The greenhouse effect is in the news a lot - what's the problem?
This diagram shows what happens to the Sun's energy when it hits the Earth.
Light and infrared rays from the Sun pass through the atmosphere, hit the Earth and warm it up. The warm Earth gives off infrared rays. Some of this travels back through the atmosphere, and out into space. But some of the Earth's infrared is absorbed by certain gases in the atmosphere. This retains heat around the Earth, causing the Earth to become warmer than it would be without the greenhouse effect.
The most important gases that do this are carbon dioxide, water vapour and methane. These gases are called greenhouse gases.
Planet Earth has always had a greenhouse effect - without it, nights would be a lot colder than they currently are. A small greenhouse effect is good for life on Earth.
Have humans affected the greenhouse effect?
The problem is that a lot of things that humans currently do are increasing the amount of greenhouse gases in the atmosphere.
We are burning more fossil fuels - to generate electricity, to make cars work, and for many other reasons. Whenever we burn fossil fuels, carbon dioxide is released.
Large areas of forest are being chopped down - to make land for farms, towns and cities. Trees are good at removing carbon dioxide from the air by photosynthesis, so deforestation (removing forests) leaves more carbon dioxide in the air.
More animal farming leads to more methane being produced. Cows produce a lot of methane because the grass they eat is difficult to digest.
Rubbish dumped in landfill sites produces methane, as microbes digest rotting food waste.
Are we sure that human activity is affecting the atmosphere?
Pretty sure. Scientists have been measuring the amount of carbon dioxide in the atmosphere directly since the 1960's (that's the red line on the graph below). By looking at indirect measurements and ice cores, we can work out what the levels of carbon dioxide were in the past (that's the thin blue line). The increase closely matches what we know about the increased use of fossil fuels.
Are we sure that human activity is affecting the climate?
Most scientists are confident that the world has got warmer in the last 100 years or so, and that increasing carbon dioxide levels are the main reason for this. There might be some other factors at work as well - there are some natural factors that change the climate, and the heat given off by buildings may make a bit of a difference in some places. Because so many factors are involved, it is hard to make accurate models to explain the past and predict the future.
Before scientists publish their work, it gets checked by other scientists. This process is called peer review, and helps to reduce mistakes. The most reliable, peer reviewed models show a strong link between carbon dioxide and global warming, which will get worse if carbon dioxide levels increase even more.
This is worrying - is there any hope?
Yes. If you look at the four things which are increasing greenhouse gas levels, they are all things which can be fixed. We can use different energy resources, so that less fossil fuel is burnt. We can increase the area of forest in the world, instead of cutting trees down. Some of these things are difficult and expensive to do (and very difficult to persuade everyone to do), but the possibilities are there, and new technologies give us the potential to make things better.
Now it's time for some questions.