Erin K. Cameron, an environmental scientist at Saint Mary’s University in Halifax, Nova Scotia, who studies the boreal incursion of earthworms, found that 99.8 percent of the earthworms in her study area in Alberta belonged Dendrobaena octaedra , an invasive species that eats leaf litter but doesn’t burrow into the soil.

In 2015, Dr. Cameron published the results of a computer model aimed at figuring out the effect on leaf-litter over time. “What we see with our model is that forest-floor carbon is reduced by between 50 percent and 94 percent , mostly in the first 40 years,” she said. That carbon, no longer sequestered, goes into the atmosphere.

Not only that, in a 2009 study she calculated that earthworms had already wriggled their way into 9 percent of the forest of northeastern Alberta, and would occupy half of it by 2049.

Ms . Shaw, of the Canadian Forest Service, found that 35 to 40 percent of the plots she examined in northern Alberta contained earthworms. The leaf litter, which can be more than a foot thick, was thin and churned up where earthworms were present.

If Dr. Cameron’s calculations bear out, it means the lowly earthworm stands to alter the carbon balance of the planet by adding to the load in the atmosphere.

The global boreal forest is a muscular part of Earth’s carbon cycle; at least one-fifth of the carbon that cycles through air, soil and oceans passes through the boreal , said Sylvie Quideau, a soil biogeochemist at the University of Alberta in Edmonton. Currently, the boreal absorbs more carbon from the atmosphere than it adds, but that is changing.

On one hand, warmer temperatures could extend the growing season, allowing trees to grow bigger and store more carbon, said Dr. Kurz, the forest researcher in British Columbia. But rising temperatures also release carbon to the atmosphere, by thawing permafrost and increasing the number of forest fires.