The Arctic is experiencing rapid climate change. Not only is it warming, it is also weirding: variability is increasing. Arctic tundra is a key player in the global carbon (C) cycle, and because carbon affects climate, tundra also affects global climate. Despite decades of research, we still don’t know whether Arctic climate change will cause a damping feedback to climate change, making it not so bad, or an amplifying feedback, making it even worse. In this project we used 17 years of measurements of how green the tundra is to estimate how climate change affects carbon cycling in the tundra. Specifically, we estimated plant growth (carbon in), respiration from the whole ecosystem (carbon out), and the balance. We found a lot of year-to-year variation in tundra carbon cycling. For example, in one type of Arctic tundra, plants grew more than twice as much in the most productive year as they did in the least productive year. Rain and light were more important for tundra carbon cycling than temperature, and climate variability was at least as important as the average. Overall, our results indicate that climate weirding will be at least as important for Arctic tundra carbon cycling as climate warming.