere, reducing the planet's capacity to absorb greenhouse gases.
• Agricultural practices - Farming activities, such as livestock rearing and rice cultivation, emit potent gases like methane, while vehicle exhausts and fertilisers contribute other harmful emissions.
• Waste management - Decomposing organic waste in landfill sites releases methane, further adding to atmospheric greenhouse gas levels.

The natural and enhanced greenhouse effects

The greenhouse effect is a critical process that regulates the Earth's temperature, but human actions have altered its balance, leading to intensified warming.

Natural greenhouse effect

This is a naturally occurring process that maintains the Earth's temperature at a level suitable for life. Solar radiation enters the atmosphere, some heat is reflected off the surface, and a layer of greenhouse gases traps a portion of this heat, preventing the planet from being approximately 33°C colder.

Enhanced greenhouse effect

Human activities have thickened the layer of greenhouse gases, reducing the amount of heat that escapes into space. This results in a more pronounced warming effect, contributing to global temperature rises.

Key greenhouse gases and their sources

Greenhouse gases are the primary drivers of the warming effect, with human activities significantly increasing their concentrations in the atmosphere. Each gas has distinct sources and varying impacts on climate change.

Major greenhouse gases and their sources

• Carbon dioxide (CO₂) - Accounts for about 77% of the enhanced greenhouse effect.
  • Burning fossil fuels such as coal, oil, and natural gas for energy.
  • Deforestation, including cutting down trees and burning timber.
  • Industrial processes, for example, cement production.
• Methane (CH₄) - Contributes around 14% to the enhanced effect.
  • Emitted by livestock during digestion and from rice cultivation in wetlands.
  • Released during the decay of organic waste in landfill sites.
• Nitrous oxides (NO_x) - Makes up approximately 8% of the enhanced effect.
  • Produced by vehicle exhausts.
  • Generated through agricultural practices and industrial processes.
• Chlorofluorocarbons (CFCs) - Represent about 1% but have a high warming potential.
  • Released from older refrigeration systems and aerosols, though their use is now largely restricted.

These gases collectively amplify the greenhouse effect, with human activities being the dominant source of their increased presence in the atmosphere since the industrial era.