The amount of carbon absorbed and released by global terrestrial ecosystems varies greatly from year-to-year. Understanding the geographic distribution of this year-to-year variability is essential in our understanding of how carbon moves around the globe, and its subsequent impacts on climate change. However, the geographic region most responsible for this year-to-year variation is currently under debate, with either global drylands or tropical forests often being identified as most responsible. Thus, we investigated whether using different definitions of drylands and tropical forests could change which of these biomes became the most responsible for the year-to-year variability. Additionally, we investigated the origin of this year-to-year variability at two additional geographical scales: individual 0.5 ° latitude x 0.5 ° longitude grid cells, and ecoregions (which are groupings of similar ecosystems, sized in-between individual grid cells and global biomes). Using simulation results from 18 dynamic global vegetation models, we found that dryland regions are a significantly greater contributor to the year-to-year variability in the global release and absorption of carbon, regardless of how drylands are defined or the scale of analysis. Our findings highlight the importance of dryland ecosystems within the context of the global carbon cycle and climate change.