What drives a warming tundra to release carbon into the atmosphere?

The world’s arctic and alpine tundra ecosystems, collectively known as the tundra, store large reservoirs of organic carbon, keeping it out of the atmosphere—but that storage mechanism is weakening as the planet warms and the tundra melts, releasing stored carbon into the atmosphere. It’s difficult to measure exactly how much carbon loss is occurring, or to predict how much may occur in the future, due to variability across sites where scientists are studying this phenomenon.

A recent meta-analysis, published in the journal Nature, compiled data from 136 datasets taken from 56 warming experiments at 28 tundra sites—including data gathered in experiments run by Regents’ professor of biological sciences Ted Schuur—to investigate the factors that either encourage or restrict ecosystem respiration. The network of experiments, all of which were conducted during the growing season (June through August) over time frames that ranged from a single summer up to 25 years, revealed that factors including nitrogen concentration in soil, soil pH and the ratio of carbon to nitrogen at a site contribute to ecosystem respiration.

Data from the experiments is helping climate scientists understand exactly how much carbon these cold regions will release into the atmosphere as they warm, providing information necessary to predict the scale of global warming in the decades to come.

“Experiments such as these expose natural ecosystems to environmental conditions that we expect to occur in the Arctic in the future,” Schuur said. “These data, collected from experiments across the entire region, give us an insight of how the Arctic region will act to accelerate future climate change as stored permafrost carbon is released to the atmosphere in the form of carbon dioxide and methane greenhouse gases.”

The data showed that a mean temperature increase of 1.4 degrees Celsius in the air and 0.4 degrees Celsius in the soil resulted in a 30% increase in ecosystem respiration. Both plant-related (autotrophic) and microbial (heterotrophic) respiration contributed to the overall increase.

“The fate of the globally important carbon (C) stock of the arctic and alpine tundra biome will be determined by the balance between climatic impacts on C uptake (plant photosynthesis) and release (ecosystem respiration),” the researchers wrote.

Researchers observed that the magnitude of effects varied substantially with varying local environmental conditions at each site. For example, the researchers note that soil pH may cause variability in ecosystem respiration because it influences how available key nutrients are to plants and organisms that help with decomposition.

Likewise, indirect effects of warming play a role in how much respiration occurs—for example, if soil dries due to warming, it may create conditions that promote respiration by making oxygen more available to microbes in waterlogged soil, or conversely it may create conditions that decrease respiration by making water less available to microbes in dry soil.

Scientists can use this new understanding of the complex mechanisms behind ecosystem respiration at the study’s research sites and scale their results across the entire tundra region, creating a more accurate estimate of how much carbon will be released in the future. The study will help scientists develop improved models predicting future carbon cycling and warming.

The researchers also noted that data from additional experiments—particularly those that track warming across the whole year—will help further improve our understanding of environmental change in the Arctic region. Other researchers may contribute their own findings to the Tundra Flux Database.