Our modern civilization is based on hydrocarbons: fuel and coal are the main sources of energy of our societies. Burning fuel and coal releases a great amount of carbon dioxide (CO2) into the Earth's atmosphere.
CO2 is a greenhouse gas, which means that its presence in the atmosphere contributes to global warming. But the role of CO2 and other greenhouse gases is ambiguous: our planet would not shelter life if greenhouse gases did not contribute to heat up our atmosphere.
The problem is that modern human activities release too much greenhouse gases into the atmosphere at the same time, which causes a swift global warming on a scale never experienced before by our planet. The risk is that it could unbalance the Earth's atmospheric and oceanic cycles on a global scale, which would have very unpredictable consequences for our climate.