Abstract A molecule of methane is nearly 30 times more powerful as a greenhouse gas than carbon dioxide is. Because of methanogens in ruminants and rice fields, agriculture is a large source of methane, double that coming from fossil fuels, but the recent rise in methane is thought to be due to wetlands and global warming–induced changes in the hydrological cycle. Methane emissions would be even higher if not for degradation by aerobic methanotrophic bacteria and anaerobic microbial symbioses. Methane emissions from the Arctic could increase as global warming continues. One concern is the melting of methane hydrates, which may house five times more methane than what is in known natural gas reserves. Thawing of permafrost in the Arctic now releases more carbon dioxide than methane, but that could change with abrupt thawing. More so than for carbon dioxide, what microbes do is essential for understanding methane and its contribution to climate change.