Findings suggest that the Arctic may be even less of a carbon sink than previously thought.
A multi-institution team of scientists measured a large pulse of greenhouse gases [carbon dioxide (CO2) and methane (CH4)] released from the frozen Arctic tundra during a two-week period in late May to early June 2014 when soils started to thaw. Little is known about such releases, and the researchers investigated the circumstances, mechanism, likelihood, and outcomes of these events. They show that the pulse was the result of a delayed mechanism, in which gases produced in fall were trapped in the frozen soils and released in spring. The team linked hydrology, biogeochemistry, and geophysics to uncover the pivotal roles of warmer fall weather and spring rain-on-snow events, implying these pulses may be more frequent in the future.
The research identified a large, underrepresented source of carbon in the Arctic. The findings suggest that the Arctic may be even less of a carbon sink than previously thought. The eddy covariance measurements imply that to calculate Arctic carbon budgets more accurately, early spring fluxes should be measured and taken into account. The dynamics of this offset in the context of climate change are not yet known, but it appears that the conditions that lead to the accumulation and abrupt emission of the stored gases may become more frequent with warming.
Measurements of a large pulse of carbon gases emitted from the tundra ecosystem were made near Barrow, Alaska, in May 2014. The pulse was large enough to offset nearly half of the following summer's net plant CO2 uptake and added 6% to the CH4 summer fluxes. A similar pulse was measured 5 km away, indicating that this was a widespread phenomenon. Examination of an array of field surveys and laboratory experiments indicated that the spring carbon pulse was a result of a delayed mechanism in which gases produced in the fall are trapped in the frozen soils and released in early spring. How do gases accumulate in the soil? As temperatures drop in late fall, the mid-soil layer remains above freezing for approximately a month after the surface layer has frozen. Microbial activity in the mid-layer produced gases that are trapped beneath the surface ice. How are gases rapidly released from the soils in spring? May 2014 was unique in that several rain-on-snow events took place, with the potential to enhance soil cracking. These cracks can serve as pathways for rapid gas release as soon as the surface ice thaws. How will things change in the future? Warmer fall seasons may lead to a longer period of gas accumulation in the soils; more rain-on-snow events in spring may increase the likelihood of spring carbon pulse events.
Contacts (BER PM)
Naama Raz Yaseef
Lawrence Berkeley National Laboratory
Margaret S. Torn
Lawrence Berkeley National Laboratory
This work was funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research, Terrestrial Ecosystem Science program's Next-Generation Ecosystem Experiments-Arctic project and Atmospheric System Research program's Atmospheric Radiation project. Snow depth and density were measured with the support of Arctic Landscape Conservation Cooperative, U.S. Fish and Wildlife Service, project number ALCC2012-07.
Yaseef, N. R., M. S. Torn, Y. Wu, D. P. Billesbach, A. K. Liljedahl, T. J. Kneafsy, V. E. Romanovsky, D. R. Cook, and S. D. Wullschleger. 2017. "Large CO2 and CH4 Emissions from Polygonal Tundra During Spring Thaw in Northern Alaska," Geophysical Research Letters, DOI: 10.1002/2016GL071220. (Reference link)
Hubbard, S. S., et al. 2013. "Quantifying and Relating Land-Surface and Subsurface Variability in Permafrost Environments Using LiDAR and Surface Geophysical Datasets," Hydrogeology Journal 21(1), 149-69. DOI: 10.1007/s10040-012-0939-y. (Reference link)
Song, C., et al. 2012. "Large Methane Emission upon Spring Thaw from Natural Wetlands in the Northern Permafrost Region," Environmental Research Letters 7(3), 34009. DOI: 10.1088/1748-9326/7/3/034009. (Reference link) .
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