The composition of the atmosphere; the difference between weather and climate and how they are measured; the greenhouse effect and the enhanced greenhouse effect; and the causes and consequences of climate change.

What this dot point is asking

The SQA wants you to state what the atmosphere is made of, explain the difference between weather and climate and how each is measured, describe the natural greenhouse effect and how humans are enhancing it, and outline the causes and consequences of climate change.

What the atmosphere is made of

The atmosphere is the layer of gases held around the Earth by gravity. Its composition is roughly:

The atmosphere supplies oxygen for respiration and carbon dioxide for photosynthesis, shields the surface from harmful radiation, and holds the heat that makes the Earth habitable.

Weather versus climate

These two terms are often confused but mean different things.

Weather is measured with instruments: a thermometer for temperature, a rain gauge for precipitation, an anemometer for wind speed, and a barometer for air pressure. Climate is worked out by averaging weather records over many years, which is why a single cold day does not disprove a warming climate: climate is about long-term averages, weather is about the moment.

The greenhouse effect

The greenhouse effect is natural and essential: without it the Earth would be far too cold for life.

• Energy from the Sun reaches the Earth and warms the surface.
• The warmed surface gives off heat (infrared radiation).
• Greenhouse gases in the atmosphere (carbon dioxide, methane and water vapour) absorb some of this heat and re-radiate it back towards the surface, trapping it and keeping the planet warm.

The enhanced greenhouse effect and climate change

Human activity is adding extra greenhouse gases, which traps more heat and raises the average temperature. This is the enhanced greenhouse effect, the cause of human-driven climate change.

The main human causes are:

• Burning fossil fuels (coal, oil, gas) for energy and transport, releasing extra carbon dioxide.
• Farming, especially livestock and rice growing, and waste in landfill, releasing methane, a powerful greenhouse gas.
• Deforestation, which removes trees that would otherwise absorb carbon dioxide by photosynthesis.

The consequences include:

• Rising sea levels, as ice melts and seawater expands, flooding low-lying land.
• More extreme weather, such as stronger storms, floods, heatwaves and droughts.
• Shifting habitats and species ranges, with some species unable to move or adapt fast enough, leading to loss of biodiversity.
• Effects on food production as growing conditions change.

Examples in context

Example 1. Methane from livestock. Cattle release methane as they digest grass. Because methane is a powerful greenhouse gas, large numbers of livestock add significantly to the enhanced greenhouse effect, showing that climate change is driven by more than just burning fuels.

Example 2. A 30-year climate record. A weather station records temperature every day. Averaging these records over 30 years shows a steady rise in mean temperature, even though individual years vary. This illustrates how climate (the long-term average) can be warming while day-to-day weather still includes cold spells.

Try this

Q1. State the two gases that make up most of the atmosphere and their approximate percentages. [1 mark]

• Cue. Nitrogen (about 78 per cent) and oxygen (about 21 per cent).

Q2. Explain why a single very cold day does not show that climate change is not happening. [2 marks]

• Cue. Weather is the day-to-day state in one place, but climate is the long-term average (about 30 years), so one cold day does not change the warming trend.