The air, explained.

Part 1Today's atmosphere — the numbers to know

For about 200 million years the proportions of gases in the atmosphere have been roughly the same as they are today. You need these figures by heart.

The air is about four-fifths nitrogen and one-fifth oxygen. More precisely: roughly 78% nitrogen (N₂) and 21% oxygen (O₂). The last 1% is small amounts of other gases — mainly argon, with carbon dioxide at a small fraction of a per cent, plus varying amounts of water vapour.

Part 2The first billion years — volcanoes and oceans

Evidence about the early atmosphere is limited, so scientists' ideas are uncertain — but one widely accepted theory goes like this.

For the Earth's first billion years there was intense volcanic activity. Volcanoes released the gases that formed the early atmosphere. The atmosphere was probably mainly carbon dioxide, with little or no oxygen — a bit like the atmospheres of Mars and Venus today. Volcanoes also released water vapour, and nitrogen, plus small amounts of methane and ammonia.

As the Earth cooled, the water vapour condensed to form the oceans. This was a turning point: once there were oceans, carbon dioxide dissolved into the water. Dissolved CO₂ then formed carbonate precipitates that made sediments on the sea bed — locking carbon away and lowering the CO₂ in the air.

Part 3How oxygen appeared

When algae and plants evolved, they began to photosynthesise. Photosynthesis takes in carbon dioxide and releases oxygen, so it did two jobs at once: it slowly raised the oxygen in the air and helped lower the carbon dioxide.

Algae first appeared about 2.7 billion years ago. Over the next billion years or so, oxygen built up to a level high enough for animals to evolve.

Carbon dioxide was also reduced because it was locked into sedimentary rocks (like limestone) and into fossil fuels — coal, crude oil and natural gas — which formed from the buried remains of ancient plants and plankton. That carbon stays locked away until we burn the fuels.

Part 1The greenhouse gases

Some gases in the atmosphere keep the Earth warm enough to support life. They are called greenhouse gases. The three you must know are carbon dioxide (CO₂), methane (CH₄) and water vapour (H₂O).

These gases are present in small amounts, but they have a big effect, because they absorb the heat (infrared) energy that the Earth radiates and stop it escaping straight back to space.

Part 2How the greenhouse effect works

Here is the chain of events you should be able to describe — in the right order:

1. Energy from the Sun reaches the Earth as short-wavelength radiation (including visible light), and much of it passes straight through the atmosphere. 2. The Earth's surface absorbs this energy and warms up. 3. The warm Earth radiates energy back out, but now as longer-wavelength infrared radiation. 4. Greenhouse gases absorb this outgoing infrared and then re-radiate it in all directions — including back towards the Earth. That trapped energy keeps the surface warmer than it would otherwise be.

Part 1More greenhouse gases, more warming

Over the last century, human activities have increased the amounts of carbon dioxide and methane in the atmosphere. The main causes are:

Carbon dioxide — from burning fossil fuels (in power stations, cars and industry) and from deforestation (fewer trees means less CO₂ is removed by photosynthesis). Methane — from farming livestock (especially cattle), landfill sites and growing rice.

Based on peer-reviewed evidence, scientists believe that this extra greenhouse gas is causing the average temperature of the Earth's surface to increase — this is global climate change.

Part 2The effects of climate change

An increase in average global temperature may lead to several potential effects. You should be able to describe some — and recognise that the size of each is uncertain:

Polar ice caps and glaciers melt, and warmer water expands, so sea levels rise — increasing flooding of low-lying land. Weather patterns change, giving more frequent and severe storms. Changes in the amount and timing of rainfall affect where crops can grow. Temperature and water stress change the distribution of wildlife and can reduce biodiversity.

Part 3Carbon footprint — and why it's hard to cut

The carbon footprint is the total amount of carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, service or event. (Notice it isn't only CO₂ — it includes methane and the rest.)

We can reduce the carbon footprint by cutting these emissions, for example by using renewable energy or nuclear instead of fossil fuels, increasing energy efficiency, capturing and storing carbon dioxide, and using taxes or laws (carbon caps) to limit emissions.

But reductions are hard to achieve in practice. Why? Because of the scientific disagreement over the causes and consequences, a lack of international cooperation, the economic cost and the impact on people's lifestyles and jobs, plus the slow pace of developing and rolling out new technology and educating the public.

Part 1Complete vs incomplete combustion

Most fuels contain carbon and hydrogen (they are hydrocarbons), and many also contain some sulfur as an impurity. When a fuel burns, the atoms in it combine with oxygen — and the products depend on how much oxygen is available.

In complete combustion, there is plenty of oxygen, so the carbon becomes carbon dioxide and the hydrogen becomes water. These aren't pollutants in the toxic sense (though CO₂ is a greenhouse gas). The trouble starts when there isn't enough oxygen — incomplete combustion — which produces carbon monoxide and particulates (soot).

Part 2The four pollutants and their effects

You need to know four pollutants, where each comes from, and the harm each causes.

Two important details: sulfur dioxide and oxides of nitrogen both dissolve in rain to make acid rain, which damages buildings, trees and lakes. And oxides of nitrogen don't come from the fuel — they form because the high temperatures inside an engine make the nitrogen and oxygen already in the air react together.