Global soils store at least twice as much carbon as Earth’s atmosphere1,2. The global soil-to-atmosphere (or total soil respiration, RS) carbon dioxide (CO2) flux is increasing3,4, but the degree to which climate change will stimulate carbon losses from soils as a result of heterotrophic respiration (RH) remains highly uncertain5,6,7,8. Here we use an updated global soil respiration database9 to show that the observed soil surface RH:RS ratio increased significantly, from 0.54 to 0.63, between 1990 and 2014 (P = 0.009). Three additional lines of evidence provide support for this finding. By analysing two separate global gross primary production datasets10,11, we find that the ratios of both RH and RS to gross primary production have increased over time. Similarly, significant increases in RH are observed against the longest available solar-induced chlorophyll fluorescence global dataset, as well as gross primary production computed by an ensemble of global land models. We also show that the ratio of night-time net ecosystem exchange to gross primary production is rising across the FLUXNET201512 dataset. All trends are robust to sampling variability in ecosystem type, disturbance, methodology, CO2 fertilization effects and mean climate. Taken together, our findings provide observational evidence that global RH is rising, probably in response to environmental changes, consistent with meta-analyses13,14,15,16 and long-term experiments17. This suggests that climate-driven losses of soil carbon are currently occurring across many ecosystems, with a detectable and sustained trend emerging at the global scale.
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We are indebted to the thousands of researchers who measured and published the data collected here. This work used eddy-covariance data acquired and shared by the FLUXNET community (see Methods). This research was supported by the US Department of Energy, Office of Science, Biological and Environmental Research as part of the Terrestrial Ecosystem Sciences Program. The Pacific Northwest National Laboratory is operated for DOE by Battelle Memorial Institute under contract DE-AC05-76RL01830. R.V. acknowledges support from NASA-CMS (80NSSC18K0173) and USDA (2014-67003-22070). C.M.G. received additional support from the National Science Foundation Division of Environmental Biology, Award 1353908.
Nature thanks A. Konings, K. Ogle and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Extended data figures and tables
The model summarized in Table 1 was repeatedly fitted to the SRDB dataset, as filtered with various first and final year assumptions (that is, the earliest and latest measurement allowed, to test whether the results were an artefact of the 1989–2014 period chosen, indicated by a black diamond in the figure). Colour shows the significance of the RH:RS temporal trend.
Blue dots show MAT and MAP of the entries in the database; lighter points indicate more recent data. Grey background indicates global climate distribution in the HadCRUT433 data, with darker shades indicating more land area, for 1991–2010.
The model examines how the dependent variable—the ratio of respiration to GPP or SIF—is related to the independent variables: climate, land cover, disturbance and SOC content; for more details, see Methods. Two respiration fluxes (RH and RS), two GPP sources (the statistically upscaled MTE dataset, and the remotely sensed MODIS product), and two SIF sources (SCIAMACHY and GOME-2) are shown. Grey bands show 95% confidence intervals. Blue lines indicate least-squares trend, while grey bands show 95% confidence intervals.
Trends are not significant for the satellite data from GOME-2 SIF (P = 0.22), MODIS GPP (P = 0.07) or SCIMACHY SIF (P = 0.49). Blue lines (solid line is statistically significant, dashed line is non-significant) lines indicate least-squares trend. The temporal trend for MTE GPP decreases significantly (P = 0.02) over time, that is, MTE GPP increasingly underpredicts GPP measured by eddy covariance tower.
Extended Data Fig. 5 Sensitivity analysis of how much GPP satellites would have to be missing to render the GPP–respiration trends shown in Fig. 2a non-significant.
We ran a sensitivity analysis that assumed a conservative (that is, high) GPP trend of 0.5% yr-1, and repeatedly re-fitted the Fig. 2a models assuming that satellites missed 0%, 10%, 20%,...100% of this gain. This figure plots the ‘missed’ percentage (x axis) versus the P-value of the linear regression (y axis) for each respiration flux (RH and RS) and GPP or SIF data product (‘GPPadjusted’, that is, adjusted for the assumed ‘missed’ percentage). Even if satellites are missing 100% of this large assumed GPP increase, most of the temporal trends shown in Fig. 2a trends remain highly significant.
Panels show different International Geosphere-Biosphere Programme (IGBP) land cover type: crop (CRO), closed shrubland (CSH), deciduous broadleaf forest (DBF), evergreen broadleaf forest (EBF), evergreen needleleaf forest (ENF), grassland (GRA), mixed forests (MF), open shrublands (OSH), herbaceous savannahs (SAV), snow and ice (SNO), wetlands (WET) and woody savannahs (WSA).
Extended Data Fig. 7 Bootstrap analysis examining effect of ratio of significant to non-significant data on likelihood of observing a trend.
What is the likelihood that the FLUXNET data subset (that included GPP measurements made in the same study site and year as site-specific RS measurements in the SRDB) is too small to detect signals of rising RH, given site-to-site variability in climate and carbon dynamics? Each point is a different random draw from the original dataset, the comparison of RS to MODIS GPP, with 1,000 bootstrap draws per fraction of artificial no-trend data. The horizontal dashed line is P = 0.05. Note log scale (to base e, which R defaults to) of y axis. Boxplots visualize five summary statistics (the median, the first and third quartiles, and two whiskers; see https://ggplot2.tidyverse.org/reference/geom_boxplot.html); points are semi-transparent to indicate data density at each point.
Extended Data Fig. 8 Bootstrap analysis examining effect of dataset size on likelihood of observing a trend.
What is the likelihood that the n = 106 FLUXNET dataset is too small to detect signals of rising RH, given site-to-site variability in climate and carbon dynamics? Each point is a different random draw from the original dataset, the comparison of RS to MODIS GPP, with 1,000 bootstrap draws per fraction of no-trend data. The vertical dashed line shows the size of the original RS:FLUXNET dataset, about 5% of the RS:GPPMODIS data, while the horizontal line is P = 0.05. Note log scale (to base e) of y axis. Boxplots visualize five summary statistics (the median, the first and third quartiles, and two whiskers; see https://ggplot2.tidyverse.org/reference/geom_boxplot.html); points are semi-transparent to indicate data density at each point.
Extended Data Fig. 9 Distribution of the longitudinal site data in climate change space relative to the main SRDB dataset.
The longitudinal data (Fig. 2c, coloured dots below) in climate change space (1991–2010 HadCRUT4 changes in air temperature and precipitation) are shown over the distribution of the main dataset (n = 1,852, grey dots). The longitudinal data cover −0.03 to +0.08 °C yr−1 and −12 to +16 mm yr−1; the main dataset covers a much broader climate change space of −0.07 to +0.17 °C yr−1 and −35 to +57 mm yr−1.
References for FLUXNET site data used in this analysis. See http://fluxnet.fluxdata.org/data/data-policy/ . Citation numbers refer to references listed in the Methods.
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Nature Climate Change (2018)