Three hundred twenty miles north of the Arctic circle in Barrow on Alaska’s northern slope, average high temperatures are at or below zero degrees Fahrenheit for 160 days per year. This cold, harsh climate creates a vast expanse of permafrost to help make this Arctic tundra biome one of the biggest carbon sinks on the planet.
These continuously frozen soils thwart the return of CO2 absorbed by plants to the atmosphere by trapping decomposed plant remains within its frozen layers for as many as several thousands of years. But as the Arctic warms at about twice the rate of the rest of the world, scientists are exploring the possibility that these regions may very well transition from carbon sink to carbon source with time.
Barrow, Alaska, in particular, the northernmost city in the United States, has become – in the words of one Alaskan politician, “ground zero for climate change.” A study site located within the Barrow Environmental Observatory has been the subject of numerous intense climate science investigations, including several by Berkeley Lab scientists as part of the Department of Energy’s (DOE) Next Generation Ecosystem Experiments (NGEE-Arctic) project.
The site’s polygon-based microtopography, which can have a significant impact on water distribution and storage across the Arctic landscape, help make it an ideal spot for NGEE-Arctic research into climate change impacts on ecosystem processes. One of these studies is described in a paper published recently in Science of the Total Environment. For their investigation, a research team led by Bhavna Arora applied a novel approach to assess the influence of environmental factors such as soil temperature or snowmelt on carbon dioxide and methane flux at locations across the site in Barrow for the years 2012 through 2014.
“In the Arctic tundra, it can be difficult to interpret how environmental factors such as soil temperature or moisture affect greenhouse gas flux because shifts in the timing of snowmelt and plant phenology can strongly influence this flux,” Arora said. “We took advantage of the Barrow site’s unique ice-wedge topographical features and data we’d collected during previous growing seasons to identify the key drivers of greenhouse gas fluctuations in carbon-rich environments such as these.”
Ice-wedge polygons are the result of frost cracks in the ground due to extremely cold temperatures. The researchers evaluated polygons characterized as high-, flat- or low-centered. Low-centered polygons have low, wet centers bordered by well-defined, typographically higher and dryer edges. High-centered polygons have topographically higher, well-drained centers and no clearly raised edges; while flat-centered polygons have an intermediate relief between high- and low-centered polygons.
“Each polygon type has a distinct pattern of vegetation distribution, snow accumulation, geochemistry, and hydrogeology. Environmental factors such as early snowmelt or higher temperatures may affect different polygon types differently,” Arora explained. “It’s important to take all these unique characteristics into account when attempting to determine how greenhouse gases are behaving in response to the impact of various environmental conditions on the different types of ice wedges.”
For their analysis, the team focused on measurements of air temperature; soil moisture, organic matter depth, and active-layer depth; and CO2 and CH4 fluxes across polygon types and features during growing seasons from the years 2012, 2013, and 2014. Despite having hundreds of reliable measurements, the degree to which soil moisture and temperatures varied over these years made it difficult to interpret their influence on GHG flux. To address this, the team developed a novel entropy classification scheme capable of unraveling the complex relationships between soil characteristics, vegetation structure, polygonal geomorphology and climatic conditions, and of identifying key factors impacting GHG flux variability from year to year.
“It was important for us to extricate the different variables by year and then analyze them in relationship to one another across the three years before attempting to make sense of their impact on CO2 and CH4 flux,” Arora said. “This is critical because without a clear understanding of dominant controls on GHG flux, it would be hard to predict how these fluxes are likely to evolve with a changing climate.”
They found that greenhouse gas fluxes varied considerably in polygonal Arctic tundra. Overall, CO2 fluxes at the Barrow site showed considerable temporal variability at the site, attributed to factors that impact soil such as temperature, moisture, and/or vegetation dynamics, which are influenced by the length of the growing season. Variability in methane fluxes was governed by seasonal vegetation and thaw dynamics, and the highest fluxes in CH4 were consistently associated with low-centered polygons.
“Our results also indicate that flat-centered polygons may become important sources of CO2 during warm and dry years, while high-centered polygons may become important during cold and wet years,” said Arora.
On a broader scale, the U.S. Climate Report issued last week detailing the dire consequences of a changing climate on the U.S. economy, urban infrastructure, and public health reinforced for Arora how important it is to be investigating changes taking place within the carbon-rich Arctic tundra. Required by Congress every four years and issued by 13 federal agencies and the U.S. Global Change Research Program, the report culminates years of research by leading U.S. climate scientists. The 2018 report is said to be one of the starkest assessments of how failing to reign in climate warming will bring about increasingly intense and frequent wildfires and storms, in effect crippling infrastructure that was not built to withstand such extremes.
“Over the last several years, our research at the Barrow, Alaska site has illuminated how significantly patterns are shifting across different polygon types,” Arora said. “The U.S. Climate Report strongly issued the warning that carbon dioxide emissions must be reigned in. Studies like ours help identify the dominant factors impacting environments that have been reliably keeping carbon dioxide out of the atmosphere for centuries. It’s important for us to keep monitoring these environments so that we can improve our predictions about how greenhouse gases are behaving in response to a changing climate, and if such environments will continue to be a significant carbon sink. Better climate models allow us to protect regions like the one near Barrow, Alaska, which are at the crossroads of the climate change crisis.”