The primary human source of carbon dioxide (CO2) in the atmosphere is from the burning of fossil fuels for energy production and transport. Changes in land use and deforestation also contribute significantly. Trees, for example, are natural 'carbon sinks' - they absorb carbon dioxide while alive and when they are destroyed, carbon dioxide is released into the atmosphere. Once in the atmosphere, most of the carbon dioxide stays there for 50 to 200 years, and some of it stays there indefinitely.
What are fossil fuels?
Oil, coal and natural gas are called fossil fuels because it is believed they are formed from the remains of plants and animals living millions of years ago. All fossil fuels are made up of hydrocarbons, and release carbon dioxide when burned.
Carbon dioxide emissions and the carbon cycle.
Currently, fossil fuels are the primary source for almost 80 percent of the industrial world's energy. They are a non-renewable resource, so we'll eventually run out of them. However, if we want to avoid dangerous climate change we can only afford to burn less than one-fourth of the known oil, coal and gas reserves - burning any more will almost certainly release enough carbon dioxide to change the climate dramatically.
Who burns the most?
The simple answer is that because industrialised nations have bigger economies and have been burning fossil fuels for a hundred years or more, they are responsible for most of the cumulative carbon dioxide emissions in the atmosphere. However, all nations are responsible to one degree or another.
This can, and should, change in the future. In some countries, it is changing today. Thanks to renewable energy technology and energy efficiency, economic development and fossil fuel use need not be coupled.
However, among the world's top economies, the US still stands out as the number one polluter. With less than 5 percent of the world's population, the US is the world's largest producer of greenhouse gases and is responsible for almost a quarter of global emissions of carbon dioxide.
But to look at carbon dioxide emissions only by country is perhaps too narrow. The same question applies per business or even individual. Someone driving a gas-guzzler of a car is burning more fossil fuels then someone with a more efficient car, for example. Of course nations and businesses must be held accountable, but as individuals we each also make decisions that affect the climate.