title: Direct human influence on atmospheric CO2 seasonality from increased cropland productivity.
abstract: 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. 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. However, the precise causal mechanisms behind the observed changes in atmospheric CO2 seasonality remain unclear. 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 CO2 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.
authors.affiliation: Department of Earth and Environment, Boston University, Boston, Massachussetts 02215, USA.
authors.forename: Josh M
authors.initials: JM
authors.lastname: Gray
authors.affiliation: Earth Systems Research Center, University of New Hampshire, Durham, New Hampshire 03824, USA.
authors.forename: Steve
authors.initials: S
authors.lastname: Frolking
authors.affiliation: Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA.
authors.forename: Eric A
authors.initials: EA
authors.lastname: Kort
authors.affiliation: Institute on the Environment, University of Minnesota, Saint Paul, Minnesota 55108, USA.
authors.forename: Deepak K
authors.initials: DK
authors.lastname: Ray
authors.affiliation: Department of Agronomy and Nelson Institute Center for Sustainability and the Global Environment, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Kucharik
authors.affiliation: Department of Geography, McGill University, Montreal, Quebec H3A 0B9, Canada.
authors.forename: Navin
authors.initials: N
authors.lastname: Ramankutty
authors.affiliation: Department of Earth and Environment, Boston University, Boston, Massachussetts 02215, USA.
authors.forename: Mark A
authors.initials: MA
authors.lastname: Friedl
mesh_terms: D000383:Agriculture / Q000706:statistics & numerical data*; D001272:Atmosphere / Q000737:chemistry*; D018533:Biomass; D002245:Carbon Dioxide / Q000032:analysis* / Q000378:metabolism; D018556:Crops, Agricultural / Q000254:growth & development / Q000378:metabolism*; D017753:Ecosystem; D004526:Efficiency; D006802:Human Activities; D012621:Seasons
chemical_list: D002245:Carbon Dioxide