Northern permafrost region on track to become carbon source instead of sink due to global warming

The northern permafrost region is emitting more greenhouse gases into the atmosphere than it is capturing, a new study has found. Historically, permafrost or permanently frozen ground has been one of the largest terrestrial carbon and nitrogen pools. However, the new research suggests that this region, a quarter of the northern hemisphere, is projected to shift from a net sink to a net source of carbon under global warming.

In the study published in journal Global Biogeochemical Cycles, researchers have prepared a first ever comprehensive estimation of quantities of emission and capturing of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) between 2000 and 2020 in and around the Arctic. 

The region emitted 38 million tonnes of CH4, 6,70,000 tonnes of N2O and 12 million tonnes of CO2 into the atmosphere between 2000 and 2020. When also including lateral fluxes such as erosion, the region was a net source of 144 million tonnes of CO2 and CH4 and 3 million tonnes of nitrogen. 

While there was a definite conclusion about the region being a net source of methane and nitrous oxide, the research was uncertain whether it was a source of a sink of carbon dioxide. On one hand, terrestrial ecosystems remained a CO2 sink, but emissions from fires and inland waters largely offset it. 

In total, atmospheric CO2 contributed 8 per cent to the total carbon released from the region, while atmospheric CH4 contributed 26.5 per cent. 

The upper 3 metres of permafrost region soils are estimated to store 1 trillion tonnes of soil organic carbon and 55 billion tonnes of soil nitrogen. But increasing temperatures lead to thawing permafrost, exposing substantial quantities of organic carbon and thus resulting in atmospheric release of greenhouse gases (GHGs) from permafrost into the atmosphere. 

“This release of GHGs to the atmosphere could have a strong impact on the global carbon cycle,” the study noted, adding that this has the potential to accelerate global climate warming, known as the ‘permafrost carbon feedback’. 

For the assessment, the scientists classified the region into five categories: Boreal forests, wetlands, dry tundra, tundra wetlands and permafrost bogs (ecosystems with near surface permafrost and thick surface peat layers). 

It was found that wetlands were the biggest source of methane, contributing 83 per cent. All terrestrial ecosystems except boreal forests were net CH4 emitters. Dry tundra was the biggest driver of N2O release, followed by permafrost bogs. 

The team of researchers from Sweden, the United States, Germany, Finland, Australia, France, Canada and Denmark, concluded that while longer growing seasons, increased CO2 concentrations, and additional nutrient release from thawing permafrost may lead to increased vegetation productivity and partly offset the release of permafrost GHGs, other processes cause rapid shifts to landscape structure and might accelerate the release of GHGs into the atmosphere.

“Large uncertainty ranges in these estimates point to a need for further expansion of monitoring networks, continued data synthesis efforts, and better integration of field observations, remote sensing data, and ecosystem models to constrain the contemporary net GHG budgets of the permafrost region and track their future trajectory,” the researchers said.