Soil respiration, RS, the flux of microbially and plant-respired carbon dioxide (CO2) from the soil surface to the atmosphere, is the second-largest terrestrial carbon flux. However, the dynamics of RS are not well understood and the global flux remains poorly constrained. Ecosystem warming experiments, modelling analyses and fundamental biokinetics all suggest that RS should change with climate. This has been difficult to confirm observationally because of the high spatial variability of RS, inaccessibility of the soil medium and the inability of remote-sensing instruments to measure RS on large scales. Despite these constraints, it may be possible to discern climate-driven changes in regional or global RS values in the extant four-decade record of RS chamber measurements. Here we construct a database of worldwide RS observations matched with high-resolution historical climate data and find a previously unknown temporal trend in the RS record after accounting for mean annual climate, leaf area, nitrogen deposition and changes in CO2 measurement technique. We find that the air temperature anomaly (the deviation from the 1961-1990 mean) is significantly and positively correlated with changes in RS. We estimate that the global RS in 2008 (that is, the flux integrated over the Earth's land surface over 2008) was 98+/-12PgC and that it increased by 0.1PgCyr-1 between 1989 and 2008, implying a global RS response to air temperature (Q10) of 1.5. An increasing global RS value does not necessarily constitute a positive feedback to the atmosphere, as it could be driven by higher carbon inputs to soil rather than by mobilization of stored older carbon. The available data are, however, consistent with an acceleration of the terrestrial carbon cycle in response to global climate change.