Why is the combustion of fossil fuels so dangerous to the atmosphere?

First look at what happens when we burn organic fuel such as wood. This produces carbon dioxide, a greenhouse gas. But this only replaces the atmospheric carbon dioxide that the tree had absorbed in order to grow the wood. There is no net addition of carbon dioxide to the atmosphere.

When we burn fossil fuels such as coal, oil and natural gas, we are also adding carbon dioxide to the atmosphere. But this is from carbon that has been sequestered underground for millions of years. This means there is an increase in the amount of carbon dioxide in the atmosphere, leading to an enhanced greenhouse effect and hence global warming. The extent of this can be seen when we realise that atmospheric carbon dioxide levels have risen by about 35 per cent from the long-term range of 260-280 parts per million (ppm) to 380 ppm since the beginning of the Industrial Age.