Planetary boundaries are a welcome new approach in the 'limits to growth' debate. For one thing, they shift our attention to the scale of planetary systems being altered by human activity. As a scientific organizing principle, the concept has many strengths. What scientists persistently ignore is the unpleasant fact that a good scientific concept isn't necessarily a good communications platform. In that sense, it will take much more than the presentation of a novel concept to spur action. It is imperative that we act now on several fronts to avert a calamity far greater than what we envision from climate change alone.

The key element in the planetary boundary framework is the provision of numerical target values for process variables that represent the boundaries. Rockström et al. (Nature 461, 472–475; 2009) provide first estimates for seven of nine environmental parameters by synthesizing available knowledge. It could be argued that with our limited understanding it is impossible to present reasonable numbers, or that the borders are much more malleable than the boundaries suggest, and with better or worse management, boundaries can be moved. Moreover, global values mask important issues at regional and local scales and conceal variability. On the other hand, the numbers are important because they provide targets for policymakers, giving a clear indication of the magnitude and direction of change. They also provide benchmarks and direction for science. As we improve our understanding of Earth processes and complex inter-relationships, these benchmarks can and will be updated.

So what are we to make of the water boundary suggested by the authors? Here at the International Water Management Institute, experience tells us that there are physical limits to human intervention into natural processes. We can also confirm that water limits have been reached or breached in many major river basins across the world, and the consequences are already manifest. For example, there is little or no additional streamflow or groundwater for further development remaining in the Murray–Darling River in Australia, the Yellow River in China, the Indus in Pakistan and India, the Amu and Syr Darya in central Asia, the Nile River, and the Colorado River in the United States and Mexico. All of these are important food-producing areas. These basins suffer from excessive pollution, river desiccation, competition for supplies and ecosystem degradation. The drying of the Aral Sea is one of the most infamous examples of ecosystem damage caused by breaching the limits of freshwater withdrawals. Freshwater biodiversity has plummeted as a result of the massive hydraulic construction era beginning in the 1960s. The main driver has been agricultural water use to meet the rising food demands of a growing population.

Johan Rockström and colleagues are suggesting that consumption of 'blue water' sources — evaporation and transpiration from rivers, lakes, groundwater reservoirs and irrigation — should not exceed 4,000 cubic kilometres per year. At present, blue water consumption is estimated at 2,600 cubic kilometres per year. The first thing to say is that the 4,000 figure is based on an analysis of a relatively small number of studies on the global supply and demand of water. When extrapolated (beyond the intentions of the original studies), they lead to a range of 4,000 to 6,000 cubic kilometres. If anything, this 4,000-cubic-kilometre value may well be too high.

Water for agriculture is one of the forces pushing us beyond our boundary limits. In many areas, dense concentrations of people living on arable land are using water at a rate that has exhausted supplies. In other parts of the world, there is ample water but its use is limited because the land or climate is not suitable for agriculture. In yet other places, such as sub-Saharan Africa, more water could be withdrawn, but expansion in water use is limited by financial and institutional capacity. These variations were not taken into account in the setting of the water boundary.

Another factor not taken into account is the widespread and erroneous assumption that useable water in nature can be readily accessed. In their quest for water and food security, many governments have devised grandiose plans to move massive volumes of water from water-rich to water-poor river basins. Examples include the Interlinking of Rivers Project in India and the South to North Water Diversion Project in China. The ecological consequences of these inter-basin water transfers remain unclear, but they are likely to be immense.

Essentially, the concept of a global limit overlooks the importance of local conditions and the role of management in magnifying or ameliorating problems. For this reason, the water boundary suggested by Röckstrom and colleagues may be too high. That said, the planetary boundaries concept and its first estimate of numeric values give us an important warning call that must be heeded. Rather than get bogged down in detailed arguments about the weaknesses of the approach or the methods of analysis, we now have a tool we can use to help us think more deeply — and urgently — about planetary limits and the critical actions we have to take.

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