As the Arctic tundra warms, soil microbes likely will ramp up CO₂ production

Climate change is warming the Arctic tundra about four times faster than the rest of the planet. Now, a study suggests that rising temperatures will spur underground microbes there to produce more carbon dioxide — potentially creating a feedback loop that worsens climate change.

The tundra is “a sleepy biome,” says Sybryn Maes, an environmental scientist at Umeå University in Sweden. This ecosystem is populated by small shrubs, grasses and lichen growing in cold soils rich with stored organic carbon. Scientists have long suspected that warming will wake this sleeping giant, prompting soil microbes to release more of the greenhouse gas CO₂ (SN: 8/11/22). But it’s been difficult to demonstrate in field studies.

Maes’ team included about 70 scientists performing measurements in 28 tundra regions across the planet’s Arctic and alpine zones. During the summer growing season, the researchers placed clear, open-topped plastic chambers, each about a meter in diameter, over patches of tundra. These chambers let in light and precipitation but blocked the wind, warming the air inside by an average of 1.4 degrees Celsius. The researchers monitored how much CO₂ microbes in the soil released into the air, a process called respiration, and compared that data with measurements from nearby exposed patches.

The study, published online April 17 in Nature, found that the 1.4 degree C temperature increase caused an average 30 percent increase in CO₂ respiration across the experimental sites compared with the exposed sites. Some of the studies the team compiled lasted only one year, but the longest provided measurements from 25 growing seasons, showing that these effects persist over time.

Though it’s clear that higher temperatures boost CO₂ respiration on average, there’s a lot of variability between field sites, Maes says. For instance, the CO₂ ramp-up is particularly pronounced in nitrogen-poor soil. As soils warm, plants become more active, and so do their symbiotic microbes, which support the plants by scavenging for nitrogen. The microbes’ heightened activity also means they produce more CO₂.

The findings provide the strongest evidence yet that warmer temperatures will increase microbial activity, releasing more CO₂, says environmental microbiologist Nicholas Bouskill of Lawrence Berkeley National Laboratory in California. Previous studies, including Bouskill’s own, were much smaller and came to contradictory conclusions.

The long-term question, Bouskill says, is: “Will these areas become carbon sources, or will they remain carbon sinks?”

NASA estimates that the Arctic permafrost stores 1,700 billion metric tons of carbon. Recent studies find that by the year 2100, degrading permafrost could release anywhere from 22 billion to 524 billion metric tons of carbon, depending on the rate of warming. 

Given the expected increase in CO₂ emissions from microbes and their potential to contribute to further global warming, “you could say this is a doom scenario,” Maes says. But she notes that the study’s results do not mean the tundra’s overall carbon emissions will inevitably skyrocket — other processes may counteract this effect. For example, plants could ramp up their photosynthetic activity, taking up more CO₂. And these studies don’t factor in what happens during other times of year.

Incorporating data that captures the nuance of what’s happening in the Arctic — such as the link between nitrogen-poor soil and microbial respiration — may help improve predictions about the tundra’s response to climate change and how that will, in turn, influence Earth’s climate. “We need to represent how nutrients are cycling in order to get the carbon right,” Bouskill says.