Influence of Vegetation Growth on the Enhanced Seasonality of Atmospheric CO2

The amplitude of seasonal fluctuations in concentrations of atmospheric CO2 has increased over recent decades. Model-based studies suggest that this increase could be the result of enhanced vegetation growth during the growing season and ecosystem respiration in the nongrowing season. Here we investigated seasonal changes in vegetation growth derived from satellite-based observations of the normalized difference vegetation index (NDVI) from 1982 to 2013. We found notable agreement between these observations and interannual variations of seasonality of global atmospheric CO2, suggesting that terrestrial vegetation growth is the dominant driver of the seasonality of the concentration of atmospheric CO2. Specifically, we found that the trend in seasonality of global vegetation growth was not continuous from 1982 to 2013 and that it increased substantially after 2001. In response, the trend of seasonality in the concentration of atmospheric CO2 stalled from 1982 to 2000 but increased from 2001 onward. This 2001 change in the growth seasonality trend was largely a result of decreased NDVI during spring and winter. CMIP5 models were unable to reproduce this observed seasonality. Our results showed the dominant role played by vegetation growth in determining atmospheric CO2 seasonality, highlighting the need to improve representation of vegetation growth in current terrestrial models to adequately indicate seasonal changes in the concentration of atmospheric CO2.