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35 Years of NDVI Increase in Nepal

Paper Reviewed
Krakauer, N.Y., Lakhankar, T. and Anadón, J.D. 2017. Mapping and attributing Normalized Difference Vegetation Index trends for Nepal. Remote Sensing 9: 986, doi:10.3390/rs9100986.

The Normalized Difference Vegetation Index (NDVI) is a remotely-sensed (primarily satellite-derived) measure of surface reflectivity that is well correlated with the presence and robustness of land vegetation. As such, it has been used in multiple studies in an attempt to evaluate the temporal change in vegetation in response to climate and non-climatic factors.

The latest group of scientists to conduct such an analysis was that of Krakauer et al. (2017). Focusing on the country of Nepal, the team of three scientists analyzed NDVI data for the region 26-31°N and 80-89°E over the period 1981 to 2015, seeking to determine if there has been any long-term trend in this index over the past three and a half decades. Thereafter, they constructed an empirical predictive model using a random forest of regression trees (hereafter, RF model) to estimate the influence of multiple potential predictor variables (including seasonal and geographical factors, temperature, precipitation, land cover and atmospheric CO2 concentration) on the NDVI record.

Results of their analysis are plotted in the figure below, which reveals a clear increase in NDVI across the period of record. This increase, as the authors correctly point out, "is [also] consistent with global trends," which also show significant increases in vegetation over the recent past (see, for example, the several studies reviewed here).

With regard to the cause of the observed vegetative increase, based upon the results of their RF model analysis, Krakauer et al. conclude that the increasing NDVI trend was "primarily due to [rising atmospheric] CO2." Indeed, as shown in their Table 2, carbon dioxide was the dominant factor driving this trend, with an influence that was more than 20 times the magnitude than that caused by temperature or precipitation. What is more, the CO2 effect was sufficiently large to overcome the NDVI-declining influence of land cover change, which opposing trend was about one-fourth the magnitude (but in the opposite direction) to that of CO2.

The above findings clearly document and demonstrate the positive effect that the aerial fertilization effect of atmospheric CO2 enrichment is having on the terrestrial biosphere, a benefit we have been highlighting on this website for over two decades now. It is high time for the deniers of the world to face the facts. Atmospheric CO2 is not a pollutant; it is the nutrient of life!

Figure 1. Time series of mean Nepal NDVI (Panel A) and NDVI anomaly (Panel B; mean seasonal cycle subtracted from data in Panel A) over the period 1981-2015. Source: Krakauer et al. (2017).

Posted 19 March 2018