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Johan Rockström and colleagues' description of planetary boundaries (Nature 461, 472–475; 2009) is a sound idea. We need to know how to live within the unusually stable conditions of our present Holocene period and not do anything that causes irreversible environmental change.

Planetary boundaries build on a long and respectable tradition of research and thinking on ecological limits, such as the 'limits to growth' thesis of 1972, as well as more recent developments, such as the idea of the ecological footprint and the Millennium Ecosystem Assessment — though Rockström and colleagues would have done well to acknowledge these foundations.

Their paper has profound implications for future governance systems, offering some of the 'wiring' needed to link governance of national and global economies with governance of the environment and natural resources. The planetary boundaries concept should enable policymakers to understand more clearly that, like human rights and representative government, environmental change knows no borders.

That said, there is much work to be done before the concept can be used practically — before it can be 'operationalized'. What policymakers need is a clear instruction that says something to the effect of 'keep off the grass'. What the planetary boundaries paper provides is closer to an index of lawn carrying capacity expressed in terms of soil engineering and grass regeneration.

One of the boundaries described is land-use change. The authors say there needs to be a limit on the amount of the world's land surface that is converted for farming or industry. They suggest that no more than 15 per cent of land should be used as cropland. Current crop cover is around 12 per cent.

Rockström and colleagues will be the first to accept that the 15-per-cent figure is not a consensus value that can be validated in the research literature, but rather is based on a sensible — though apparently arbitrary — expansion factor. In that regard, they need to be prepared for at least two critical questions. First, if a figure of 15 per cent cannot be authenticated scientifically, policymakers will want to know why they should pay attention to it. Why shouldn't, say, 20 per cent of land surface be used for farming? Or indeed, why not 10 per cent?

Second, readers will want to know the basis for the authors' contention that land-use change undermines human well-being. If anything, the opposite has probably been more true: converting land for farming and for industry has clearly delivered a great deal of well-being, and populations will continue to find such land-use change both attractive and desirable.

What research does tell us is that the sustainability of land use depends less on percentages and more on other factors. For example, the environmental impact of 15 per cent coverage by intensively farmed cropland in large blocks will be significantly different from that of 15 per cent of land farmed in more sustainable ways, integrated into the landscape.

The boundary of 15 per cent land-use change is, in practice, a premature policy guideline that dilutes the authors' overall scientific proposition. Instead, the authors might want to consider a limit on soil degradation or soil loss. This would be a more valid and useful indicator of the state of terrestrial health. More satisfactory policy guidelines on land use could subsequently be constructed, based on this and other relevant planetary boundaries.

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