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Scientific American Magazine - March 19, 2010 Boundaries for a Healthy Planet Scientists have set thresholds for key environmental processes that, if crossed, could threaten Earth's habitability. Ominously, three have already been exceeded By Jonathan Foley, Gretchen C. Daily, Robert Howarth, David A. Vaccari, Adele C. Morris, Eric F. Lambin, Scott C. Doney, Peter H. Gleick and David W. Fahey For nearly 10,000 years—since the dawn of civilization and the Holocene era—our world seemed unimaginably large. Vast frontiers of land and ocean offered infinite resources. Humans could pollute freely, and they could avoid any local repercussions simply by moving elsewhere. People built entire empires and economic systems on their ability to exploit what seemed to be inexhaustible riches, never realizing that the privilege would come to an end. But thanks to advances in public health, the industrial revolution and later the green revolution, population has surged from about one billion in 1800 to nearly seven billion today. In the past 50 years alone, our numbers have more than doubled. Fueled by affluence, our use of resources has also reached staggering levels; in 50 years the global consumption of food and freshwater has more than tripled, and fossil-fuel use has risen fourfold. We now co-opt between one third and one half of all the photosynthesis on the planet. This wanton growth has also expanded pollution from a local problem to a global assault. Stratospheric ozone depletion and greenhouse gas concentrations are obvious complications, but many other deleterious effects are rising. The sudden acceleration of population growth, resource consumption and environmental damage has changed the planet. We now live in a “full” world, with limited resources and capacity to absorb waste. The rules for living on such a world are different, too. Most fundamentally, we must take steps to ensure that we function within the “safe operating space” of our environmental systems. If we do not revise our ways, we will cause catastrophic changes that could have disastrous consequences for humankind. What would cause these changes? And how can we avoid them? A worldwide team of scientists—led by Johan Rockström of the Stockholm Resilience Center in Sweden,
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with colleagues from Europe, the U.S. (including me) and Australia—recently sought answers through a larger, related question: Are we nearing planetary “tipping points” that would push the global environment into dangerous new territory, outside anything seen during human history? After examining numerous interdisciplinary studies of physical and biological systems, our team determined that nine environmental processes could disrupt the planet’s ability to support human life. We then set boundaries for these processes—limits within which humankind can operate safely. Seven of the processes have clear boundaries, scientifically defined by a single number (that of course carries some uncertainy). Three
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