What environmental problem can result from burning coal to generate electricity?

Burning fossil fuels like coal releases extra carbon dioxide (CO2) that is over and above the carbon movements of the earth's carbon cycle. This is extra CO2 because it has been out of the way for millions of years, since the time the fossil fuels were laid down. The earth's carbon cycle, which moves carbon out of the atmosphere, largely through vegetation's photosynthesis, is unable to cope with this extra CO2, (and man's deforestation activities didn't help either!). So carbon dioxide and other greenhouse gases build up in the atmosphere, increasing the normal greenhouse effect to an accelerated greenhouse effect, which is causing global warming.