What is carbon dioxide? Carbon, in when it joins with oxygen, is a gas. Humans release it into the atmosphere all the time. We breathe in oxygen, and breathe out carbon dioxide. Natural sources of atmospheric carbon include gases emitted by volcanoes, and respiration of living things. Plants take it in to build their leaves, stems, trunks and flowering parts. Solid carbon dioxide, made by freezing the liquid below 109 degrees below zero, is sometimes called dry ice. Dry ice can be used to carbonate beer and sodas. It can even be used to quickly freeze cream and sugar into ice cream.