Touchdown on the gravel runway at Cherskii in remote northeastern Siberia sent the steel toe of a rubber boot into my buttocks. The shoe had sprung free from gear stuffed between me and my three colleagues packed into a tiny prop plane. This was the last leg of my research team’s five-day journey from the University of Alaska Fairbanks across Russia to the Northeast Science Station in the land of a million lakes, which we were revisiting as part of our ongoing efforts to monitor a stirring giant that could greatly speed up global warming.
These expeditions help us to understand how much of the perennially frozen ground, known as permafrost, in Siberia and across the Arctic is thawing, or close to thawing, and how much methane the process could generate. The question grips us—and many scientists and policy makers—because methane is a potent greenhouse gas, packing 25 times more heating power, molecule for molecule, than carbon dioxide. If the permafrost thaws rapidly because of global warming worldwide, the planet could get hotter more quickly than most models now predict. Our data, combined with complementary analyses by others, are revealing troubling trends.
Leaving the Freezer Door Open
Changes in permafrost are so worrisome because the frozen ground, which covers 20 percent of the earth’s land surface, stores roughly 950 billion tons of carbon in the top several tens of meters. (More permafrost can extend downward hundreds of meters.) This carbon, in the form of dead plant and animal remains, has accumulated over tens of thousands of years. As long as it stays frozen beneath and between the many lakes, it is safely sequestered from the air.
But when permafrost thaws, the carbon previously locked away is made available to microbes, which rapidly degrade it, producing gases. The same process happens if a freezer door is left open; given long enough, food thaws and begins to rot. Oxygen stimulates bacteria and fungi to aerobically decompose organic matter, producing carbon dioxide. But oxygen is depleted in soil that is waterlogged, such as in lake-bottom sediments; in these conditions, anaerobic decomposition occurs, which releases methane (in addition to some carbon dioxide). Under lakes, the methane gas molecules form bubbles that escape up through the water column, burst at the surface and enter the atmosphere.
Anaerobic decomposition is the primary source of methane in the Arctic. Melting ice in permafrost causes the ground surface to subside. Runoff water readily fills the depressions, creating many small, newly formed lakes, which begin to spew vast quantities of methane as the permafrost that now lines their bottom thaws much more extensively. Scars left behind reveal that this process has been going on for the past 10,000 years, since the earth entered the most recent interglacial warm period. Satellite recordings made during recent decades suggest, however, that permafrost thaw may be accelerating.
Those recordings are consistent with observations made at numerous field-monitoring sites across Alaska and Siberia maintained by my Fairbanks colleague Vladimir E. Romanovsky and others. Romanovsky notes that permafrost temperature at the sites has been rising since the early 1970s. Based on those measurements, he calculates that one third to one half of permafrost in Alaska is now within one degree to one and a half degrees Celsius of thawing; in some places worldwide, it is already crossing that critical zero degrees C threshold.
Ongoing observations, made by my research team during trips to Cherskii and numerous other sites and by our colleagues, reinforce the sense that thawing is accelerating and indicate that the emissions could be much greater than anticipated. My group’s latest estimates are that under current warming rates, by 2100 permafrost thawing could boost methane emissions far beyond what would be produced by all other natural and man-made sources. The added greenhouse gas, along with the extra carbon dioxide that exposed, thawing ground would release, together could raise the mean annual temperature of the earth by an additional 0.32 degree C, according to Vladimir Alexeev, also at Fairbanks.