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

Abstract

[1] 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) (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.

Greening of the Earth and its drivers

“…The contributions of CO 2 fertilization and climate change are reliable according to the optimal fingerprint analysis, whereas the effects of LCC and nitrogen deposition should be interpreted with caution. Our estimation of CO 2 fertilization effects on vegetation growth is more prominent than Los 6 , probably owing to the different attribution approaches. When using only those ecosystem models (five out of ten) that incorporate nitrogen limitations and nitrogen deposition effects (Supplementary Table 1), the fraction of the LAI trend that is unambiguously attributed to CO 2 fertilization is slightly smaller (66.2 ± 13.2%, 0.045 ± 0.009 m 2 m −2 yr −1 ) than when using models that ignore nitrogen processes (75.0 ± 42.6%, 0.051 ± 0.029 m 2 m −2 yr −1 ).…”

“…None of these driving factors can be considered in isolation, given their strong interactions with one another. Previously, a few studies had investigated the drivers of global greenness trends 6,7,11 , with a limited number of models and satellite observations, which prevented an appropriate quantification of uncertainties 12 .Here, we investigate trends of leaf area index (LAI) and their drivers for the period 1982 to 2009 using three remotely sensed data sets (GIMMS3g, GLASS and GLOMAP) and outputs from ten ecosystem models run at global extent (see Supplementary Information). We use the growing season integrated leaf area index (hereafter, LAI; Methods) as the variable of our study.…”

“…None of these driving factors can be considered in isolation, given their strong interactions with one another. Previously, a few studies had investigated the drivers of global greenness trends 6,7,11 , with a limited number of models and satellite observations, which prevented an appropriate quantification of uncertainties 12 .…”

Analysis of CO<sub>2</sub> mole fraction data: first evidence of large-scale changes in CO<sub>2</sub> uptake at high northern latitudes

“…In contrast, our analysis of t CO 2 time series shows more uptake of CO 2 in spring and early summer and earlier onset of net release of CO 2 between mid-summer and autumn. A number of studies have linked increases in NDVI and subsequent carbon uptake with a CO 2 fertilization effect (Lim et al, 2004;Kaufmann et al, 2008;Los, 2013) which may be partly responsible for the observed increases in carbon uptake during this period. Our analysis of NDVI data shows that increases of vegetation greenness in spring and autumn have led to significant lengthening of the photosynthetic growing season over the measurement period, where autumn greening is changing in most regions at a greater rate than spring greening.…”

Earth System Model Needs for Including the Interactive Representation of Nitrogen Deposition and Drought Effects on Forested Ecosystems

“…Effects from elevated CO 2 can vary; CO 2 fertilization can help mitigate extreme heat and drought [146], or fertilization effects on productivity may be eliminated during drought and nutrient limitation [147]. These additional impacts on ecosystems are important, considering that CO 2 fertilization currently plays a larger role than N deposition in increasing productivity [148].…”