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Burning fossil fuels releases carbon dioxide, a human cause of climate change
Earth's climate has always changed. The spec says climate change is the result of natural factors and human factors. Natural factors have driven the swings between cold and warm periods over thousands of years. Human factors matter most for the rapid warming of recent times.
To understand both, we start with the greenhouse effect.
The Sun's energy passes through the atmosphere and warms the Earth's surface. The surface gives off some of this energy as heat. Certain gases in the atmosphere trap some of that heat and send it back down. This keeps the planet warm enough for life. Without it, the average temperature of the Earth would be around -18 degrees C instead of about 15 degrees C.
The gases that trap heat are called greenhouse gases. They include carbon dioxide, methane, nitrous oxide and water vapour.
Key terms:
The Earth's path around the Sun and the tilt of its axis change slowly over tens of thousands of years. This changes how much sunlight reaches different parts of the Earth. Over long cycles, it can tip the planet into a glacial period or an interglacial period.
Big eruptions throw huge amounts of ash and gas high into the atmosphere. These particles block some sunlight, so the Earth can cool for a year or two. Volcanoes also release carbon dioxide, but only a small amount compared with human activity.
The Sun does not give out exactly the same amount of energy all the time. It goes through cycles. When solar output is higher, more energy reaches the Earth and temperatures can rise. When it is lower, temperatures can fall.
These natural causes work slowly, or in short bursts. Orbital changes take thousands of years. A volcanic cooling effect fades within a few years. Neither can explain the speed of warming seen since the late 1800s. That is why human factors are so important.
Farming adds greenhouse gases: cows release methane as they digest food
People have added large amounts of greenhouse gases to the atmosphere since the Industrial Revolution. Three main human factors are in the spec.
Fossil fuels are burned to make electricity, to heat buildings, to run factories and to power cars, lorries, ships and planes. Burning them releases carbon dioxide that was locked underground for millions of years. Fossil fuel use is the biggest single human cause of climate change.
Farming releases greenhouse gases in several ways:
As the world population grows, more food is needed, so farming gases grow too.
Trees take in carbon dioxide as they grow and store the carbon in their wood. When forests are cut down, two things happen. Fewer trees are left to absorb carbon dioxide. Also, if the trees are burned or left to rot, the stored carbon goes back into the air as carbon dioxide. You will study the causes of deforestation in the Amazon rainforest in a later lesson.
Carbon dioxide in the atmosphere has risen from about 280 parts per million before the Industrial Revolution to over 420 parts per million today. The rise is mainly due to burning fossil fuels and cutting down forests.
The natural greenhouse effect is helpful. The problem is that human activities keep adding extra greenhouse gases. More gas means more heat trapped, so the planet warms further. This is the enhanced greenhouse effect.
Do not say the greenhouse effect is bad. It is natural and we need it. The problem is the enhanced greenhouse effect. Also, do not say the ozone layer causes climate change. Climate change is about greenhouse gases trapping heat. Finally, a good answer names the gas and says where it comes from, such as methane from cattle.
(a) Name one natural factor that can cause climate change. [1 mark]
(b) Explain how agriculture contributes to the enhanced greenhouse effect. [3 marks]
(a) Any one of: volcanic activity, orbital changes or changes in solar output (1).
(b) Farm animals such as cattle release methane, which is a greenhouse gas (1). Fertilisers used on crops release nitrous oxide (1). These gases trap more heat in the atmosphere, which adds to the enhanced greenhouse effect (1).
In an "explain" question, link each point with a reason. Name a gas, say where it comes from, then say what it does to heat in the atmosphere.