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Soils store about four times as much carbon as plant biomass (1), and soil microbial respiration releases about 60 petagrams of carbon per year to the atmosphere as carbon dioxide (2). Short-term experiments have shown that soil microbial respiration increases exponentially with temperature (3). This information has been incorporated into soil carbon and Earth-system models, which suggest that warming-induced increases in carbon dioxide release from soils represent an important positive feedback loop that could influence twenty-first-century climate change (4). The magnitude of this feedback remains uncertain, however, not least because the response of soil microbial communities to changing temperatures has the potential to either decrease (5-7) or increase (8,9) warming-induced carbon losses substantially. Here we collect soils from different ecosystems along a climate gradient from the Arctic to the Amazon and investigate how microbial community-level responses control the temperature sensitivity of soil respiration. We find that the microbial community-level response more often enhances than reduces the mid- to long-term (90 days) temperature sensitivity of respiration. Furthermore, the strongest enhancing responses were observed in soils with high carbon-to-nitrogen ratios and in soils from cold climatic regions. After 90 days, microbial community responses increased the temperature sensitivity of respiration in high-latitude soils by a factor of 1.4 compared to the instantaneous temperature response. This suggests that the substantial carbon stores in Arctic and boreal soils could be more vulnerable to climate warming than currently predicted., Short-term experiments have demonstrated that the rate of soil microbial respiration increases exponentially with temperature, and this general relationship has been used to parameterize soil carbon (C) and Earth-system models [...]