Approximately twice as much soil carbon is stored in the northern circumpolar permafrost zone than is currently contained in the atmosphere. Permafrost thaw, and the microbial decomposition of previously frozen organic carbon, is considered one of the most likely positive feedbacks from terrestrial ecosystems to the atmosphere in a warmer world. Yet, the rate and form of release is highly uncertain but crucial for predicting the strength and timing of this carbon cycle feedback this century and beyond. New insight brought together under a multi-year synthesis effort by the Permafrost Carbon Network helps constrain current understanding of the permafrost carbon feedback to climate, and provides a framework for newly developing research initiatives in this region. A newly enlarged soil carbon database continues to verify the widespread pattern of large quantities of carbon accumulated deep in permafrost soils. The known pool of permafrost carbon is now estimated to be 1330-1580 Pg C, with the potential for ~400 Pg C in deep permafrost sediments that remain largely unquantified. Laboratory incubations of these permafrost soils reveal that a significant fraction of this material can be mineralized by microbes and converted to CO2 and CH4 on time scales of years to decades, with decade-long average losses from aerobic incubations ranging from 6-34% of initial carbon. Variation in loss rates is depended on the carbon to nitrogen ratio, with higher values leading to more proportional loss. Model scenarios show potential C release from the permafrost zone ranging from 37-174 Pg C by 2100 under the current climate warming trajectory (RCP 8.5), with an average across models of 92±17 Pg C. Furthermore, thawing permafrost C is forecasted to impact global climate for centuries, with models, on average, estimating 59% of total C emissions after 2100. Taken together, greenhouse gas emissions from warming permafrost appear likely to occur at a magnitude similar to other historically important biospheric C sources, such as land use change, but that is only a fraction of current fossil fuel emissions. Permafrost C emissions are likely to be felt over decades to centuries as northern regions warm, making climate change happen even faster than we think based on projected emissions from human activities alone.