Dec 8, 2008
Freezing tundra gives off methane
An extended measurement season in northeast Greenland undertaken as part of International Polar Year has revealed that the region’s tundra emits methane as it freezes in the autumn. Scientists generally stop data collection in the area at the end of the growing period, which may have masked this behaviour.
“Nobody would have believed anyone saying these emissions are taking place if the measurements were not so solid and carefully carried out,” Torben Christensen of Lund University in Sweden told environmentalresearchweb. “It is a classical basic-research finding of a surprise in Nature that probably has always been active, but no-one has had the logistical access before to the right tundra environments combined with the right techniques and high time-resolution measurements to be able to capture it.”
Emissions from terrestrial wetlands are the largest single source of methane, which is a greenhouse gas. In high latitudes there’s typically a late-autumn shoulder in atmospheric methane concentrations, but the cause for this is as yet unclear.
Christensen and colleagues from Denmark’s University of Copenhagen and University of Aarhus, NOAA Earth System Research Laboratory, in the US, SRON Netherlands Institute for Space Research, and Utrecht University in the Netherlands used automated flux chambers to take hourly measurements of methane with laser-spectroscopy equipment in the Zackenberg Valley.
The scientists found that methane emissions fell following the growing season but then increased as freezing started, continuing at high levels for several weeks while complete freeze-in of the soil and root zone took place. This may be caused by methane in the active layer being squeezed out by frost, say the researchers. During freezing at lower latitudes, in contrast, the absence of such freezing may allow methane to diffuse downwards.
The autumn methane flux was highly variable over small distances, probably because of differences in peat and vegetation structure providing different pathways for emission to the atmosphere. The freeze-in emissions were also more variable than summer emissions, peaking at 112.5 mg of methane per m2/hour – the highest rate ever reported from a tundra ecosystem (excluding hotspot emissions from thermokarst lakes). Over the summer the ecosystem emitted roughly 4.5 g of methane per m2.
Incorporating these autumn emissions in atmospheric transport models improved the simulated seasonal cycle of atmospheric methane.
“[This is] a basic research finding that may help us understand how the high northern latitudes are connected to the atmosphere and how seasonal dynamics of methane concentrations in the atmosphere may be better explained if the emissions we have observed are indeed a general phenomena,” said Christensen. “But the implications for climate change are only through this improved understanding of how the natural system works. Through this we may better understand permafrost melting in the margins of the Arctic and how changing methane emissions in these regions may have feedback effects on climate.”
The researchers believe there is no reason that such a mechanism shouldn’t happen everywhere that there are similar ecosystems. Applying the fluxes measured at Zackenberg to all wet-meadow tundra would give a pulse of around 4 Teragrammes of methane from the highest latitudes at what was previously thought to be an inactive time of year.
“This does not greatly increase emissions estimates from high northern latitudes, but it revises our view of the seasonal distribution of known emissions,” write the researchers in Nature.
Now the team is investigating the mechanism behind the emissions, both in the lab and in the field. “But to improve our possibilities of arriving at answers, we are pivotally dependent on making sure that the research station Zackenberg is kept open for a much longer period every year than has previously been the case,” said Christensen. “We believe it is in the spring and autumn periods that the major unanswered research questions lie, not only in this particular research field, and we must make sure that a station we have such exciting observations from is kept open permanently at least from April through November.”
About the author
Liz Kalaugher is editor of environmentalresearchweb.