The seasonally affected active layer (the surface layer of soil just above the permafrost that freezes and thaws each year) and the underlying permafrost contain a substantial amount of organic matter whose carbon is frozen and locked away. As temperatures rise so does the possibility that more and more of this carbon may be released in to the atmosphere, further enhancing the impacts of climate change.
Scientist Tingjun Zhang of the National Snow and Ice Data Center and his colleagues are investigating changes to the thickness of the active layer, as well as how permafrost is degraded, how much carbon it contains and how fast the carbon is released from permafrost to the atmosphere. They are also developing new satellite-based techniques to expand their research on the dynamics of the active layer to a broader geographic region.
Zhang is a professor at the University of Colorado, Boulder and at the College of Earth and Environmental Sciences at Lanzhou University in Gansu Province, China where he leads a group conducting permafrost studies on the northern Qinghai-Tibetan Plateau.
Zhang spoke with Field Notes during his travels in China about his National Science Foundation-funded project Exploring the Dynamics of the Active Layer and Near-Surface Permafrost across the North Slope of Alaska.
Field Notes (FN): What the active layer and why is it important?
Tingjun Zhang (TZ): The active layer is actually not part of permafrost because it freezes in winter and thaws in summer. Permafrost is defined by temperature. Anything in the soil—even rocks—that has a temperature at or below 0°Celcisus continuously for two consecutive years is considered permafrost. The active layer is seasonal and is not part of permafrost.
Our study looks at the surface all the way down to the permafrost. Permafrost can be anywhere from a few meters deep to more than several hundred meters deep. In Prudhoe Bay, Alaska, the permafrost extends about 620 meters below the surface. In Barrow, Alaska, the permafrost extends down roughly 350 meters. In Siberia, we have permafrost deeper than 1,500 meters!
FN: What questions do you hope to answer about the dynamics of the active layer?
TZ: We’ve been working on the North Slope of Alaska since 1996 to look at permafrost and climate change—my primary interest. We want to know what are the responses of the active layer to changes in climate condition. We are also looking at the changes in temperature, the changes to the thickness of the active layer and other factors.
FN: You are using novel techniques to collect data. Please tell us about that.
TZ: We are now using remote sensing and radar data to measure the thickness of the active layer. We developed a new method using satellite data that can detect surface deformations and changes.
When the water in the active layer undergoes a phase change—or changes from water to ice—it causes the active layer to expand. It’s just like when you put water in the freezer—the bottle may break because the water expands as it freezes. It’s the same in nature. If the active layer is half a meter and it freezes, the volume expansion of the ice will make the surface rise because underneath is solid permafrost. This surface deformation is about 4 cm and is called frost heave. The satellites measure the up and down movement of the active layer on a regional scale and at a very high resolution, about 15 meters to 30 meters. From there, we can calculate the active layer thickness and any changes.
In the past, to study the active layer, we would probe locations using a metal rod. This layer is very soft so you can put a metal rod down in the soil to see how deep it was in the summer. But that technique is very site specific, gave very limited data and was limited by manpower.
Using satellite data to map the active layer thickness is better because you can cover a broader area. We’ve done this work in Barrow, Prudhoe Bay, and around Toolik Lake August. We hope to eventually do this for the whole North Slope region.
FN: How are changes in the active layer key to understanding the potential impacts of climate change in northern Alaska and other regions?
TZ: Our field measurements, as well as the modeling community, say that the affects of climate change will be greatest in the Arctic compared with the rest of the globe. So if warming is already happening, the active layer will thaw deeper. If ground ice in the active layer melts and we have a larger scale of surface deformation, this would have a number of serious impacts.
First, the permafrost contains substantial organic material. The amount of carbon frozen in the permafrost right now is about two times the carbon in the atmosphere. The carbon is frozen in the permafrost, but if the climate warms and it’s released in to the atmosphere it will further enhance global warming. We call this the “carbon bomb.”
We also have engineering aspects in the Arctic to consider. If the ground ice melts, any construction—like highways, roads, airports, you name it—their foundations could be damaged. This would have an impact on communities and economies. Permafrost thaw would also create coastal erosion and collapse. This would also affect communities in the region.
FN: This research and your techniques may be relevant to other regions. Do you have any plans to expand your current project or conduct similar studies elsewhere?
TZ: We are doing modeling work here in China at the Qinghai-Tibetan Plateau [in Central Asia]. Permafrost here is warmer than in Alaska by a couple of degrees. If global warming accelerates permafrost thaw here at the Qinghai-Tibetan Plateau, this will dramatically impact the release of carbon in to the atmosphere.
The Qinghai-Tibetan Plateau contains about six to seven percent of the Northern Hemisphere’s total permafrost, but has roughly the same amount of carbon (about four or five percent).
We have measurements for how the permafrost is thawing and how the carbon is being released on the Qinghai-Tibetan Plateau. In Prudhoe Bay and Barrow, permafrost temperatures are still quite cold, at -7 or -8°C on average. On the Qinghai-Tibetan Plateau, it’s -.5 or -0.2°C.
So warming will affect permafrost here first and will have a dramatic impact on carbon release. That is why I’m dong further research at Qinghai-Tibetan Plateau. —Alicia Clarke
For more information about Tingjun Zhang and his research, visit: http://nsidc.org/research/bios/zhang.html