When fossil fuels are burned, for example by industry, in power stations or in vehicles and aeroplanes, gases are released which enter the atmosphere. Although these gases have always been present in the Earth's atmosphere their concentration is gradually increasing as more and more fossil fuels are burned.
Scientists believe that the build-up of these so-called greenhouse gases in the atmosphere, acts like a blanket around the planet, trapping heat inside the Earth's atmosphere. This is the greenhouse effect and the resulting increase in global temperatures is called global warming.
It is thought that the build-up of greenhouse gases impacts on global temperature in two ways.
The gases allow more of the Sun's rays to enter the atmosphere. Some solar radiation is still reflected back into space by the outer parts of the atmosphere, but it is believed that the amount reflected back is gradually reducing.
At the same time, the greenhouse gases absorb more of the solar radiation that is reflected back from the Earth's surface trapping heat and keeping it in the atmosphere. The ability of the atmosphere to capture the Sun's warmth is essential for life on Earth, but if significantly more warmth is being captured this could have a negative impact on the planet.
CFCs have been used in aerosols, (eg hairspray cans), fridges, and in making foam plastics. The resulting ozone holes let harmful ultra-violet radiation in and add to the problems of the greenhouse effect and global warming. CFCs were banned in many countries in the mid-1990s after it was found that they were breaking up the Earth's ozone.
Some scientists say the hole in the ozone layer over Antarctica could disappear within 50 years.