Remote‐Sensing Derived Trends in Gross Primary Production Explain Increases in the CO2 Seasonal Cycle Amplitude.

An increase in the seasonal cycle amplitude (SCA) of atmospheric CO2 since the 1960s has been observed in the Northern Hemisphere (NH). However, the underlying dominant drivers are still debated. The peak season CO2 uptake by vegetation is critical in shaping the CO2 seasonality. Using satellite‐upscaled gross primary production (GPP) from FLUXCOM and near‐infrared reflectance of vegetation (NIRV), we demonstrate that peak GPP has increased across the NH over the last two decades. We relate this productivity increase to changes in the CO2 SCA using an atmospheric transport model. The increased photosynthesis has strongly contributed to CO2 SCA trends, but with substantial latitudinal and longitudinal variations. Despite a general increase in the CO2 SCA, there are distinct regional differences. These differences are mainly controlled by regional biosphere carbon fluxes, with the remainder explained by non‐biome factors, including large‐scale atmospheric transport, changes in fossil fuel combustion, biomass burning and oceanic fluxes. Using the global flask and in situ CO2 measurement sites, we find that SCA trends at high latitude are mainly driven by increasingly productive natural ecosystems, whereas mid latitude sites around the Midwest United States are mainly impacted by intensified agriculture and atmospheric transport. Averaging across the 15 long‐term surface sites, forests contribute 26% (7%) to the SCA trends, while crops contribute 17% (24%) and the combined shrubland, grassland and wetland regions contribute 23% (37%) for simulations driven by FLUXCOM (NIRv) ecosystem fluxes. Our findings demonstrate that satellite inferred trends of ecosystem fluxes can capture the observed CO2 SCA trend. Plain Language Summary: An increase in the seasonal cycle amplitude (SCA) of atmospheric carbon dioxide (CO2) since the 1960s has been observed in the Northern Hemisphere (NH). However, dominant drivers of the amplified CO2 seasonality are still debated. In this study, we employ satellite‐based remote sensing observations to track spatial and temporal changes in global gross primary production (GPP) of different vegetation types. Then, we use a state‐of‐art atmospheric transport model to examine whether our bottom‐up estimates of ecosystem fluxes can capture the magnitude of CO2 SCA trends at multiple surface sites. Further, we explore dominant drivers of the observed CO2 SCA trends across different locations of sites. To our best knowledge, this paper makes the first effort to link long‐term satellite remote sensing observations with ground atmospheric CO2 measurements across the NH to explore how terrestrial carbon fluxes shape and change spatiotemporal pattern of atmospheric CO2. Key Points: Peak growing season gross primary production has increased across the northern extratropics over the last two decades based on satellite observationsThe remote sensing‐based global terrestrial carbon fluxes well capture the observed CO2 seasonal cycle amplitude trends at surface sitesMajor factors driving the observed increase in the atmospheric CO2 seasonal amplitude trends vary across the location of sites [ABSTRACT FROM AUTHOR]