Temperature-associated increases in the global soil respiration record

The carbon dioxide generated underground by plants and microbes and released into the atmosphere — termed soil respiration — comprises the second largest terrestrial carbon flux. It has been suggested that the flow of CO2 from this source should change with climate, but this has been difficult to confirm observationally. Ben Bond-Lamberty and Allison Thomson use a two-decade database of soil respiration measurements to show that not only is soil respiration increasing over time, but also that this increase is strongly associated with temperature changes. They estimate that total global soil respiration is increasing by about 0.1% per year, implying a moderate sensitivity to air temperature. This is consistent with an acceleration of the terrestrial carbon cycle in recent decades. Soil respiration (RS) is the flux of microbial- and plant-respired carbon dioxide from the soil surface to the atmosphere, and constitutes the second-largest terrestrial carbon flux. It has been suggested that RS should change with climate, but this has been difficult to confirm observationally. It is shown here, however, that the air temperature anomaly (the deviation from the 1961–1990 mean) correlates significantly and positively with changes in RS. 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 flux1,2,3. However, the dynamics of RS are not well understood and the global flux remains poorly constrained4,5. Ecosystem warming experiments6,7, modelling analyses8,9 and fundamental biokinetics10 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 ± 12 Pg C and that it increased by 0.1 Pg C yr-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.