Every spring, in alpine regions around the world, one of Earth’s tiniest migrations takes place. The migrants are single-celled green algae; they are kin to seaweed, but instead of living in the sea they live in snow. (Snow weed, maybe?) They spend the winter deep in the snowpack, atop last summer’s snow, as dormant cysts. In the spring, they wake and swim up through the trickle of snowmelt to the surface, dividing and photosynthesizing as they go. Then, at the top, they turn red. This creates what scientists call pink snow or watermelon snow—drifts and glaciers that look like Slush Puppies and eventually reduce to rivulets of crimson.

The color comes from astaxanthin, a molecular cousin of the chemical that makes carrots orange. The algae produce it seemingly as a sunscreen; it absorbs UV light, warming the organisms, and, critically, melting the surrounding snow. “The melting helps them a lot,” Roman Dial, a biologist at Alaska Pacific University, told me recently. “The surface of a snowfield can be a very dry place; the liquid water drains away. And life just can’t use frozen water. It’s like if you were out camping and your water bottle was frozen, you’d be thirsty until it melted.”

Watermelon snow is a perfectly natural phenomenon, but in an age of disappearing glaciers it is also problematic. Last year, scientists discovered that the algae had reduced the amount of sunlight reflected by some glaciers in Scandinavia—and increased the amount of sunlight absorbed—by thirteen per cent. The result, as Dial and his colleagues demonstrated in this month’s issue of Nature Geoscience, is faster melting. As in other parts of the warming planet—particularly the Arctic, where scientists fear that thawing permafrost may be triggering a climatic feedback loop—the effect is likely self-perpetuating. Ice sheets are already being darkened by dust, soot, and ash, which hasten melting and add nutrients on which algae can flourish. As the organisms proliferate, they melt even more snow, which allows them to proliferate again. “It spreads more rapidly than people realize, once it gets established,” Dial said.

Chlamydomonas algae turns from green to red when placed on snow. Photograph by Wilbur E. Garrett / National Geographic via Getty

Watermelon snow was known to Aristotle, two thousand years ago, and its biological origins became apparent in the early nineteenth century. Snow algae have since drawn the attention of climate scientists and of biologists keen to understand the conditions under which life might develop on other planets. Three genera have so far been identified—Coenochloris, Chloromonas, and Chlamydomonas—encompassing perhaps dozens of species; there are orange snow algae and yellow snow algae, and many have a laxative effect if you eat them. In frozen ecosystems, they are primary producers—the grass of the snow, grazed upon by the glacial equivalent of cattle. In Iceland, the cattle are tardigrades, plump microscopic animals that are sometimes called water bears but resemble origami caterpillars. In Chile, the top consumers on the ice fields are stoneflies; in the Himalayas, they’re wingless midges. And in the Pacific Northwest, from the Oregon mountains up to Alaska, they’re ice worms.

Some facts about ice worms: they can grow as long as an inch and are black. This protects them from the sun, although they rarely encounter it; in the morning, they migrate down into the glacier, returning to the surface at the end of the day. (The name of one species, Mesenchytraeus solifugus, translates from the Latin as “flees from light.”) Ice worms don’t migrate horizontally, however, so every glacier is an island, its population genetically distinct. To capture food, the animals can anchor themselves in the ice and dangle their mouths into streams of snowmelt; they eat pollen, the spores of ferns, and snow algae. They die at temperatures much below fifteen degrees Fahrenheit or warmer than fifty degrees. The Web site for the North Cascade Glacier Climate Project notes that “it is hard to walk in the evening onto the snowpack . . . without squashing an ice worm.”

Dial learned about ice worms thirty years ago, while participating in a hundred-and-fifty-mile wilderness race across Alaska’s Kenai Peninsula. A shortcut took him across the Harding Ice Field. “It got dark and I was looking down at the snow and thought, Wow, there’s a lot of plant material,” he recalled. “There were these squiggly threads. I thought it was lichen, but, when I looked closer, there were hundreds, thousands, of worms on the surface.”

For the Nature Geoscience study, Dial and his colleagues set up a large experimental plot on the same ice field, dividing it into thirty-metre squares. They showered some squares with plain water, which increased the algae’s growth by half, and others with nutrient-rich water, which increased growth fourfold. Then they assessed the density of the snow in each square, measured how much the surface had lowered, and calculated the amount of snow the algae were melting. “Lo and behold, the more algae there were, the more melt there was,” Dial said. His team designed an algorithm for calculating melt, then used satellite imagery to generalize it across the entire field. “Seventeen per cent of all the water that was melting was from snow that had algae on it,” Dial said. “That’s a pretty big chunk.”

It’s probably too early to get alarmed about snow algae. And, anyway, they aren’t causing climate change; that’s on us. But they clearly thrive on it, and they are yet another indicator that humankind has barely begun to understand the resonances of its own existence, much less the steps it must take to insure its survival. And, of course, snow algae need snow; when that’s gone, which seems to be the direction of things, the snow algae will go, too, along with the ice worms and the wingless glacier midges. (Tardigrades could stick around longer; a recent study suggested that, if the world ends in a nuclear holocaust, they might be the only organisms that weather it.) Before the snow algae vanish, though, and while there’s still some glacier left, it’s entirely possible that the last snow we’ll see on Earth will be pink or even red—a blush, a bruise, a rush of blood.