The arctic has long held a large reserve of methane gas which is being released as the permafrost and ice melts, which can lead to troubling short and long term affects.
First off, Methane captures heat many times greater then CO2, and all though it doesnt last as long, the amount of methane that'll be released if action isnt taken [Up to 50gt+ over time] is akin to doubling the amount of CO2 in our air, this can lead to sudden rises in temperature that would last for quite a while.
If we act now, or begin to act, we can stop the methane release before it becomes a cycle of heating in the arctic, which in turn would release more into the atmosphere, raising temperatures dangerously.
This issue was discovered only a few years ago, and it needs to be dealt with before it becomes an issue.