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