Analysis of trends in fused AVHRR and MODIS NDVI data for 1982–2006: Indication for a CO₂ fertilization effect in global vegetation

Recent studies report an increase in vegetation greenness in mid‐to‐high northern latitudes. This increase is observed in leaf‐out data in Europe and North America since the 1950s and in satellite data since the 1980s. Increased vegetation greenness is potentially a factor contributing to a land CO 2 sink. Various causes for increased vegetation greenness are suggested, but their relative importance is uncertain. In the present study, the effect of climate and CO 2 fertilization on increased vegetation greenness and the land CO 2 sink are investigated. The study is organized as follows: (1) A model is used to simulate monthly global normalized difference vegetation index (NDVI) fields for 1901–2006. The model is derived from NDVI, precipitation, and temperature data for 1982–1999. The modeled fields, referred to as reconstructed vegetation index (RVI), are tested back in time on phenological data (1950s–1990s) and forward in time on Moderate Resolution Imaging Spectrometer (MODIS) data (2001–2006). The RVI represents the response of NDVI to variations in climate. (2) Residuals between RVI and NDVI are analyzed for associations with variations in downwelling solar radiation, nitrogen deposition, satellite‐related artifacts, and CO 2 fertilization. CO 2 fertilization was the only factor that improved RVI modeling. (3) The effect of climate variations and CO 2 fertilization on the land CO 2 sink, as manifested in the RVI, is explored with the Carnegie Ames Stanford Assimilation (CASA) model. Climate (temperature and precipitation) and CO 2 fertilization each explain approximately 40% of the observed global trend in NDVI for 1982–2006. For 1901–2006, estimated trends in NDVI related to CO 2 fertilization are four to five times larger than climate‐related trends. CASA simulations indicate that the CO 2 fertilization effect on vegetation greenness contributes about 0.7 Pg C per year to the recent land CO 2 sink. This is a conservative estimate and is likely larger. This effect of CO 2 fertilization would be a large component of the land carbon sink. In the supporting information the RVI is used as a common standard to fuse MODIS and advanced very high resolution radiometer (AVHRR) NDVI data. This fusion compares well with SeaWiFS data.