Has Plant Life Reached Its Limits? In 1972, a junior at Oregon State University bought a small paperback for $2.75 titled "The Limits of Growth." Citing the rate at which the population was expanding and the spread of industrialization, it projected a dismal future for the planet. The book made a deep impression on the student, Steven Running, a botany major. Forty years later, the prediction is being borne out, said Dr. Running, who is now a forest ecologist at the University of Montana and directs the university's Numerical Terradynamic Simulation Group. Humans are approaching the limits of the globe's finite plant life, he argues in an article published online on Thursday in the journal Science. Since 2000, Dr. Running and his colleagues have monitored how much plant growth covers terra firma, using two NASA satellites in the agency's Earth Observing System. After they crunched the numbers, combining the current monitoring system's data with satellite observations dating back to 1982, they noticed that terrestrial plant growth, also known as net primary production, remained relatively constant. Over the course of three decades, the observed plant growth on dry land has been about 53.6 petagrams of carbon each year, Dr. Running writes in the article. This suggests that plants' overall productivity - including the corn that humans grow and the trees people log for paper products - is changing little now, no matter how mankind tries to boost it, he said. Dr. Running calls plant productivity a new "planetary boundary," a concept set forth in a much-discussed paper published in 2009 in the journal Nature. In that article, Johan Rockstrom of Stockholm University and 28 other researchers identified nine "planetary boundaries," or thresholds that humans cannot cross without grave consequences for the planet. Some researchers, however, have challenged the idea that "planetary boundaries" can be clearly defined. The concept is "hopelessly naïve and simplistic," Stuart Pimm, a conservation ecologist at Duke University, argues. The current rate of species extinction is about 100 times the natural rate, said Dr. Pimm, whose research focuses on biodiversity. In the Science article, Dr. Rockstrom and his co-authors propose a boundary of 10 times the natural rate of extinction; beyond that, the Earth's ecosystems may become less resilient to climate change, they suggest. But Dr. Pimm said the multiple of 10 was an arbitrary choice. He also faults the notion that there is a clear threshold for the environmental impact of humans rather than gradual progressive consequences. "It's not as if we can keep doing business as usual until we hit a planetary boundary, and all hell will break loose," he said. It's already breaking loose now. Dr. Running's own research shows how manmade climate change is affecting plant productivity. His team's data show that in recent years plant growth has increased in high-latitude regions because climate change is extending growing seasons there, he said. At the same time, in areas where drought has struck, plant growth has declined. Although plant productivity for the globe has remained more or less constant, he and his team have observed regional differences that reflect the consequences of human activity, he added. Dr. Running emphasized that his overall conclusion is not one of doom and gloom. "The message is not that we're facing mass starvation in the future," he said, but rather to resuscitate an argument in that little book he read four decades ago and see that discussion advance. "There may be planetary limits, and we may be approaching them," Dr. Running said. We better start paying attention if that's true.