Indeed, the distinction is important because the level of worry among scientists differs depending on the source—by land or by sea—of the permafrost emissions. Following Connor’s article, Andrew Revkin, who writes the New York Times Dot Earth blog, argued that:

If you read the Independent of Britain, you’d certainly be thinking the worst. The newspaper has led the charge in fomenting worry over the gas emissions, with portentous, and remarkably similar, stories in 2008 and this week.

If you read geophysical journals and survey scientists tracking past and future methane emissions, you get an entirely different picture.

Revkin cited a recent paper in the Journal of Geophysical Research, which described a worrisome feedback loop wherein climate change warms Arctic waters and reduces ice extent, thereby lengthening the summer open-water season and facilitating even more solar heating of the water column. However, the research also found that subsea permafrost and methane hydrates along the northern coast of Siberia, much of which lie deep beneath the seafloor, are relatively stable and that the observed thawing has been underway for thousands of years.

As Revkin put it in a 2010 post about oceanic Arctic methane, it’s uncertain whether these emissions are new or just newly observed. It’s also uncertain whether the thawing permafrost portends a “tipping point” beyond which there will be a rapid, inexorable melt or a more gradual, mitigable melt.

This was point addressed by Revkin’s New York Times colleague Justin Gillis in a long and wonderfully nuanced front-page article that brought the focus back to terrestrial permafrost deposits. According to his report:

In the minds of most experts, the chief worry is not that the carbon in the permafrost will break down quickly — typical estimates say that will take more than a century, perhaps several — but that once the decomposition starts, it will be impossible to stop.

Nonetheless, Gillis stressed that with an uptick in Arctic previously rare Artic wildfires, scientists fear “extensive burning could lead to a more rapid thaw of permafrost” on land. A 2008 paper in Geophysical Research Letters suggested sea-ice loss could also speed terrestrial melting. Scientists don’t seem to exhibit the same level of worry about subsea permafrost (which is not to say that they are unconcerned).

A couple days after his front-pager, Gillis followed up with a blog post explaining the discrepancy and why he chose to focus on terrestrial permafrost:

In my article over the weekend about the climate risks from buried Arctic carbon, I omitted any discussion of one issue that sometimes appears in the news: methane deposits under relatively shallow seawater near the coasts of Siberia, Canada and Alaska. It was a purposeful omission because my piece focused on carbon buried on land, which presents a climate risk if it eventually emerges as methane or carbon dioxide.

However, given the alarming headlines about methane in the ocean, as seen here and here for example, I did some additional reporting. What I learned about ocean methane was reasonably reassuring, with the caveat that scientists would like to know a great deal more about these deposits before declaring for certain that the hazard is minimal.

… Nobody regards the case as closed, and more research is necessary, but most of the methane deposits lining the margins of continents would seem to be fairly low on the list of scientific concerns about global warming.

Gillis’s post disparaged the notion, popularized by the news media, that there is a methane “time bomb” waiting to go off in the ocean. Even on land, there is little support for this idea, Revkin pointed out in yet another recent post on the subject.

“It’s not a time bomb; it’s an important additional cumulative emissions source,” Edward Shurr—one of the forty-one scientists who wrote the Nature analysis describing the larger-than-realized store of Arctic carbon—told him.

It’s important to keep these details—including the difference between terrestrial and oceanic permafrost emission—in mind, because they have direct bearing on the “list of scientific concerns about global warming” that Gillis mentioned, and on how we might prioritize various measures to address climate change.

Curtis Brainard is the editor of The Observatory, CJR's online critique of science and environment reporting. Follow him on Twitter @cbrainard.