It’s no mystery why carbon dioxide (CO2) levels fluctuate with the seasons: As greenery grows in the spring and summer, it soaks up the planet-warming gas, and when trees shed their leaves in the autumn, some of that gas returns to the atmosphere. But scientists haven’t figured out why the differences between summer and winter concentrations of CO2 have been growing substantially at Arctic latitudes since the 1960s—in some regions, the fluctuations have increased as much as 25%. A new computer simulation fingers long-term warming in the Arctic, which has led to the proliferation of plants across large swaths of the landscape. The simulation was calibrated by using satellite observations, which have long showed increased greening across much of the Arctic since the early 1980s (including tundra sites such as those in eastern Russia, above). If the effects of climate change weren’t included in the model, the trends toward bigger seasonal variations in CO2 at Arctic latitudes disappeared, researchers report online today in Science. For now, the increases in CO2 soaked up by new vegetation—including trees now growing where shrubs used to dominate—more than compensate for the amounts of the gas released by thawing permafrost, the team says. But in the future, if soil nutrients are exhausted by the flush of new growth, CO2 generated by the decomposition of organic matter long trapped in the soil could end up adding to the overall concentrations of that planet-warming gas.