Causes of Climate Change » Humans and the Enhanced Greenhouse Effect

The natural greenhouse effect

The last lesson, Natural Causes of Climate Change, showed that nature alone cannot explain the fast warming of recent decades. The main cause is something people have added to the air. To understand it, start with the greenhouse effect that has always been there.

Key terms:

• Greenhouse gas: a gas in the atmosphere that absorbs heat given off by the Earth and sends some of it back towards the surface.
• Natural greenhouse effect: the warming of the Earth by the greenhouse gases that are naturally in the atmosphere.
• Enhanced greenhouse effect: extra warming caused by the greenhouse gases that human activity has added to the atmosphere.
• Fossil fuels: coal, oil and natural gas, formed from the remains of living things over millions of years.
• Parts per million (ppm): a way of measuring how much of a gas is in the air. 400 ppm means 400 molecules of the gas in every million molecules of air.

Think of a car parked in the sun. The windows let sunlight in, but they stop some of the heat getting out, so the car warms up. The atmosphere does something similar. Here are the steps:

1. Energy from the Sun passes through the atmosphere as short-wave radiation.
2. The Earth's surface absorbs this energy and warms up.
3. The warm surface gives off heat, which travels upwards as long-wave radiation.
4. Greenhouse gases in the atmosphere absorb some of this heat.
5. The gases send some of the heat back towards the surface, and the rest escapes to space.
6. The surface is warmed again, so the Earth stays warmer than it would be without the gases.

This natural effect is helpful. Without it, the average temperature of the Earth would be about -18°C instead of about 15°C, and most life could not survive.

The enhanced greenhouse effect

The problem is not the greenhouse effect itself. The problem is that people have added a lot more greenhouse gas, mainly since the Industrial Revolution. The more gas there is, the more heat is absorbed and sent back down. That is the enhanced greenhouse effect, and it is why the climate is now warming.

The main greenhouse gases and where they come from

Human sources of greenhouse gases

Source	Gas released	How it happens
Burning fossil fuels for energy	Carbon dioxide	Coal, oil and gas are burnt in power stations to make electricity and in homes for heat. The carbon in them joins with oxygen.
Transport	Carbon dioxide	Cars, lorries, ships and planes burn petrol, diesel and aviation fuel.
Deforestation	Carbon dioxide	Trees store carbon. When they are burnt or rot, it goes into the air, and fewer trees are left to take carbon dioxide in.
Cattle	Methane	Cattle produce methane in their stomachs as they digest grass, and release it into the air.
Rice paddies	Methane	Flooded fields have little oxygen, so microbes in the mud release methane.
Fertilisers	Nitrous oxide	Microbes in the soil turn some of the nitrogen in fertiliser into nitrous oxide.
Industry	Carbon dioxide and others	Factories burn fuel for heat and power. Cement making and some chemical processes also release gases.
Landfill	Methane	Buried rubbish, such as food waste, rots without much oxygen and gives off methane.

Burning fossil fuels for energy and transport is the biggest source of carbon dioxide. Farming is a major source of both methane and nitrous oxide.

Rising carbon dioxide

Air bubbles trapped in Antarctic ice show that before the Industrial Revolution the amount of carbon dioxide was about 280 ppm. Direct measurements at Mauna Loa in Hawaii showed an average of about 425 ppm in 2024. That is more than 50% higher than before the Industrial Revolution, and the level is still rising each year.

When you describe this, say that the rise is rapid. A natural change of this size would normally take thousands of years, but the recent rise has happened since the 1700s.

Putting it together

1. People burn fossil fuels, clear forests, farm animals and crops, run industry and bury rubbish.
2. This puts extra carbon dioxide, methane and nitrous oxide into the atmosphere.
3. More greenhouse gas absorbs and returns more heat to the surface.
4. The Earth warms more than it would naturally, and the climate changes.

Exam-style question

Fig. 1.1 is a table of made-up data. It shows the average concentration of carbon dioxide in the air above Region X in five years.

Year	1960	1980	2000	2010	2020
Carbon dioxide (ppm)	315	338	369	390	414

Fig. 1.1

(a) Identify two greenhouse gases, other than carbon dioxide, that people add to the atmosphere. [2 marks]

(b) Describe the change in carbon dioxide shown in Fig. 1.1. [3 marks]

(c) Explain how burning fossil fuels leads to the enhanced greenhouse effect. [4 marks]

(d) Discuss how agriculture contributes to the enhanced greenhouse effect. [6 marks]