In the comment, he looks back at the scientific work in the field of methane flux research since more than 20 years, and worries about the fact that Arctic methane emissions remain almost as uncertain as they were then, and points out that the research on methane fluxes needs to be extended.
"Methane is a powerful greenhouse gas — it is some 25 times stronger per tonne than carbon dioxide in warming the climate. So not knowing why it varies in the atmosphere is a serious problem. Worse, tipping points in the climate system that cause sudden surges in methane release from the Arctic would have a substantial effect on global temperatures", he writes.
Read the comment here (link to Nature)
Read an interview with Torben R. Christensen about the comment and his research (in Swedish)