23 October 2020

Higher temperatures and more snow lead to higher release of CO2 from heaths in Greenland


A new study by scientists from University of Copenhagen shows that increased air temperature and more snow can lead to increased CO2 emission in the short term. The climate change manipulations were made in a heath ecosystem in Greenland as part of a PhD project by Nynne Rand Ravn, supervised by Bo Elberling and Anders Michelsen at Center for Permafrost (CENPERM).

Experiment with snow fence which accumulates snow and open top chambers that increase temperature in heath in Greenland (Photo: Nynne Ravn)

Low temperatures in the Arctic have kept the turnover of organic material and CO2 emission at a low level. Consequently, the release of CO2 by ecosystem respiration has been lower than the amount of CO2 which has been taken up by the ecosystem from the atmosphere, through photosynthesis by plants. In this way arctic ecosystems have removed CO2 from the atmosphere and carbon has accumulated in soil, as dead organic matter.

Climate change can via changes in temperature and precipitation in arctic areas potentially cause increased decomposition and respiration. If photosynthesis is unaffected, and “old  carbon” in the soil to a larger degree is turned over by microbes, this can lead to increased emission of CO2.  This scenario may possibly lead to further warming when the ecosystem releases more CO2.

A new study published in Soil Biology and Biochemistry shows the results of a large scale experiment in a dry heath in West Greenland. Here, six snow fences increases snow depth by ca. 1 m on the leeward side and cause 5-8 °C soil warming in late winter, because the snow is insulating against the cold air, but it also causes later snowmelt. Furthermore the ecosystem is warmed by transparent open top chambers causing 1.3 °C higher temperature in the snow free period. The treatments are also combined so the climate change factors can be investigated separately, and together.

Ecosystem respiration and photosynthesis was investigated together with the balance between these, net ecosystem production, to reveal potential effects on CO2 balance in arctic heath ecosystems. This was supplemented with measurements of the natural abundance of the stable isotopes of carbon which could indicate if ”fresh” carbon from photosynthesis or “old” carbon from organic deposits is released.

According to Nynne Ravn the emission of CO2 from the ecosystem was increased by all treatments: “The effect of increased snow on ecosystem CO2 balance was due to a delay in onset of growing season, and warming caused increased ecosystem respiration”. Anders Michelsen adds that: “Due to the isotopic composition of emitted CO2, it seems that warming caused release of CO2 from older organic deposits stored in the soil”.

The study showed a fast response in the CO2 balance resulting from the climate manipulations. Increased temperature and snowfall can increase CO2 emission in the short term.  In the longer run, increased growth of plants that tolerate larger amounts of snow, for instance willow shrubs, could mitigate the increased soil emission through enhanced photosynthesis, which may be a focus of future research. Bo Elberling concludes that: “The results show the importance of interdisciplinary collaboration on field experiments to study future changes in arctic ecosystems, but also that measurement over longer periods and across vegetation types are prerequisites to predict the long-term consequences climate change in the Arctic. This is exactly the ambitions behind the activities in Center for Permafrost”.

Snow fences in tundra landscape in Greenland changes the amount of snow and causes shorter growing season for plants (Photo: Nynne Ravn)

Link to scientific paper

Ravn N.R., Elberling, B., Michelsen, A. (2020)  Arctic soil carbon turnover controlled by experimental snow addition, summer warming and shrub removal. Soil Biology & Biochemistry  https://doi.org/10.1016/j.soilbio.2019.107698 

Link to outreach in Ingeniøren (in Danish)