Global countdown: topics for class discussions
The hole in the ozone layer
This natural filter around the planet had always prevented the Sun's harmful ultraviolet radiation from reaching the Earth's surface. It is naturally created and naturally destroyed. However, chemicals (CFCs and HCFCs) used in fridges, aerosols and some industrial processes are now making holes in the filter, increasing cases of skin cancer and cataracts, and killing plankton - which forms the basis of the oceans' food chain.
Ozone (O3) is a gas that is found in the stratosphere, 10 to 50 kilometres above our heads. For every million molecules of air, there are just three molecules of ozone. All the Earth's ozone would form one layer a few millimetres thick.
A natural blanket of gases surrounds the Earth, including water vapour, carbon dioxide and methane. They trap heat, which would otherwise be lost - and the trapped heat maintains a temperature in a range that supports life.
Human activity releases carbon dioxide (CO2) into the atmosphere, magnifying the natural greenhouse effect and raising the global temperature. This increases the effects of hurricanes, high tides and droughts and causes rising sea levels and loss of habitats. Burning fossil fuels - coal, oil and gas - causes most CO2 emissions. Transport, especially air travel, is a big contributor.
The effect of these increases could be a worldwide rise in average global temperature of 1 to 3.5 C by the end of this century. And it's getting worse. Climate change and ozone depletion have always been seen as separate problems. But in 2002, a Nasa study linked greenhouse gases to ozone destruction. The ozone layer was expected to make a full recovery by 2040 because CFC use has been cut. But increased temperatures and moisture in the stratosphere slow the recovery of the ozone layer. So there will be little improvement by 2040.
Pollutants damage the health of people and animals and destroy landscapes.
Landfill sites give off methane gas, which is 20 times more powerful than CO2 and adds to the greenhouse effect.
Water droplets are formed by sulphur dioxide emitted from burning fossil fuels, particularly in coal-fired power stations. It falls as rain, sometimes hundreds of miles from its source, killing lakes and forests and harming animals and plants.
Unfair distribution of global wealth is a major cause of environmental damage such as rainforest destruction and overfishing in our oceans. People living at subsistence levels find it hard to look after their land in a sustainable way, using up today what they should be saving for tomorrow.