• Short wavelength radiation from the Sun (mostly visible light) passes through the atmosphere and warms the surface.
• The warm surface gives off longer wavelength infrared radiation.
• Greenhouse gases (carbon dioxide, methane, water vapour) absorb infrared and re-emit it in all directions, some back towards the surface. Nitrogen, oxygen and argon do not.

The greenhouse effect itself keeps the Earth warm enough for life. The problem is adding more greenhouse gases, which warms it further.

• Carbon dioxide: burning fossil fuels, deforestation (burning, and fewer trees to absorb it).
• Methane: cattle, rice paddies, rotting waste in landfill.
• Most climate scientists agree, from peer-reviewed evidence, that human activity is warming the Earth. Models are simplified and the data is complex, so predictions carry uncertainty, and media reports can oversimplify.
• Possible effects: rising sea levels, more severe storms, changed rainfall and food production, species moving.
• A carbon footprint is the total greenhouse gas emitted over the life cycle of a product, service or event. Reduce it with renewable energy, efficiency, carbon capture and fewer journeys.

percentage change = (new value − original value) ÷ original value × 100

1. Carbon dioxide rose from 318 ppm (1960) to 412 ppm (2020)
2. increase = 412 − 318 = 94 ppm
3. percentage increase = 94 ÷ 318 × 100 = 29.6% (3 s.f.)

Always divide by the starting value. Dividing by the final value gives a smaller, wrong answer.

Pollutant	Where it comes from	Problem
carbon monoxide	incomplete combustion	toxic, colourless, odourless: the blood carries less oxygen
particulates (soot)	incomplete combustion	global dimming, lung damage
sulfur dioxide	sulfur impurities in fuels	acid rain, breathing problems
oxides of nitrogen	nitrogen and oxygen from the air react in hot engines	acid rain, breathing problems

1. S + O₂ → SO₂: 32 g of sulfur gives 32 + 2 × 16 = 64 g of sulfur dioxide
2. 100 tonnes of coal with 2% sulfur contains 2 tonnes of sulfur
3. so it makes 2 × 64 ÷ 32 = 4 tonnes of sulfur dioxide