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Studies indicate that, historically, terrestrial ecosystems of the northern high‐latitude region may have been responsible for up to 60% of the global net land‐based sink for atmospheric CO₂. However, these regions have recently experienced remarkable modification of the major driving forces of the carbon cycle, including surface air temperature warming that is significantly greater than the global average and associated increases in the frequency and severity of disturbances. Whether Arctic tundra and boreal forest ecosystems will continue to sequester atmospheric CO₂ in the face of these dramatic changes is unknown. Here we show the results of model simulations that estimate a 41 Tg C yr⁻¹ sink in the boreal land regions from 1997 to 2006, which represents a 73% reduction in the strength of the sink estimated for previous decades in the late 20th century. Our results suggest that CO₂ uptake by the region in …