On Friday, 2 October we read the feature article ”A safe operating space for humanity” by Johan Rickström et al. (2009, Nature 46: 472–475). The paper outlines different biophysical processes that human action have affected and seeks to define thresholds – planetary boundaries – that we shouldn’t cross in order to keep the Earth habitable for our societies. The authors argue that three of nine of these thresholds have already been crossed: 1) the atmospheric concentration of carbon dioxide causing climate change, 2) the extinction rate of species and 3) the amount of nitrogen extracted from the atmosphere for human use, affecting nitrogen cycle in the nature have been increasing at a dangerous rate in the recent past. In addition, the authors suggest indicative thresholds for altering phosphorus cycle, stratospheric ozone depletion, ocean acidification, global freshwater use and change in land use. Atmospheric aerosol loading and chemical pollution are also among the processes that are likely to have thresholds, but these the authors did not define.
“Safe levels” of greenhouse gas concentrations is a well established concept – certain concentration leads to certain warming, and from the desired concentration, one can calculate how much greenhouse emissions we “can afford”, still avoiding the most catastrophic consequences of climate change. This can be divided further between nations and emission reduction targets relevant for policy-making. Here, the authors attempt to extend similar way of thinking to other environmental problems.
Defining these thresholds an ambitious exercise, as the processes are interlinked in many ways, and the ultimate limiting factors might be hard to track down. As the discussion continues, the relative importance of the different sectors could be accounted for, and the links between them could be highlighted more. The threshold parameters used here are sometimes confusing: instead of using extinction rate as a threshold parameter for biodiversity loss, one could consider e.g. the rate of habitat loss and degradation as that type of target would be easier to convert into political measures.
We were wondering about the values of some thresholds. For example, the global freshwater use is far from its safe boundary according to the paper, and yet the lack of drinking water is something you hear about quite often. Maybe it is because the consumption and supply of freshwater varies very much at an areal scale. One could consider dividing the global targets into more local ones in cases where the resource or threshold indicator varies spatially.
The nonlinear nature of biophysical responses is often hard to grasp. Being aware of it is crucial for sound policies. The paper by Rockström et al. is a good opening for discussion about these issues. In the run-up to Copenhagen climate summit, it is also a good reminder of other environmental problems aside global warming – these ought to be taken into account when planning for climate change mitigation and adaptation.
Link to the paper:
http://www.nature.com/nature/journal/v461/n7263/full/461472a.html
Johan Rockström, Will Steffen, Kevin Noone, Åsa Persson, F. Stuart Chapin, III, Eric F. Lambin, Timothy M. Lenton, Marten Scheffer, Carl Folke, Hans Joachim Schellnhuber, Björn Nykvist, Cynthia A. de Wit, Terry Hughes, Sander van der Leeuw, Henning Rodhe, Sverker Sörlin, Peter K. Snyder, Robert Costanza, Uno Svedin, Malin Falkenmark, Louise Karlberg, Robert W. Corell, Victoria J. Fabry, James Hansen, Brian Walker, Diana Liverman, Katherine Richardson, Paul Crutzen & Jonathan A. Foley