One of the more comforting trends in our changing climate has reversed itself, researchers report, and now may be heading in the wrong direction.

From 1982 to 1999, plants around the world's continents responded to increasing concentrations of carbon dioxide in the atmosphere by growing more vigorously, and soaking up more CO2.

Now University of Montana researchers analyzing measurements taken during the last 10 years by instruments aboard NASA's Terra satellite report that other impacts of higher CO2 concentrations — large-scale regional droughts — have overtaken that global trend, especially in the tropics and the Southern Hemisphere.

The image above, courtesy of Maosheng Zhao and Steven W. Running of the University of Montana, who report their findings in the current issue of the journal Science, is what the last decade's trend in global growth — what scientists call "Net Primary Production" (NPP) — looks like. The brown depicts regions of reduced plant growth and the green is increased growth. Overall, in the last decade the world's plants returned to the atmosphere slightly more CO2 than they absorbed.

The researchers observed that the trend reversal came during a decade when global temperatures were the warmest since instrument-based recording began in the 1880s. The decade saw large-scale droughts in North America and China in 2000, in North America and Australia in 2002, in Europe in 2003, in the Amazon, Africa and Australia in 2005, and over large parts of Australia from 2007 through 2009.

Zhao and Running conclude:

Over the past 10 years, large-scale periodic regional droughts and a general drying trend over the (Southern Hemisphere) reduced global terrestrial NPP. Under a changing climate, severe regional droughts have become more frequent, a trend expected to continue for the foreseeable future. The warming-associated heat and drought not only decrease NPP, but also may trigger many more ecosystem disturbances, releasing carbon to the atmosphere. Reduced NPP potentially threatens global food security and future biofuel production and weakens the terrestrial carbon sink.

The scientists say only close monitoring in the coming years will answer the question: Is this reversal the result of "decadal variation, or a turning point" in the response of global vegetation in a changing climate?