Arctic permafrost has become a recent star in the climate change conversation, capturing the attention of scientists, activists and policymakers alike because of its ability to emit large quantities of carbon dioxide as well as methane -- a particularly potent though relatively short-lived greenhouse gas -- when it thaws. As temperatures rise in the Arctic, scientists are increasingly concerned that permafrost will become a major contributor to the greenhouse gas emissions driving global warming.
Studies of permafrost emissions are important in both estimating current levels of greenhouse gas emissions and making predictions for the future. So far, most studies have focused on the way permafrost behaves in the summer, when Arctic temperatures are at their highest. But a new paper in Proceedings of the National Academy of Sciences says we've been overlooking the importance of cold-season emissions of methane gas in particular -- and possibly underestimating their impact in the future.
"The cold period in general is the time of the year that is warming the fastest in these Arctic ecosystems," said the new study's lead author Donatella Zona, an assistant professor at San Diego State University and research fellow at the University of Sheffield.
Until recently, scientists have known very little about how much methane is released by permafrost during the cold winter months, she said. But she noted, "Really, if we're thinking about the future of climate change, we need to understand if this time of the year is important."
Currently, most of the models that scientists use to predict future methane emissions only factor in warm-season methane emissions, assuming that the vast majority of permafrost emissions will occur when temperatures are at their highest. These models are important because they allow scientists to make projections about how severe global warming will be in the future and help policymakers make decisions about how much -- and how quickly -- global carbon emissions need to be reduced.
So Zona, along with a group of nearly 20 other scientists, decided to investigate whether cold-season methane emissions were really as negligible as the models have assumed. They examined data collected from five different sites in Alaska between June 2013 and January 2015, as well as data collected from aircraft in the same region.
"Donatella and her team are to be commended for making the first year-round measurements of [methane] in the Arctic," said Stan Wullschleger, an environmental scientist at Oak Ridge National Laboratory, in an email to The Post. ". . .The fact that this was done not just at one site, but multiple sites, is a breakthrough in our ability to quantify [methane] budgets for tundra ecosystems."
The researchers found that cold-season methane emissions are not only not negligible -- they're pretty significant. While emissions varied somewhat from one site to the next, Zona said that, overall, emissions from September to May accounted for about half of all the methane emitted from those sites throughout the entire year.
This might seem a little baffling when you consider the fact that methane is generally released as Arctic soil thaws -- a process that should be most pronounced during the warmest part of the year. Zona said the key to understanding where cold-season emissions come from lies in the way Arctic soil is structured and how it reacts to changes in temperature.
Arctic soil layers are structured kind of like a sandwich in the winter, Zona said. There's a top layer (the very surface of the soil) and a bottom layer that both freeze as temperatures drop. In between them, there's a layer of soil -- found just below the surface -- that can remain unfrozen for months, even as the temperature drops. This period of time is known as the "zero curtain" period, because temperatures in the unfrozen middle layer tend to hover right around zero degrees Celsius. The researchers believe that the majority of methane emissions produced during the winter occur during this zero curtain period, while the middle soil layer is still unfrozen.
The researchers also discovered another characteristic of cold-season methane emissions that isn't well reflected in current models. According to the authors, most models assume that wetter tundra sites produce more methane than drier sites -- but they found that dry sites actually seemed to be producing the most methane.
These are all important points when it comes to predicting how much methane the Arctic will release in the future.
Estimates of current Arctic methane emissions are more or less accurate, Zona said. But she believes the models are likely to underestimate how much methane will be produced in the future, if they don't take cold-season emissions into account. This is because the zero curtain period will likely exist for longer and longer amounts of time if winter temperatures continue to rise in the Arctic. Future increases in snowfall could also help extend the zero curtain period, since snow tends to insulate the soil and keep it warm.
"The problem with modeling is that there's not much data available from sites," said Martin Heimann, director of the Max Planck Institute for Biogeochemistry, noting that different areas in the Arctic emit methane at different rates. Expanding the database with more on-the-ground measurements, such as those collected in this study, will be crucial to coming up with the most accurate understanding of the processes going on in the Arctic and the way they will affect Earth's future climate.
In the meantime, the study identifies some key aspects of Arctic methane emissions that, until now, have been largely overlooked - and suggests that a major updating of climate models may be overdue. The paper encapsulates "fascinating research that is neither captured in previous measurements or in our models," Wullschleger said. "We still have a lot to learn."