Credit: © ISTOCKPHOTO / LEE CHIN YONG

In their definition of planetary boundaries that humans should not transgress for fear of “deleterious or even catastrophic consequences for large parts of the world's inhabitants”, Rockström et al. (Nature 461, 472–475; 2009) consider ocean acidification as an essential part of the equation. This may be true whether we consider “inhabitants” to be all life or only humans, for the ocean and its resources are deeply embedded in human culture. But the authors' suggested boundary, based on aragonite saturation — a measure of the extent to which seawater is saturated with the carbonate mineral — needs careful examination.

The term 'ocean acidification' has become the recognizable phrase to encompass the ensemble effects of elevated CO2 levels on marine life. Much as climate is understood to mean much more than temperature change, so too ocean acidification means more than simple changes in pH. Other consequences of warming and the great CO2 invasion of the ocean also need consideration as boundaries. All aerobic life in the sea, not just calcareous animals, will be affected to some degree by the 'acidification' challenge as oxygen levels fall and carbon dioxide levels rise.

Aragonite is the most common form of calcium carbonate used by coralline animals and is the basic building block of coral reefs. Thus, it might be reasonable to expect that if we transgress the proposed boundary for ocean acidification, so that waters at increasingly shallow depths become depleted of aragonite, coral reef formation will slow substantially. Strong evidence that this can happen has come from many laboratory CO2-manipulation experiments, but there are few comparable field observations of a decline in the growth of large corals at reduced pH.

In fact, many animals form calcareous shells in waters that are well under-saturated with aragonite; the existence of freshwater pearls and deep-sea corals attests to this. These animals have the ability, at a modest physiological cost, to work against the temperature and pressure gradient for dissolution of aragonite.

It is not well-known whether such abilities are latent in reef-forming corals faced with a slow change in pH over many decades. But the chances are that the species familiar to the reefs we marvel at today will not survive, and we can ill afford to try this global experiment. The limit given by Rockström et al. — an aragonite-saturation state equivalent to at least 80 per cent of the average global pre-industrial level of 3.44 — therefore seems reasonable.

But is it truly useful to create a list of environmental limits without serious plans for how they may be achieved? Without recognition of what would be needed economically and politically to enforce such limits, they may become just another stick to beat citizens with. Disruption of the global nitrogen cycle is one clear example: it is likely that a large fraction of people on Earth would not be alive today without the artificial production of fertilizer. How can such ethical and economic issues be matched with a simple call to set limits? Although peak-oil concerns could be allayed by 'clean' coal technologies, among other solutions, the same cannot be said of phosphate — and food is not optional.

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