This article discusses the issue of methane and CO2 being released into the atmosphere as the Arctic permafrost melts. When it does, it will release lots and lots of GHGs into the atmosphere. We do not know how bad it will be, but the risk is enormous. Here is a quote:
“The permafrost contains twice as much carbon as is currently in Earth’s atmosphere,” Douglas says. “That’s 1,600 billion metric tons.” That’s an awful lot.