The cycle of carbon into and out of the tundra is, in one sense, a race between what's happening on top of the soil, where photosynthesizing plants are absorbing carbon dioxide, and what's happening below the surface, where active microbes are decomposing sequestered carbon and releasing it into the air.

High latitudes skew that race, making it difficult for plant growth to keep up with tundra carbon respiration, Miller said.

"At some point, there's only so much light during the day," he said.

When the Arctic night arrives and the daylight is gone, photosynthesis is impossible — but increasingly warm temperatures allow microbial activity to continue below the surface, he said. That can happen even if the tundra surface is snow-covered and the soil temperatures fall below freezing, as there can be pockets of unfrozen water, he said.

"As long as there's a little trace of liquid water available, the microbes might be able to churn out a little bit of carbon dioxide," he said.