Nitrous Oxide Emissions from Warming Arctic Pose Global Threats
Impact of the potent greenhouse gas is poorly understood, revealing an urgent need for further research
Despite the many gases in our atmosphere, studies of greenhouse gas emissions driving climate change have primarily focused on carbon dioxide and methane. Although carbon dioxide exists at much higher concentrations, the global-warming potential of other gases, molecule for molecule, is often staggeringly more potent. One of those gases is nitrous oxide, which is now being released from a warming Arctic and contributing to the earth’s warming.
photo by Christina Biasi
Nitrous oxide as a driver of climate change is usually absent from mainstream discourse, yet it has been well-established that human activity is a major source of emissions of this greenhouse gas that has 300 times the warming potential of carbon dioxide over a hundred year time span. Evidence is building that nitrous oxide emissions from natural sources may be increasing due to elevating global temperatures. For example, Researchers from the University of Eastern Finland (UEF) recently investigated nitrous oxide emissions from Arctic peatlands following permafrost thaw. Their findings, published in the Proceedings of the National Academy of Sciences, indicate that current nitrous oxide emissions from the Arctic have been underestimated, and that future release is potentially high as the permafrost thaws.
“Not only carbon dioxide and methane, but also nitrous oxide needs to be considered in research on climate feedbacks from Arctic ecosystems,” says Christina Biasi, research director at UEF. “We believe that nitrous oxide plays a bigger role than currently suggested, especially in a future warmer world.”
With Arctic lands expected to warm by 5.6 to 12.4 degrees Celsius by 2100, there’s reason for concern about melting permafrost. It’s estimated that more than 67 billion tons of nitrogen stocks are stored in the upper three meters of permafrost soil, a vast sum accumulated over thousands of years through the nitrogen cycle. Some of that nitrogen already exists in the form of “old” nitrous oxide, built up during the permafrost formation, while other mineralized nitrogen interacts with microbes to produce “new” nitrous oxide.
Because nitrous oxide is such a potent greenhouse gas, a little goes a long way in terms of its warming effect in the atmosphere. This is worrisome in the context of rising global temperatures and increasing emissions. While it’s currently held that our planet has reached an atmospheric carbon dioxide concentration of about 405 parts per million, NOAA’s Annual Greenhouse Gas Index illustrates that the combined effect of nitrous oxide, carbon dioxide, and methane emissions, results in a 489 parts per million carbon dioxide equivalent. The scientific consensus is that these greenhouse gas emissions have and continue to increase the earth’s temperature leading to deleterious effects such as droughts, more frequent extreme weather events, and higher rates of species extinction, to name a few. However, results from the sparse research that has been conducted on nitrous oxide have led scientists to believe that the degree of emissions from both the Arctic lands and ocean waters into the atmosphere is underestimated. Without adequate research to address this blindspot, the true rate of planetary warming could also be underestimated.
Nitrous oxide that is released into the atmosphere poses another threat. After remaining in the troposphere for about 120 years functioning as a greenhouse gas, it migrates to the stratosphere, where it depletes the ozone layer. The ozone layer serves as a protective barrier absorbing solar radiation; without it the sun’s ultraviolet rays would severely damage biological organisms, including humans. Although the Montreal Protocol, an international treaty signed in 1987 to protect the ozone layer, helped reduce several ozone depleting substances, nitrous oxide is not one of the substances it regulates.
Loss of nitrogen to the atmosphere may also reduce plant biomass, further contributing to climate change.
“The nitrogen story is important…because if nitrogen is lost to the atmosphere, plant growth won't be able to compensate for carbon being lost from permafrost soils,” explains Ben Abbott, a postdoctoral fellow at Michigan State University. “The potential for nutrient loss to slow plant growth is one of the reasons most experts believe biomass increases won't be able to offset permafrost carbon emissions.”
Abbott mentions that hydrological conditions directly impact how much carbon and nitrogen are released from permafrost, and that there’s abundant evidence from field research that thawing permafrost affects hydrology in a way that causes nitrogen loss. Permafrost thaws in different ways, but if there are masses of ice in the soil, thermokarst formations develop, which are marked by a combination of dry and wet areas across the landscape —ideal conditions for nitrous oxide production.
The UEF team found that Arctic peatlands also have the potential for high nitrous oxide emissions. Their lab experiment measured nitrous oxide discharge from 16 peat mesocosms collected from Arctic permafrost peatlands simulated under near-field conditions using intact soil-plant systems incubated in a climate chamber. Some samples included vegetation while others were naturally barren.
“In our study, the highest post-thaw emissions occurred from bare peat surfaces, a typical landform in permafrost peatlands, where permafrost thaw caused a fivefold increase in emissions. These emissions rates match those from tropical forest soils, the world’s largest natural terrestrial N2O source,” they wrote.
Carolina Voigt, lead author of the study, states that prior field research had already shown that Arctic peat soils especially contain hot spots of nitrous oxide release, and that warming increases these emissions not only from bare soils but also from vegetated areas. According to Voigt, a direct result of the findings from their study is that nitrous oxide is now a higher priority for research on climate feedbacks from Arctic ecosystems.
Despite previous models that suggested future greening of the Arctic with increased carbon dioxide levels, the UEF team cited recent reports that show the region is actually browning, a trend triggered by factors such as tundra wildfires, warming during the winter months, and other unusual weather events. Such browning could increase the amount of bare soils, possibly fueling nitrous oxide emissions.
There are many documented sources of nitrous oxide emissions globally, including fossil fuel burning, numerous commercial uses, biomass burning, and agriculture. It’s also used as an anesthetic in dentistry and medicine, referred to as “laughing gas.” However, the latest research shows that rapidly changing conditions are making the Arctic a formidable contender as a significant source of nitrous oxide emissions.
Jeremy Mathis, director of Arctic research at NOAA, explains that although he was struck by the findings of the study conducted by UEF scientists, there’s need for further field research on Arctic nitrous oxide emissions to better understand their extent, as the data is currently too sparse to draw broad conclusions. The most alarming phenomenon in the Arctic is the loss of the highly reflective snow and ice cover, known as the albedo, so if greater warming results from increased nitrous oxide and other greenhouse gases, we have a serious problem, he says.
“There’s an extreme lack of observational infrastructure in the Arctic because it’s a very challenging place to work, which makes it a very expensive place to work,” says Mathis. “I hope we have research efforts going forward to increase nitrous oxide measurements in the Arctic. We need further investment beyond what the US has now, and this study gives us the justification and motivation to do it.”
According to Biasi, the infrastructure necessary for measuring nitrous oxide is not as developed as the carbon-monitoring network, and nitrous oxide is also more challenging to measure, requiring more advanced instruments. However, she asserts that the main reason there is so little data is the current lack of awareness of nitrous oxide emissions from Arctic soils.
The dearth of data regarding Arctic nitrous oxide emissions extends beyond permafrost sources; a gap in research also exists for the Arctic Ocean itself as a source of the greenhouse gas. Last year, Annie Bourbonnais, assistant professor at University of Massachusetts, received a $290,608 grant from the National Science Foundation to study nitrous oxide cycling in the western Arctic Ocean.
According to Bourbonnais, the Arctic Ocean may represent an important source of nitrous oxide to the atmosphere. “High nitrous oxide saturations were recently observed in productive shallow Arctic shelf waters following significant reduction in sea ice cover and its associated increase in primary productivity,” she explains. “Our proposed research is guided by the overarching hypothesis that productive shelf areas of the Arctic support significant net fluxes of nitrous oxide to the atmosphere.”
Nitrous oxide is produced in the ocean water column and sediments by two pathways. The first is in oxygen-rich conditions in which ammonium is converted to nitrate, creating nitrous oxide as a byproduct. The second occurs in deoxygenated sediments where nitrous oxide is produced in an intermediate stage between the conversion of nitrate to nitrogen gas. The loss of sea ice cover in Arctic shelf waters such as the Chukchi and Beauford seas has resulted in increased sunlight absorption, fueling phytoplankton growth. Following phytoplankton death, organic material is remineralized to ammonium, which can be converted to nitrate and ultimately produce nitrous oxide.
Bourbonnais’s research project aims to better distinguish nitrous oxide production pathways between surface water and sedimentary sources in both coastal and offshore waters by using a stable isotope approach. She hopes that the research will provide a foundation for future work with a better understanding of nitrous oxide ocean cycling in light of the sea ice changes driven by global warming, acknowledging that the nitrous oxide fluxes from ocean to atmosphere create a positive feedback loop for further warming.
While scientists have a conservative estimate for the total amount of nitrogen in the Arctic, the full potential amount of nitrous oxide emissions from the region is presently unknown. The UEF team is currently working on field research that aims to gather knowledge on the extent of nitrous oxide-emitting surfaces in the Arctic.
“We are at the moment measuring nitrous oxide fluxes from Pleistocene-aged yedoma permafrost soils, which comprise one-third of the Arctic soil carbon pool and also display large nitrogen stocks,” says Maija Marushchak, a postdoctoral researcher at UEF. “We are also currently running a field-warming experiment, to study the effects of raised temperature on nitrous oxide emissions.”
“We want the American people to understand that what happens in the Arctic doesn’t stay in the Arctic,” adds Mathis. “The rate of warming there is at least twice as fast as the rest of the planet. We’re seeing that the changes are having a propagating effect on weather and climate patterns. People need to understand that what happens in the Arctic has an impact on their lives and the economy.”