Is preserving the general environmental conditions that allowed civilization to flourish—a moderate climate, a rich array of species, rivers that reach the sea—necessary to ensure humanity endures? Or is minimizing alterations to the global environment introduced by human activity—rising levels of CO2 from fossil-fuel burning, widespread extinction, dams that impound water—more important to our success? Choosing the right approach is vital as the scale of human impact on the planet becomes so large that scientists are calling this new epoch in Earth's history the Anthropocene (when human activity alters global climate and ecosystems).

One bid for preservation initiated in 2009 by 29 scientists from around the world focused on the concept of planetary boundaries. They identified 10 environmental limits we might not want to transgress in the Anthropocene: aerosol pollution; biodiversity loss; chemical pollution; climate change; freshwater use; changes in land use (forests to fields, for example); nitrogen and phosphorus cycles; ocean acidity; and the ozone hole. That meme has now spread to the United Nations, and is driving ongoing global talks to address environmental problems, including the much-anticipated Rio+20 summit meeting that begins next week in Rio de Janeiro.

But a new analysis from environmental policy think tank the Breakthrough Institute released June 12 argues that such a focus on environmental restoration is actually counterproductive when it comes to overall human welfare. "The planetary boundaries framework is not a useful guide for policy or environmental management in any concrete sense, as it does not capture the challenges involved in most of the environmental problems it lists," argues geographer Linus Blomqvist, policy associate at the institute's Conservation Program and co-author of the review. "They should be discarded."

Specifically, Blomqvist and his colleagues argue that six of the 10 boundaries—land use, biodiversity, nitrogen cycle, freshwater use, aerosol and chemical pollution—do not have a hard limit at planet-scale physical thresholds that, if transgressed, would tip them into functioning differently. For example, managing a given watershed may make more sense than managing the amount of global freshwater consumption to stay below an arbitrary, sustainable "limit" of 4,000 cubic kilometers per year.

Further, breaking any of these boundaries might not have any negative impact on humanity. Indeed, cutting back on, say, nitrogen fertilizer could significantly set back human welfare given that more than half of the people on the planet are fed by food grown with synthetic fertilizer. A pristine rainforest, after all, provides less direct benefit to humanity than additional food production. Instead, society should focus on environmental trade-offs, the review argues. "The real limitations for sustainability are rather our ability to grow enough food, maintain a healthy climate and so on," Blomqvist says, although the analysis fails to offer its own limits or policy suggestions other than focusing more on climate change.

Reducing the emissions of the greenhouse gases that cause global warming makes the most sense in the context of planetary boundaries, and many of the other thresholds collapse into it, Blomqvist and his colleagues note. That makes climate change the defining boundary of the Anthropocene. "Arguably, the single most important measure to ensure climate stability is reform of our energy and transport systems," Blomqvist says. "There is every reason to regard the Holocene climate as desirable." (The Holocene epoch extends from about 12,000 years ago to the present.)

Although the 2009 study's authors also noted no thresholds exist for some of their planetary boundaries they proposed limits on land-use change, freshwater, nutrients and biodiversity based on two criteria. First, they help determine the resilience of ecosystems on land and at sea, which in turn impact whether larger boundaries, such as climate change, are transgressed. Second, they are associated with tipping points at the local scale. After all, when a large enough number of local ecosystems transform, a global shift occurs, notes Johan Rockström, one of the authors of the planetary boundaries concept and a natural resource management professor at Stockholm University.

"It would be good to define planetary boundaries at multiple scales—local, regional and global," adds ecologist Jonathan Foley, director of the Institute on the Environment at the University of Minnesota and a co-author of the planetary boundaries concept. But "if there are major changes in the global environment, beyond what we have experienced in the Holocene, then this could represent a serious disruption to our civilization."

Many of the criticisms offered by the Breakthrough Institute were raised in the original paper that presented the planetary boundaries concept, published in Nature on September 24, 2009. (Scientific American is part of Nature Publishing Group.) Blomqvist concedes that point, yet he says: "Given the sometimes naive reception of the [planetary boundaries] concept, we thought it was worthwhile pointing out."

Other scientists have criticized the planetary boundaries as too generous (for example, allowing too much human appropriation of freshwater flows) or employing the wrong metric (atmospheric concentrations of CO2 rather than cumulative emissions of greenhouse gases).

At the same time any effort to identify a safe operating space for humanity must grapple with the fact that humans, on the whole, have never been better off—whether the metric be population, wealth or some other measure. Any ecological degradation has not led to a collapse in human welfare.

The point of the planetary boundaries, according to its authors, is to enable an enduring human prosperity that doesn't destroy the planet's natural resources in the process—ultimately undercutting that good fortune. After all, the present wealth and attendant short-term boost in population, consumption and technological growth may be largely founded on longer-term deterioration of the planet, including declining fish populations, acidifying oceans, degrading soils, remnant forests, polluting watersheds and a transforming climate. "Future generations will pay the price for this," Foley argues, unless human activity is redirected. "I'm still hopeful that we can do this, but it will represent a massive shift from our current way of running the world."

Blomqvist agrees: "We must not destroy the ability of future generations to enjoy a healthy, good life by depleting resources for short-term gain." In addition, he says, "humans everywhere want food as well as beautiful landscapes and a rich biological heritage."

The difference in approach comes down to how best to manage the Anthropocene: through planetary boundaries suggested by the environmental systems that allowed the epoch to come about or through local or regional efforts aimed at weighing the complex trade-offs among human resource use, ecological needs and a global push to combat climate change. "Arbitrary boundaries are not helpful and, if anything, can be dangerously misleading on local and regional levels," Blomqvist maintains. And "critical transitions" or "tipping points" as suggested by the planetary boundaries concept may not exist for many of the cases because those shifts, if they exist, have already happened. "Earth has already been fundamentally and thoroughly transformed by humans," Blomqvist notes. "Saying that land-use change has a tipping point in this respect is like saying that there's a tipping point for methane-farting cows."

But now that planetary management is no longer a luxury but a necessity, many policymakers, including some of the nations gathering in Rio this month, have adopted the 10 boundaries as their negotiating framework. That suggests that such planetary lines in the sand are of at least some utility, although researchers will continue to revise them in a bid to help create an enduring home for humanity. "I see it as a useful framework for dealing with the complexities of managing the planet," Foley says. "We will see how the ideas are useful—or not—in the coming weeks and months."