It contains an estimated 1,400-1,850 gigatons of carbon -- man's carbon emissions to date are about 385 GtC, from burning fossil fuels and land use changes.
That's 3.5 to 5 times as much as has been emitted so far.
Of course, this thawing will occur over centuries.
And it's why it's ridiculous to look only at the global average change in surface temperatures.
This is something you'd completely miss if you buy that global warming paused 17 years 4 months ago, as Lord Monckton recently claimed. It's not true anyway, but even if it was, such a broad brush stroke misses important regional changes, such as the fact that the North Pole region has warmed by 0.80°C in that time period, according to UAH data for the lower troposphere.
The NASA scientists say:
"Permafrost soils are warming even faster than Arctic air temperatures - as much as 2.7 to 4.5 degrees Fahrenheit (1.5 to 2.5 degrees Celsius) in just the past 30 years.... As heat from Earth's surface penetrates into permafrost, it threatens to mobilize these organic carbon reservoirs and release them into the atmosphere as carbon dioxide and methane, upsetting the Arctic's carbon balance and greatly exacerbating global warming."They don't give any numbers -- I guess they will have to wait for their papers. But there is a surprisingly small amount of methane in our current atmosphere: only about 5.2 Gt.
Are atmospheric methane levels increasing? Yes, but only a little. Here is the graph of monthly CH4 measurements from a station in Yonagunijima, Japan. It just happens to be a station I found that updates only a month or to after the present, and is relatively far north:
So after whatever caused last decades pause in CH4 levels -- the consensus seems to be related to the implosion of the Soviet Union, though no one I've read seems especially sure -- methane levels are on the rise again.