Fossil fuels contain carbon which has been buried in the ground for millions of years. When burnt, the carbon is released into the atmosphere in the form of carbon dioxide , a colourless odourless gas.
Carbon dioxide is one of the gasses known as greenhouse gasses , so-called because they trap the sun's heat within the atmosphere in much the same way as the glass in a greenhouse traps the heat.
As the concentration of greenhouse gasses in the atmosphere increases, the global temperature rises. The effects of this are wide ranging, and include:
- Melting of polar ice caps and glaciers in mountainous regions
- Rising sea levels due to melted ice
- Reduced albedo of the planet due to melted ice
- Flooding due to melting of glaciers
- Flooding due to increased sea levels
- More extreme weather, like Hurricane Katrina
Collectively, these effects are known as climate change.
Our planet exists in a delicate equilibrium with negative feedback mechanisms to keep it in balance. If the proportion of greenhouse gasses in the atmosphere gets too high, the planet gets too hot, it could set off all manner of positive feedback cycles, sending our climate and weather systems completely out of control.
The June 2010 Zero Carbon Britain report includes a chapter with a very good explanation of the current state of climate change science.