The frigid Arctic tundra has soaked up carbon dioxide from the atmosphere for millennia, an essential balancer for our planet’s climate. Now, it appears to be emitting more heat-trapping gasses than it absorbs as global temperatures rise. The switch threatens to amplify warming even further and shows that climate change isn’t always a gradual process, but one that can accelerate with sudden steps.

The National Oceanic and Atmospheric Administration released its annual Arctic Report Card this week, pooling research from the planet’s north. As what is likely the hottest year on record draws to a close, researchers have found the Arctic region is warming four times faster than the global rate, with adverse consequences for everything from the soil, the ice, the plants that live there and the animals that then eat those plants to the communities that rely on them, and everyone else on Earth.

“This is yet one more sign, predicted by scientists, of the consequences of inadequately reducing fossil fuel pollution,” NOAA administrator Rick Spinrad said in a statement.

About 4 million people live north of the Arctic Circle. It’s a critical geopolitical choke point that’s beginning to become more accessible to shipping as sea ice recedes, creating territorial tensions between countries and a race for minerals and oil.

The Arctic is also critical for global weather as it drives oceanic and atmospheric currents in lower latitudes. It’s one of the regions where scientists can observe discrete shifts as average temperatures rise, like when reflective sea ice yields to open, sunlight-absorbing ocean, or when precipitation falls as rain rather than snow.

Now scientists can add the Arctic tundra to that list.

In most ecosystems, plants absorb carbon dioxide from the atmosphere. Those plants grow, die, or get eaten by animals that also grow and die. The carbon in their corpses — lignin, cellulose, fatty acids, proteins, and so on — feeds microorganisms that break down large molecules and return carbon dioxide to the air, completing the carbon cycle.

The tundra, a frozen treeless Arctic biome with long dark winters and short summers, alters the balance of this cycle. While it doesn’t have the dense, fast-growing biomass of a tropical rainforest, its vegetation does breathe in about a billion metric tons of carbon dioxide per year, about a fifth of humanity’s total annual output. Beneath the tundra, permafrost, a layer of soil that remains frozen year-round, keeps microbes at bay that would ordinarily decompose vegetation. The net result is that Arctic soils store enormous amounts of carbon, upward of 1.6 trillion metric tons across the region. That’s about double the amount of carbon in the atmosphere.

However, as average global temperatures rise, the permafrost isn’t so “perma” and once-sleepy microbes start waking up with an appetite, chewing up organic matter in the soil. That leads to more emissions of carbon dioxide as well as methane, an even more potent greenhouse gas. Over the years, the thawing permafrost has shifted the balance of carbon such that the Arctic is absorbing less and emitting more (sidenote: ice “melts,” permafrost “thaws”). This year, nine out of 20 permafrost monitoring sites reported record-high temperatures.

It’s not always a straightforward mechanism, however. “Warming temperatures tend to increase the uptake of carbon dioxide during the summer,” said Brenden Rogers, a scientist at the Woodwell Climate Research Center who contributed to the Arctic report card, during a press conference. “But they also generate increased net carbon dioxide emissions in soil microbes in the fall, winter, and spring.”

But one more factor has firmly flipped the Arctic tundra from being a carbon sink to a carbon source: wildfires. Remarkably, wildfires have occasionally ignited in Arctic regions in the past, but in recent years, their frequency and severity have picked up. Looking at the years between 2001 and 2020, researchers found that fires are a key source of greenhouse gases and present an urgent perennial concern for Arctic residents. Last year was the worst wildfire season in the Arctic on record, and this year was the second-biggest year for wildfire emissions.

Fire can then create a feedback loop that leads to more thawing permafrost, priming more of the landscape to burn.

It’s possible that the tundra could flip back in the other direction, toward being a net sink, if global temperatures come down.

Rogers noted that there is a lot of year-to-year variability in how much carbon the region absorbs, indicating that there are short-term drivers at work. But the results highlight that humanity can’t take the traditional carbon-absorbing biomes for granted. It’s not just the greenhouse gas emissions from burning fossil fuels that are driving warming, but also the loss of critical natural systems that used to offset heat-trapping gasses. And the effects are already beginning to manifest.

“We are seeing impacts together now in real time in the Arctics, and it’s a call to action,” Spinrad said.