Abstract
Long-term trends in photosynthetic capacity measured with the satellite-derived Normalized Difference Vegetation Index (NDVI) are usually associated with climate change. Human impacts on the global land surface are typically not accounted for. Here, we provide the first global analysis quantifying the effect of the earth’s human footprint on NDVI trends. Globally, more than 20% of the variability in NDVI trends was explained by anthropogenic factors such as land use, nitrogen fertilization, and irrigation. Intensely used land classes, such as villages, showed the greatest rates of increase in NDVI, more than twice than those of forests. These findings reveal that factors beyond climate influence global long-term trends in NDVI and suggest that global climate change models and analyses of primary productivity should incorporate land use effects.
Highlights
Climate change research often focuses on long-term productivity trends in vegetation at regional [1,2,3] to continental and global scales [4,5]
When comparing ecoregions with the fastest Normalized Difference Vegetation Index (NDVI) increases against those showing the largest NDVI decreases, we found that the ecoregions exhibiting the fastest NDVI increases had a greater average population density
We used NDVI data from the Advanced Very High Resolution Radiometer (AVHRR) instrument developed into the GIMMS3g dataset (Global Inventory Modeling and Mapping Studies) [33,34]
Summary
Climate change research often focuses on long-term productivity trends in vegetation at regional [1,2,3] to continental and global scales [4,5]. Several regional and local analyses demonstrate significant impacts on NDVI trends from changes in human land-use practices [14,15,16], global-scale analyses relating trends of NDVI to these direct anthropogenic effects have received little attention [5]. We provide one of the first global analyses of direct anthropogenic effects on long-term trends in NDVI from 1981 to 2010 ( see [17] for an analysis that include land use but on a different scale). Anthromes were introduced by Ellis and Ramankutty [13] and classify the earths land surface based on a combination of data on population density, land-use, and land cover [13] They characterize land surface based on global patterns of sustained, direct human interaction with ecosystems. We tested for specific mechanistic linkages that may underlie spatial variation in trends of NDVI among ecoregions and investigated effects of human land conversion (measured as percentage village, urban areas and croplands per ecoregion), nitrogen deposition, and irrigation
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