Planetary boundaries
Despite the apparent stress that humanity is causing to the Earth system, defining sustainable limits for our own existence has proved to be something of an intractable problem. But what if we could define global sustainability numerically?
In this issue of Nature, a group of renowned earth system and environmental scientists led by Johan Rockström of the Stockholm Resilience Centre make a first attempt at estimating boundaries for the biophysical processes that determine the Earth’s capacity for self-regulation.
Using existing data, Rockström and colleagues put ‘acceptable’ upper limits on seven environmental parameters: climate change, ocean acidification, stratospheric ozone depletion, freshwater use, biodiversity loss, the global cycles of nitrogen and phosphorus, and land-use change. Crossing even one of these boundaries, they say, would risk triggering abrupt or irreversible environmental changes. And if one boundary is transgressed, then others are at serious risk of being breached.
For some parameters, such as nitrogen loading and atmospheric CO2 concentrations, we may have already stepped out of our safety zone and need to back-pedal quickly. For others, such as ocean acidification, we may still have enough time to avoid catastrophic change if we act wisely.
But do we understand the Earth system well enough to know the real limits to environmental degradation? And if we can define them, even roughly, would doing so would ultimately help or hinder efforts to protect the planet? We posed these questions to seven leading experts, who were invited to respond to the ‘planetary boundaries’ proposal. Each author brings specific expertise to evaluating one aspect of the proposed framework. Their responses can be freely accessed at Nature Reports Climate Change. We’ve weighed in with our own thoughts in an editorial in Nature, and with a podcast. All of Nature’s coverage, plus a full length version of the paper by Rockström and colleagues, can be accessed here.
The commentaries are available individually at the following links:
William H. Schlesinger, President of the Cary Institute of Ecosystem Studies in Millbrook, New York comments on the boundary for global nitrogen and phosphorus cycles (html|pdf).
Steve Bass, senior fellow at the International Institute for Environment and Development, UK comments on the boundary for land-use change (html|pdf).
Myles Allen, physicist and climatologist at the University of Oxford, UK comments on the boundary for climate change (html|pdf).
Mario J. Molina, director of the Mario Molina Center for Strategic Studies in Energy and the Environment in Mexico City comments on the boundary for stratospheric ozone depletion (html|pdf).
David Molden, deputy director general for research at the International Water Management Institute in Sri Lanka comments on the boundary for freshwater availability (html|pdf).
Peter Brewer, ocean chemist and Senior Scientist at the Monterey Bay Aquarium Research Institute in Moss Landing, California comments on the boundary for ocean acidification (html|pdf).
Cristián Samper, Director of the Smithsonian National Museum of Natural History in Washington DC comments on the boundary for biodiversity loss (html|pdf).
For the most part, our respondents agree that the ‘planetary boundaries’ framework is a useful and worthwhile endeavour. But they also issue words of caution in choosing upper limits on environmental degradation. Some such as ocean chemist Peter Brewer question whether we know enough to choose the right parameters; on ocean acidification, for example, does an upper bound on aragonite saturation fully represent the potential detriment of loading the ocean with CO2?
Similarly, climatologist Myles Allen warns that setting a limit on long-term atmospheric carbon dioxide concentrations may distract from the much more immediate challenge of limiting warming to 2°C. Whatever the long-term target, keeping temperatures to no more than 2°C above pre-industrial values will require substantial emissions reductions over the coming decades, says Allen. Without knowing what future generations might do, and without a good understanding of the long-term behaviour of the carbon cycle, perhaps we should focus on what’s achievable in the near-term?
Others, such as Schlesinger say there are dangers associated with setting boundaries. While thresholds are comforting for policymakers, they can also be risky, says Schlesinger. After all, waiting for thresholds to be crossed can merely allow the continuation of misbehaviour that might be better nipped in the bud. And, as Molden points out, defining boundaries on a global scale, by necessity, overlooks the role of local circumstances in exacerbating or ameliorating the problem of managing scarce resources.
But the planetary boundaries concept and its first estimate of numeric values also gives us an important warning call of how close we are to overstressing the Earth.
Do you agree? Is the planetary boundaries framework a useful way of measuring sustainability? Join the discussion here on Climate Feedback.
Image: Shutterstock
Olive Heffernan
Comments
Myles Allen has written a blog post at Peer-to-Peer about his Nature Reports Climate Change commentary on the Nature Feature. See: http://blogs.nature.com/peer-to-peer/2009/09/post_4.html
Posted by: Maxine | September 23, 2009 06:55 PM
Wonderful, profound, vitally important, beautifully presented stuff. Thank you, Nature!
And facing this science, let's not mince words. It's not just "... an important warning call of how close we are to overstressing the Earth."
It's a blueprint of how we are now, seriously, already, overstressing the Earth. Our one and only Earth.
This science is a wayfinding guide for us to back off dramatically, especially and most urgently in the most over-stressed dimensions.
Posted by: Kevin Matthews, ArchitectureWeek | September 23, 2009 07:11 PM
How interesting! Great stuff, Nature, thanks a lot! (I'm lucky, because I've just began to dig into this stuff with "The Limits to Growth".
At first sight, they seem a bit strict with CO2 boundary. They set 350 ppm, but the usual value for 2ºC is 400 ppm (eg. Steering Committee). AFIK, Hansen is the only scientist that has suggested such a lower limit.
With that (more common) boundary, at the current rate of 2 ppm/year, we would overstep the limit in 7 years.
On the other hand, that 350 ppm boundary would give us a radiative forcing boundary of 1.2 W/m2 (instead of 1). With the usual 400 ppm it would be 1.9 W/m2) [F = 5.35ln(C/C0)].
Posted by: Jesus Rosino | September 23, 2009 09:18 PM
I found this feature by Nature, the associated commentaries, and the original paper by the Stockholm Resilience Centre to be incredibly depressing reading. Not because of the information they contain but for what they do not say. The entire motivation for examining these limits is because the size of the human population and its associated resource consumption and production of waste products have now become so huge that they threaten our own survival. Indeed, with our total ecological footprint now >1.25 planet Earths we are already in a state of population overshoot. However, the topic of population growth is such a vexed and contentious issue that not even a supposed centre of objectivity such as Nature appears able to handle it openly (though full credit to commentators David Molden and Peter Brewer for at least mentioning it). The scientific community seems to tie itself in knots trying to avoid talking about population issues. The language at the Stockholm Resilience Centre website is typical. Phrases such as “rapid expansion of human activities” and “the expanding human enterprise” do, of course, encapsulate the idea that environmental impact is a product of both population size and consumption. However, with the world population expanding linearly by 79 million people per year (the growth rate is no longer decreasing, www.worldwatch.org/node/6257) and each person needing about 2000 kcal from food per day to survive there truly are limits to growth that we appear to be rapidly approaching (especially in light of concerns that we have probably passed the peak of world oil production last year, see www.energybulletin.net/50077). In language better suited to a report by a management consultant the paper by Rockström et al. states, “The proposed concept of ‘planetary boundaries’ lays the groundwork for shifting our approach to governance and management, away from the essentially sectoral analyses of limits to growth aimed at minimizing negative externalities, towards the estimation of the safe space for human development”. This perpetuates the illusion that there is still uncertainty about the suicidal trajectory on which the human species is set and how close the edge of the cliff is. If we cannot talk openly and clearly about population growth (i.e. if we cannot, as a scientific community, recognise the scale and urgency of the problem) then there is no hope that the world can take the necessary action to avoid – or even ameliorate – the approaching ecological disaster (population crash).
Posted by: Michael Lardelli | September 24, 2009 02:20 AM
This model makes a lot of major unfounded assumptions, perhaps the biggest being that humans are incapable of thriving under climate scenarios that differ greatly from the Holocene.
Under current population growth scenarios its quite clear humans will have an increasing impact on the planet but this is not necessarily a negative.
Posted by: Marc Hendrickx | September 24, 2009 05:41 AM
Playing with phantasy numbers is just bad science
Posted by: Jarl Ahlbeck | September 24, 2009 07:16 AM