Melting permafrost increases the emission of greenhouse gases in Arctic lakes

Groundwaters that circulate through the subsoil as a result of the melting permafrost can transport carbon dioxide and methane —gases with a strong greenhouse effect— to Arctic lakes and this increases the effects of climate change. This process of transporting gases to the lakes, which culminates with their emission into the atmosphere, is now quantified for the first time in a paper published in the journal Nature Communications. The study was led by Carolina Olid, lecturer at the Faculty of Earth Sciences and member of the Geomodels Research Institute of the University of Barcelona.

Researchers Jordi Garcia-Orellana, Valentí Rodellas, Marc Diego-Feliu and Aaron Alorda-Kleinglass, from the Department of Physics (UAB) and the Institute of Environmental Science and Technology (ICTA-UAB); Gerard Rocher-Ros and Jan Karlsson (Umeå University, Sweden) and David Bastviken (Linköping University, Sweden) also participate in the study.

Permafrost is the soil that remains frozen in different parts of the planet, such as the Arctic and Antarctica, as well as in the high mountains and high plateaus around the world. According to this study, climate change will enhance the process of methane runoff from permafrost to Arctic lakes through groundwater, which is why it is important to include this process in climate prediction models, as stated by Carolina Olid, first author of the study and member of the Department of Earth and Ocean Dynamics of the UB.

The Arctic, the sentinel of climate change

The Arctic is one of the most sensitive areas to climate change and it has warmed nearly four times faster than the rest of the world over the past forty years. The new study focuses on Arctic lakes located in the sporadic permafrost region of northern Sweden. In this area —a typical tundra ecosystem— between 10 and 50% of the soils are permanently frozen. The top layer of the soil above the permafrost —the active layer— is the one that freezes and thaws annually.

"In this region, the average annual temperature has risen considerably in recent years and is now above 0ºC. This causes permafrost —with its abundant organic matter rich in carbon and greenhouse gases— to thaw. Through the circulation of groundwater, the thawing permafrost acts as a spring that supplies the hydrologically connected lakes in the region with greenhouse gases. The gases are finally emitted into the atmosphere and contribute to amplifying global warming", says lecturer Carolina Olid.

To date, the effects of permafrost thawing in groundwater discharge into Arctic lakes and its relevance on greenhouse gas emissions were still unknown and had not been studied in detail. To answer these questions, the team used radon —a gas abundant in subsoil currents— as a tracer of groundwater discharge into the lakes.

However, the quantification of radon to quantify the transport of greenhouse gases to the lakes does have technical and operational difficulties. "Radon is a radioactive gas, it disintegrates very quickly, and this made us to install measuring equipment at the Abisko scientific station in order to measure the samples on the same day they were collected", says the researcher, also former member of Umeå University.

"The concentrations of radon and methane in the water of the subsoil are highly variable, which means that it is necessary to work with a large number of samples to be able to determine their magnitude with good precision", says Valentí Rodellas, a member of the ICTA-UAB research team. "The soils of these rich, dense regions, which have a great capacity to retain water, make it even more difficult to obtain a sufficient volume of water to carry out the analyses. We also had to transport tens of kilos of material in our backpacks over relatively long distances to reach the reservoirs, as there was no motorised transport", he adds.

More greenhouse gas emissions in summer

The study reveals that the inflow of methane gas through groundwater into the lakes is more intense in the summer than in autumn due to the greater abundance of water circulating in the subsoil (melting, rain, etc.). The higher temperatures in the summer also favour the production of methane in the subsoil and, therefore, enrich the gas content of the groundwater that goes to the lakes.

"Climate change and accelerated permafrost melting will increase the amount of greenhouse gases that can be transported to the lakes through groundwater. The increase in precipitation —up to 40% in the Arctic over the next decade— will also increase the flow of groundwater and, therefore, the discharge of methane into the lakes", says researcher Carolina Olid.

A greater emission of greenhouse gases into the atmosphere is not the only effect caused by the melting of permafrost. "Permafrost water contains high concentrations not only of greenhouse gases such as carbon dioxide (CO2) or methane (CH4), but also of other compounds such as nutrients, and pollutants (mercury, etc.). The arrival of these compounds in the lakes generates harmful impacts on the natural environment and the organisms that live in lake ecosystems, as well as in terrestrial systems (bogs, etc.)", says Gerard Rocher-Ros, postdoctoral researcher at Umeå University.

"These are effects with implications for the entire aquatic trophic chain of both the lake itself and the network of interconnected rivers and streams, which have an impact on the ecosystem services provided by natural resources to society”, adds Carolina Olid.

Improving climate change prediction in the future

It is still difficult to determine those areas of the planet where the process of permafrost melting will be more pronounced or faster. In some areas, some lakes will dry up because the water will flow out through the drains or new channels formed by the melting. In other cases, the lakes will expand their margins as the melting progresses —a process known as thermokarst— and new lakes will emerge in relief depressions.

In this scenario of climate uncertainty, the introduction of groundwater discharge into climate models will improve predictions of future methane emissions from lakes.

"This will allow us to assess the real role of lakes in the carbon cycle and whether they really function as sources or as potential reservoirs of greenhouse gases. This information is key to developing appropriate conservation policies to mitigate climate change", concludes lecturer Carolina Olid.

We would like to have a special word for Jordi Garcia-Orellana, who left us recently. He was more than a mentor for all of us. He was the one who encouraged us to pursue research and a best friend when we needed support. This project resulted from his interest in the role of groundwater discharge on biogeochemical cycles. This study and many others could not have been carried out without him.