The soils that encircle the northern reaches of the Arctic are a vast repository for carbon in the form of undecayed organic matter from dead vegetation. The enormous amount of material trapped in the permafrost contains enough carbon to double the current amount of carbon dioxide in the Earth’s atmosphere.
Most of the blame for climate change has been placed on the growing levels of carbon dioxide in the atmosphere, but methane also plays a major role. Estimates are that about 1/5 of greenhouse effect warming is caused by methane in the atmosphere. There is far less of it than carbon dioxide, but methane is tremendously more effective at trapping heat.
Hydrogen is the most abundant element in the universe. Estimates are that it comprises 75% of all matter. There is plenty of it here on earth too, but almost none of it is in its elemental form. It is mostly bound up in compounds like water.