Poo releases methane which contributes to global warming.
Barry's Response - Around us and all over the world, we have tons and tons of microbes decomposing organic matter and emitting more tons of carbon and methane (which is 21 times more effective than carbon dioxide as a greenhouse gas) into the air every year. Poo is just one example of that natural process.
Decomposition occurs at a faster rate when the ambient temperature increases, but as long as the amount of rotting material stays the same, the total amount of GHG release should neither increase nor decrease. One would simply expect that it would happen faster and be over sooner.
But does the amount change? If the stuff currently trapped under ice becomes free for compost, then the total pool increases. It has happened before and will again repeatedly over the cycles of millions of years.
Another by-product of decomposition is mold, and especially the spores it releases. This can lead to air-quality related illnesses. Read more about them here.