How are underground fossil reserves both a "carbon source" and a carbon sink"?

Underground fossil reserves are a carbon source, because as an example coal is made of very compressed fossilized plants, and when it burns it releases tons of carbon into the air. This is why burning coal, oil and other natural gases is not great for our atmosphere. This is even worse because we're cutting and burning down forests, so plants can’t take in the extra carbon, and by doing this we’re adding even MORE carbon to the atmosphere! - Piper

Underground fossil reserves are carbon “sources” when they are extracted from the ground.  The majority of the earth’s natural carbon supply is stored deep within the earth’s crust within underground fossil reserves.  When natural gases, coal, and oil are extracted from the ground and burned, the carbon within them is released.  

Underground fossil reserves are carbon “sinks” when they are left in the ground. If all this carbon was left in the ground the effect would be extraordinary.  Carbon would begin to return to the earth and our climate would start the process of recovering from climate change.  Sadly, however, these companies want money for themselves so they will continue to pump these natural recourses out of ground, further contributing to climate change. - Aidan

Drilling for and burning our underground fossil fuels is one of the biggest problems facing the world today; it is the main contributor to global climate change. Underground fossil fuels contain most of the planet's stored carbon and yet we keep burning them for energy. The underground stores would be a great thing because they store so much carbon but we burn the fuel stored in them, leading to our warming ecosystems. - Josh

Fossil reserves are pure carbon sinks, and when left untouched they stay that way and go about their own cycle. When people mine fossil fuels, carbon no longer remains in the fossil reserves, it is then burned and sent into the atmosphere. This makes it nearly impossible for the carbon to go back into the fossil reserves. - Ryan

Burning underground fossil reserves is one of the main things that is causing global warming by transferring carbon from deep underground into the atmosphere. The underground reserves are definitely changing from a great carbon sink into a great carbon source for the atmosphere. - CJ

Underground fossil reserves (coal, oil, peat and gas) are carbon sinks because it takes carbon to make them in the first place.  But they are a carbon source when they are burned because they release fossil fuels into the atmosphere as carbon dioxide (which is really bad). - Alan

Coal underground can easily store a great amount of carbon. That means coal is a carbon sink. Coal power plants that burn coal release carbon dioxide and methane into the atmosphere, and that is very, very bad. If you haven’t caught my drift yet, I mean that when coal is burned and/or used in a coal power plant it turns coal into a carbon source for the atmosphere. - Emily

Underground fossil reserves of coal, oil, and gas are by themselves simply just big hunks of carbon sitting in the ground. That makes them a carbon sink. However when they are burned, the carbon that was just sitting there is now released into the atmosphere. This makes underground fossil reserves both a carbon sink and a carbon source. - Owen

Oil is a carbon source because when you burn it, it releases carbon dioxide, making it a carbon source. But when you leave it in the ground, it stores carbon, making it a carbon sink. - Hudson

Underground fossil reserves hold enormous amounts of carbon, but they are no longer considered a carbon sink. Now they are considered a carbon source due to the large amounts of excess carbon released when fossil fuels are burned. - Leo