The atmospheric carbon dioxide (CO2) record displays a prominent seasonal cycle that arises mainly from changes in vegetation growth and the corresponding CO2 uptake during the boreal spring and summer growing seasons and CO2 release during the autumn and winter seasons1,2,3,4. The CO2 seasonal amplitude has increased over the past five decades, suggesting an increase in Northern Hemisphere biospheric activity2,5,6. It has been proposed that vegetation growth may have been stimulated by higher concentrations of CO2 as well as by warming in recent decades, but such mechanisms have been unable to explain the full range and magnitude of the observed increase in CO2 seasonal amplitude2,6,7,8,9,10,11,12,13. Here we suggest that the intensification of agriculture (the Green Revolution, in which much greater crop yield per unit area was achieved by hybridization, irrigation and fertilization) during the past five decades is a driver of changes in the seasonal characteristics of the global carbon cycle. Our analysis of CO2 data and atmospheric inversions shows a robust 15 per cent long-term increase in CO2 seasonal amplitude from 1961 to 2010, punctuated by large decadal and interannual variations. Using a terrestrial carbon cycle model that takes into account high-yield cultivars, fertilizer use and irrigation, we find that the long-term increase in CO2 seasonal amplitude arises from two major regions: the mid-latitude cropland between 25° N and 60° N and the high-latitude natural vegetation between 50° N and 70° N. The long-term trend of seasonal amplitude increase is 0.311 ± 0.027 per cent per year, of which sensitivity experiments attribute 45, 29 and 26 per cent to land-use change, climate variability and change, and increased productivity due to CO2 fertilization, respectively. Vegetation growth was earlier by one to two weeks, as measured by the mid-point of vegetation carbon uptake, and took up 0.5 petagrams more carbon in July, the height of the growing season, during 2001–2010 than in 1961–1970, suggesting that human land use and management contribute to seasonal changes in the CO2 exchange between the biosphere and the atmosphere.
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We thank all data providers, especially the NOAA CO2 and CarbonTracker team, and the Jena inversion team. M. Heimann suggested the flux data site comparison. This research was supported by NOAA (NA10OAR4310248 and NA09NES4400006), the NSF (AGS-1129088), and NASA (NNH12AU35I).
The authors declare no competing financial interests.
Extended data figures and tables
Extended Data Figure 1 Management intensity (relative to year 2000) and harvest index change over time as used in the model.
The analytical functions are hyperbolic tangent (see text). The parameter values correspond to a management intensity in 1961 that is 10% smaller than in 2010, and a harvest index of 0.31 in 1961 and 0.49 in 2010, based on literature review34.
FTA (in Pg C yr−1) for the period 2001–2010 from the VEGAS model, compared to atmospheric inversions of Jena81, Jena99 and CarbonTracker.
a, Mauna Loa Observatory CO2, in parts per million, p.p.m. b, Mauna Loa Observatory CO2 growth rate, dCO2/dt; c, FTA from VEGAS; d, FTA from the atmospheric inversion of Jena81. Trends and high-frequency variations have been removed following ref. 23. The seasonal amplitude in Figs 2 and 3 is calculated as the difference between the maximum and minimum FTA each year.
Fluxes are summed over latitude bands for the VEGAS model and from two atmospheric CO2 inversions: Jena99 and the CarbonTracker. The Northern Hemisphere mid-to-high-latitude region, driven by the winter–summer temperature contrast, is the main contributor. The Southern Hemisphere has the opposite phase to that of the Northern Hemisphere, but its contribution to the global total is small owing to its small land area. The two subtropical maxima around 10° N and 10° S are due to the wet–dry seasonal shift in the Inter-Tropical Convergence Zone (ITCZ) and monsoon movement; these are out of phase and largely cancel each other out in terms of their net contribution to the global total FTA seasonal amplitude. The results are resampled into 2.5° latitude bands from the original resolutions of 0.5° × 0.5° for VEGAS, 5.0° × 5.0° for Jena99 and 1.0° × 1.0° for CarbonTracker.
Modelled linear trends (in units of kg C m−2 over 50 years, upper panel) from 1961 to 2010 show major increases in the agricultural areas of North America, Europe and Asia (the lower panel shows the crop fraction in 2000). There are also widespread increases in much of the Northern Hemisphere, especially the high-latitude regions in response to warming and the CO2 fertilization effect. Together, they are mostly responsible for the increase in FTA and the CO2 seasonal amplitude. Decreases in some regions are due to climate trends. Detailed regional patterns may not be well captured, for example, in the former Soviet Union, because of the simplified model representation of temporal changes.
Measurements of SIF with the GOME-2 instrument on board the MetOp-A satellite platform are compared to GPP estimates from the data-driven model from MPI-BGC35, and four mechanistic carbon cycle models (VEGAS, LPJ, Orchidee, and LPJ-Guess) from the TRENDY international project.
Comparison of VEGAS model (line) and FLUXNET observations (circles) at an agricultural site Bondville, Illinois (88.290398° W, 40.006199° N). a, GPP; b, Re, c, NEE ( = Re minus GPP = FTA). Shown are seasonal cycles averaged over the period 1996–2007.
Uncertainties in the CO2 seasonal amplitude trend due to model parametric errors in representing agricultural NPP. The upper and lower uncertainty range are labelled ALL + u and ALL − u respectively where u is uncertainty (standard deviation).
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Zeng, N., Zhao, F., Collatz, G. et al. Agricultural Green Revolution as a driver of increasing atmospheric CO2 seasonal amplitude. Nature 515, 394–397 (2014). https://doi.org/10.1038/nature13893
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