Every year, migratory seabirds paint the rocks of Arctic nations with their guano—or poop—as they nestle in for the summer. And that high volume of ammonia-rich poop, it turns out, actually has a slight cooling effect on the atmosphere, researchers report today in Nature Communications. Using model simulations combined with summertime observations of the atmosphere around Nunavut in Canada, the team found that the emissions of ammonia from guano unleash summertime bursts of particles in the atmosphere. Once it goes airborne, the ammonia interacts with sulfuric acid and water, forming particles that become the nuclei around which water condenses, giving birth to clouds. Clouds play a complicated role in the climate, but the low-lying clouds seeded by the ammonia reflect incoming sunlight and ultimately have a cooling effect on the region, the researchers found. It’s a significant regional effect across the Arctic, they note—but not enough to counteract its ongoing rapid warming.