Arctic tundra ecosystems are hot spots for production and storage of methane, a potent greenhouse gas. As air temperatures rise, tundra soils may release more and more methane into the atmosphere. These soils freeze for several months each year, but new research by Arndt et al. suggests that the Alaskan tundra is freezing later each year, resulting in higher methane emissions in the fall.
Prior research has shown that Arctic tundra soils release a significant amount of methane well into the fall season during a period known as the “zero curtain.” In this window, the timing of which varies depending on location and from year to year, air temperatures are below 0°C, but underlying soils remain unfrozen—sometimes through January—because of slow latent heat release. However, because of limited data, zero-curtain methane emissions have been poorly understood.
The new investigation examined soil temperature and methane emissions data from four monitoring sites across the tundra in Alaska’s North Slope. One station is located at a National Oceanic and Atmospheric Administration (NOAA) observatory just outside Utqiaġvik, the northernmost city in the United States. There, atmospheric methane concentrations in late fall and early winter have been rising above background levels for 2 decades.
The researchers found that from 2001 to 2017, soils froze later, with the zero curtain extending further into winter by about 2.6 days per year. NOAA observatory data suggested that this later freezing was linked to higher above-background methane concentrations in the fall. The team’s data also showed that from 2013 to 2017, methane emissions dropped each year after the zero curtain closed.
The findings suggest that later soil freezing may increase methane release from Arctic tundra ecosystems. Because soils remained unfrozen after air temperatures dropped well below 0°C, the researchers concluded that air temperature is a poor predictor of methane emissions. Instead, long-term records of soil temperature could be essential for predicting the effects of climate change on methane release in the tundra. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2019JG005242, 2019)
—Sarah Stanley, Freelance Writer
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