By Kennedy Maize

Washington, D.C., August 25, 2011 — A year ago, a peer-reviewed report in Science magazine concluded that a warmer planet over the past decade has led to drought that has reduced plant production and threatened worldwide food security. Today, a new, peer-reviewed study in the same journal argues that the results in the first report are bogus, the result of a flawed model and contradicted by empirical evidence.

The original research bolstered those who claim that man-made carbon dioxide emissions are already causing severe consequences and will result in widespread food shortages in the years ahead. The new work undercuts that linkage.

In the original report, Maosheng Zhao of the University of Montana and Steven Running, using a computer model of “net primary production” (NPP) of plants, found a decline over 2000-2009, compared to the prior decade. The decline was found mostly in a reduction in plants in the Amazon basin, resulting from a prolonged drought.

In today’s article, an international team of scientists from the U.S. and Brazil counters that the original work was driven by a flawed NPP model that does not reflect real data. “Satellite observations of vegetation activity show no statistically significant changes in more than 85% of the vegetated lands south of 70 degrees N during the same 2000 to 2009 period,” says the most recent analysis.

A press release from Boston University, where the lead author of the debunking team was a graduate student, said the most recent work refutes “earlier alarmist claims that drought has induced a decline in global plant productivity during the past decade and posed a threat to global food security.” Lead author Arindam Samanta, now at Atmospheric and Environmental Research Inc. in Lexington, Mass., said, “Their model has been tuned to predict lower productivity even for very small increases in temperature. Not surprisingly, their results were preordained.”

Coauthor Marcos Costa, coordinator of global change research at the Ministry of Science and Technology in Brazil and a professor at the Federal University of Vicosa, said, “The large (28%) disagreement between the model’s predictions and ground truth imbues very little confidence in Zhao and Running’s results.” Coauthor Liang Xu, a Boston University graduate student, said the Zhao and Running work did not take into account the way the clouds and aerosols corrupt NASA satellite measurements of ground cover. “Analyzing the same satellite data products after carefully filtering out clouds and aerosol-corrupted data,” said Liang, “we could not reproduce the patterns published by Zhao and Running. Moreover, none of their reported productivity trends are statistically significant.”

In a Science “technical comment” accompanying today’s article,  Zhao and Running defend their work, saying further analysis and data supports their original conclusions. “Our continuous monitoring shows that global NPP in 2010 was lower than in 2009, largely due to two large-scale droughts in Amazon and Europe. We expect that the strongest impacts of changing climate on terrestrial ecosystem productivity will continue to be manifested through the hydrologic cycle, but whether these current trends continue can only be answered by global monitoring.”

Another “technical comment” accompanying today’s article, by Belinda Medlyn  of Macquarie University in New South Wales, Australia, comments that the Zhao and Running findings are the result of a flawed computer models that are driven by temperature, not by actual plant data. The original researchers, she says, have not shown that NPP “has decreased over the last decade. Rather, they have shown that, if NPP is assumed to be affected by climate as specified in their model, then NPP would have declined over the past decade. It is important to make this distinction, because otherwise, we run the risk of mistaking model outcomes for reality.”