Direct human influence on atmospheric CO2 seasonality from increased cropland productivity
Author(s): Josh M. Gray (corresponding author) ; Steve Frolking ; Eric A. Kort ; Deepak K. Ray ; Christopher J. Kucharik ; Navin Ramankutty ; Mark A. Friedl 
Ground- and aircraft-based measurements show that the seasonal amplitude of Northern Hemisphere atmospheric carbon dioxide (CO2 ) concentrations has increased by as much as 50 per cent over the past 50 years [1, 2, 3]. This increase has been linked to changes in temperate, boreal and arctic ecosystem properties and processes such as enhanced photosynthesis, increased heterotrophic respiration, and expansion of woody vegetation [4, 5, 6]. However, the precise causal mechanisms behind the observed changes in atmospheric CO2 seasonality remain unclear [2, 3, 4]. Here we use production statistics and a carbon accounting model to show that increases in agricultural productivity, which have been largely overlooked in previous investigations, explain as much as a quarter of the observed changes in atmospheric CO 2 seasonality. Specifically, Northern Hemisphere extratropical maize, wheat, rice, and soybean production grew by 240 per cent between 1961 and 2008, thereby increasing the amount of net carbon uptake by croplands during the Northern Hemisphere growing season by 0.33 petagrams. Maize alone accounts for two-thirds of this change, owing mostly to agricultural intensification within concentrated production zones in the midwestern United States and northern China. Maize, wheat, rice, and soybeans account for about 68 per cent of extratropical dry biomass production, so it is likely that the total impact of increased agricultural production exceeds the amount quantified here.
Changes in the seasonality of Northern Hemisphere atmospheric CO2 concentrations were first noted three decades ago using data from atmospheric monitoring sites at Mauna Loa, Hawaii and Barrow, Alaska [1, 7, 8]. Parallel evidence from remote sensing, ecosystem models, and eddy covariance measurements have established that Northern Hemisphere extratropical growing seasons have become longer, with concomitant changes in species composition, photosynthetic activity, and ecosystem respiration in boreal and arctic terrestrial ecosystems [4, 5, 9]. Hence, to explain observed increases in CO2 seasonality, most studies have focused on the role of climate-induced changes to the terrestrial biosphere in Northern Hemisphere mid- to high latitudes [2, 5, 6].
Graven et al.  recently compared Northern Hemisphere atmospheric CO2 concentrations collected from aircraft around 1960 with similar measurements collected around 2010. Their results not only confirm patterns observed from ground stations, but also reveal a strong latitudinal gradient in changes to the amplitude of CO2 seasonality, with measurements collected over boreal and arctic regions showing larger increases than measurements collected at lower latitudes. On the basis of the shape of the seasonal CO2 cycle at higher latitudes, Graven et al.  suggested that longer growing seasons are insufficient to explain the observed changes in atmospheric CO2 seasonality, and that enhanced uptake of CO2 during the middle of the growing season must also be occurring. Consistent with these results, our analyses show that changes in mid-latitude cropland production, with shorter and more intense carbon uptake periods than natural ecosystems , and...