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Natural causes or human activity?

The world's climate varies naturally as a result of interactions between the ocean and the atmosphere, changes in the earth's orbit, fluctuations in energy received from the sun and volcanic eruptions.

However, there is now strong evidence and almost universal agreement that significant global warming cannot be explained just by natural variations. The changes seen over recent years, and those which are predicted over the next 80 years, are thought to be mainly as a result of human behaviour.

The Intergovernmental Panel on Climate Change (IPCC) is a scientific body set up by the UN to look at climate change. They say that human activity is the main driver of the changes seen in climate.

• The Intergovernmental Panel on Climate Change Opens new window
• Watch a short film on the causes and effects of climate change Opens new window
• Transcript of a short film on the causes and effects of climate change

The greenhouse effect

The earth is surrounded by a layer of gases which act like the glass walls of a greenhouse: they let the sun’s rays enter, but stop much of the heat from leaving. This is a natural process, and it is this layer of ‘greenhouse gases’ (mainly carbon dioxide and water vapour) that keeps the planet warm enough for people and animals to live.

However, as humans emit more greenhouse gases into the atmosphere, the greenhouse effect becomes stronger. More heat is trapped, and the earth's climate begins to change unnaturally.

Since the industrial revolution, which began in the 18th century, the amount of carbon dioxide (CO2) in the atmosphere has increased by 35 per cent. In fact, the concentration of CO2 is now higher than at any point in the past 650,000 years.

• United Nations Framework Convention on Climate Change - climate change science Opens new window

Why greenhouse gases have increased

Human activity has changed the amount of greenhouse gases in the atmosphere in three important ways:

1. Forests have been cut down. Trees absorb carbon dioxide so, with fewer trees, more carbon dioxide builds up in the atmosphere. Also, the agriculture and industry that replaces the forests can often be a source of emissions. The Intergovernmental Panel on Climate Change (IPCC) estimates that deforestation produces 5.9 billion tonnes of CO2 per year or 18 per cent of global CO2.
2. Burning fossil fuels like coal, oil and gas releases greenhouse gases. In 2005, burning fossil fuels emitted about 27 billion tonnes of carbon dioxide.
3. The world’s population is growing. This is leading to an increased demand for food, livestock and energy, which is in turn leading to increased emissions.
   

• More on deforestation from the Prince's Rainforest Project Opens new window

The main contributors to climate change in the UK

In the UK, around:

• 4 per cent of emissions come from industrial processes
• 7 per cent are from agriculture – for example methane emissions from livestock and manure, and nitrous oxide emissions from chemical fertilisers
• 21 per cent are from transport 
• 65 per cent come from the consumption of fuel to generate energy (excluding transport)

About 40 per cent of emissions in the UK are the result of decisions taken directly by individuals. The biggest sources of emissions for most people are likely to be:

• the energy you use in your home (the main use is heating)
• driving
• air travel

Other things in people's homes contribute to climate change indirectly. Everything, from furniture to computers, from clothes to carpets, uses energy when it is produced and transported – and this causes emissions to be released.

• Greener living: a quick guide to what you can do
• View the 'My CO2' film

More useful links

• The Department for Environment, Food and Rural Affairs - climate change Opens new window
• Climate change frequently asked questions from the G8, Gleneagles 2005 Opens new window
• Climate change explained by the Energy Saving Trust Opens new window
• Watch a humorous short film about the impact of climate change on one farmer Opens new window
• Transcript of humorous short film on the effects of climate change on one farmer