"The large (28%) disagreement between the model's predictions and ground truth imbues very little confidence in Zhao and Running's results," said Marcos Costa, coauthor, Professor of Agricultural Engineering at the Federal University of Viosa and Coordinator of Global Change Research at the Ministry of Science and Technology, Brazil.
This new study also found that the model actually predicted increased productivity during droughts, compared to field measurements, and decreased productivity in non-drought years 2006 and 2007 in the Amazon, in contradiction to the main finding of the previous report. "Such erratic behavior is typical of their poorly formulated model, which lacks explicit soil moisture dynamics," said Edson Nunes, coauthor and researcher at the Federal University of Viosa, Brazil.
The new study indicates that Zhao and Running used NASA's MODIS satellite data products, such as vegetation leaf area, without paying caution to data corruption by clouds and aerosols. "Analyzing the same satellite data products after carefully filtering out cloud and aerosol-corrupted data, we could not reproduce the patterns published by Zhao and Running. Moreover, none of their reported productivity trends are statistically significant," said Liang Xu, coauthor and graduate student at Boston University.
In any case, the trends in plant productivity reported by Zhao and Running are minisculea 0.34% reduction in the Southern Hemisphere offset by a 0.24% gain in the Northern Hemisphere for a net decline of 0.1% over a ten-year period from 2000 to 2009. "This is the proverbial needle in a haystack," said Simone Vieira, coauthor and researcher at the State University of Campinas, Brazil. "There is no model accurate enough to predict such minute changes over such short time intervals, even at hemispheric scales."
Any investigation of trends in plant growth requires
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