Setting Boundaries: 10 Guidelines to Save Earth
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Scientists propose a set of safe limits for human impacts on Earth
The scale of humanity’s impact on the globe is becoming ever more apparent: we have wiped out species at a rate to rival great extinction events of all geologic time as well as contributing to a rapidly acidifying ocean, dwindling ice caps and even sinking river deltas. Now an international group of 29 scientists has taken a preliminary stab at setting some concrete environmental thresholds for the planet.
Johan Rockström of Stockholm University and his colleagues have proposed nine “planetary boundaries” online in the September 23 Nature. (Scientific American is part of the Nature Publishing Group.) The boundaries, dealing with climate change, ocean acidification, chemical pollution and others, are meant to set thresholds, or safe limits, for natural systems with respect to human impact, although exact numbers have not yet been determined for some.
“We have reached the planetary stage of sustainability, where we are fiddling with hard-wired processes at the global Earth-system scale,” Rockström says. “What are the Earth-system processes that determine the ability of the [planet] to remain in a stable state?”
The research takes as its desired stable state the Holocene epoch, the 10,000 years since the last ice age during which human civilization has flourished, and attempts to identify the key variables that might push planetary cycles past safe thresholds. So, for example, the key variable for climate change is atmospheric carbon dioxide concentration as well as its attendant rise in the amount of trapped heat. At present, atmospheric CO2 has reached 387 parts per million (ppm), well above the preindustrial figure of 280 ppm. The estimated safe threshold identified by the scientists, including NASA climatologist James Hansen, is 350 ppm, or a total increased warming of one watt per square meter (current warming is roughly 1.5 watts per square meter).
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