New measurements from northern Sweden show less methane emissions than feared – ScienceDaily
It is well known that thawing of permafrost can lead to the release of significant amounts of methane. New research shows, however, that in some areas this methane emissions can be one tenth of the amount predicted from a thawing. The research has been carried out in Sweden by an international group that includes researchers from the University of Copenhagen. A crucial, yet open question is how much precipitation the future will provide.
A typical landscape in northern Sweden. To the left is a pond vegetation controlled by high water levels and to the right a drier tundra vegetation, which is becoming more common in areas where the water level is low and the soil dries out in the summer. In areas where permafrost disappears, new studies show that methane emissions can be reduced by a factor of 10 due to changes in hydrology, plant society and the composition of microorganisms in the soil (photo: Bo Elberling)
Permafrost runs like a frozen belt of soil and sediment around the earth’s northern arctic and subarctic tundra. When the permafrost thaws, microorganisms can break down thousands of years old accumulations of organic material. This process releases a number of greenhouse gases. One of the most critical gases is methane; the same gas that cattle emit when they burp and fart.
Because of this, researchers and public authorities have long feared that methane emissions from permafrost will rise in line with global temperatures. But in some places it turns out that methane emissions are lower than previously assumed.
In a comprehensive new study of a collaboration from the University of Gothenburg, the Ecole Polytechnique in France and the Center for Permafrost (CENPERM) at the University of Copenhagen, researchers measured methane emissions from two locations in northern Sweden. The permafrost disappeared from one of the sites in the 1980s and 10-15 years later in the other.
The difference between the two areas shows what can happen when a landscape gradually adapts to the absence of permafrost. The results show that the first area to lose its permafrost now has ten times less methane emissions than in the second location. This is due to gradual changes in the drainage and spread of new plant species. The results of the study were recently published in the journal Global change biology.
“The study has shown that there is not necessarily a large explosion of methane that could have been expected in the wake of a thaw. In fact, in areas with sporadic permafrost, much less methane can be released than expected,” said Professor Bo Elberling of CENPERM (Center for Permafrost), at the University of Copenhagen’s Department of Earth Sciences and Natural Resource Management.
Water, plants and microbes all play a role
According to Professor Elberling, water runoff explains why much less methane was released than expected. When layers of permafrost at a depth of a few meters begin to disappear, the water in the soil above begins to drain off.
“Permafrost works much like the bottom of a bathtub. When it melts, it is as if you have pulled the plug, which allows water to seep through the now thawed soil. Drainage allows new plant species to establish themselves, plants that are better adapted to “Dryer soil conditions. That’s exactly what we see in these places in Sweden,” he explains.
Grasses that are typical of very wet areas with sporadic permafrost have developed a straw-like system that transports oxygen from their stems down to their roots. These straws also act as a conduit through which methane in the soil quickly finds its way to the surface and then out into the atmosphere.
When the water disappears, these grasses also disappear. Gradually, they are replaced by new plant species, which due to the dry soil conditions do not need to transport oxygen from the surface via their roots. The combination of more oxygen in the soil and reduced methane transport means that less methane is produced and that the methane that is produced can be better converted to CO2 in the soil.
“As grass is outcompeted by new plants such as dwarf shrubs, willow and birch, the transport mechanism disappears, which means that methane can quickly escape up through the ground and into the atmosphere,” explains Bo Elberling.
The combination of dry soil and new growth also creates more favorable conditions for soil bacteria that help to break down methane.
“When methane can no longer flow out through the straws, soil bacteria have more time to break it down and convert it to CO2,” says Bo Elberling.
As a result, it is conceivable that as microorganisms reduce methane emissions, the process will lead to more CO2 being emitted. Nevertheless, no significant increase in CO2 emissions was observed by the researchers in their study. This is interpreted as a result of the CO2 balance, which is determined more by plant roots than the CO2 released from the microorganisms that degrade methane. Crucially, even if methane ends up as CO2, it is considered less critical in the context of climate change because methane is at least 25 more potent greenhouse gas compared to CO2.
Where is permafrost?
Areas with sporadic permafrost cover the southern part of the Arctic around the world, where temperatures are usually between minus five and zero degrees. This means that a rise in temperature can cause the permafrost to disappear completely.
Future rainfall will be crucial
According to Professor Elberling, the future’s largest unknown amount is future precipitation. Because while thawing permafrost makes it easier for soil to drain in areas with sporadic permafrost, increased rainfall or poor drainage can prevent an area from drying out. Where the latter is the case, we should not expect a corresponding dehydration and reduction of methane released.
“The balance between precipitation and evaporation will be crucial for greenhouse gas emissions and absorption. However, predicting Arctic precipitation is fraught with uncertainty. In some areas we are seeing increased precipitation, while in others it is drying up – especially in summer, says Elberling.
The study focuses on data from two locations in northern Sweden. Professor Elberling is therefore cautious in concluding that analogous conditions extend to other areas with similar permafrost, such as in Canada or Russia.
The study contributes to a new understanding of a process that must be considered whenever future methane emissions are assessed in permafrost-affected areas.
“In its latest report on the Arctic’s future methane budget, the IPCC (Intergovernmental Panel on Climate Change) did not consider the conditions highlighted by us in the study. Our study reverses the general perception that thawed permafrost should be consistently associated with increased levels of methane out “, concludes Professor Elberling.
The main author Mats Björkman from the University of Gothenburg adds:
“Our research shows that methane emissions from areas where permafrost thaws are not the same everywhere. The new observations represent an important component in a more comprehensive picture of climate impact in the Arctic. Our results also underline the importance of including hydrological, vegetation, and microbial changes when studies the long-term effects of permafrost thawing and disappearing. “
In the future, Mats Björkman wants to determine which areas will either become wetter or drier and see how they are affected when the permafrost thaws.