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7 Tipping points
Humans must stay within certain boundaries if they hope to avoid environmental catastrophe, a leading group of environmental scientists says. Crossing those limits may not rock the Earth itself, but would lead to harsh consequences for human existence on the planet as we know it.
There are two kinds of boundaries, the researchers proposed in October 2009. "One represents a tipping point — you cross that and irreversible, catastrophic bad stuff happens," said Jonathan Foley, an ecologist at the University of Minnesota. "The other would involve more gradual changes, but still well outside the range of anything we've seen in human history."
Humans have already pushed the planet beyond some of the limits, such as those related to climate change and the nitrogen cycle. But some scientists who responded in the journal Nature questioned the threshold idea, and others commented that such limits seem arbitrary. Still, many applauded the idea of limits as benchmarks or starting points.
Here are the seven planetary boundaries that have been put on the table for discussion.
Stratospheric ozoneSlide 2 of 15
Earth's ozone layer might have eroded to the point where people get sunburned within minutes, if political leaders and scientists had not rallied to regulate the chemicals destroying the ozone, which protects us from solar radiation. The Montreal Protocol banned chlorofluorocarbons (CFCs) in 1989, and helped banish the specter of a future world with a permanent ozone hole yawning above Antarctica.
Environmental scientists have proposed a tipping point of a 5-percent decrease in ozone in the stratosphere (an upper layer of the atmosphere), based on ozone levels from 1964-1980.
A more realistic tipping point for stratospheric ozone might be higher, said Mario Molina, a physical chemist who heads the Center for Strategic Studies in Energy and the Environment in Mexico City. Truly catastrophic ozone depletion all across the globe would be something like a 60-percent decrease. But Molina added the lower limit on ozone destruction makes sense, given the damage to human health and the environment beyond ozone loss of 5 percent.Slide 3 of 15
Land useSlide 4 of 15
Agriculture and industry have long formed the bedrock of human civilization, so that the current crop cover supporting today's population has reached about 12 percent of land. Now environmental scientists have proposed a 15-percent land use limit, leaving some wiggle room, but still protecting animals and plants from losing valuable real estate.
The limit is a "sound idea" but also premature, according to Steve Bass, a senior fellow at the International Institute for Environment and Development in London. Bass pointed out that the arbitrary limit might leave policymakers unconvinced. After all, converting land to farming and industry has delivered huge benefits for human populations.
A better boundary of environmental health might be a limit on soil degradation or soil loss, Bass said. That could gauge the environmental impact of different types of land use, such as intensively farmed cropland versus more sustainable agriculture. Poor land-use practices have historically led to loss of soil and have also created terrible dust storms, whether in the 1930s Dust Bowl or in modern-day China.Slide 5 of 15
Land useSlide 6 of 15
Drinking water represents a basic necessity for life, but humans also use huge amounts for growing crops. Foley and his colleagues suggested that use of “blue water” sources — evaporation from rivers, lakes, groundwater reservoirs and irrigation — should not go beyond 960 cubic miles (4,000 cubic kilometers) per year, or just a little less than the entire volume of Lake Michigan. Humans currently use 624 cubic miles (2,600 cubic kilometers) each year.
But that global limit on freshwater might be too high, said David Molden, deputy director general for research at the International Water Management Institute in Sri Lanka. Molden contends the global view overlooks local conditions that limit how easily people can access freshwater, whether it's lack of infrastructure or lack of money, as well as the proportion of its water each region uses.
Intense agriculture might use up most of the freshwater in one region, not to mention a growing demand for biofuel crops that stresses water supplies. Another part of the world with plenty of freshwater might not use much for farming at all. So water limits might have to be customized for the region. Still, Molden called the idea of planetary boundaries an "important warning call" and a starting point to think about limits.Slide 7 of 15
Ocean acidificationSlide 8 of 15