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The environmental impact of meat is something of a well-done dish.

, head of the Intergovernmental Panel on Climate Change (IPCC), and are just two of the public figures who have called on us all to eat less meat in order to curb the rate at which the world warms.

The number most commonly cited in this arena - that the meat industry is responsible for about 18% of global greenhouse gas emissions - comes from a UN Food and Agriculture Organization report entitled .

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The figure is a somewhat curious entity, in that it its veracity is easily challenged, and yet manifestly incomplete.

Easily challenged, because like all kinds of accountancy, you can cut the numbers in a variety of ways - .

Yet manifestly incomplete, because raising livestock clearly impacts the world in a number of other ways - climate-changing gases are not its only legacy.

Time then, perhaps, for a more holistic assessment.

And here - as opportune as a spoonful of apple sauce to a dishful of roast pork - , from Nathan Pelletier and Peter Tyedmers of Dalhousie University in Canada, in this week's Proceedings of the National Academy of Sciences (PNAS).

What they've attempted is to calculate the environmental impact of livestock in 2050, and relate that back to "" - the points beyond which it would be unwise to pass if humanity wishes to avoid serious degradation of environmental systems.

They calculate that the total amount of meat we'll be eating in 2050 is about double today's levels, given the growth in the human population and the rate at which we're getting richer (and thus spending more on high-value foods).

Factoring in likely changes in the efficiency of farming systems, they estimate that the climatic impact of meat production on this scale would "occupy 52% of humanity's suggested safe operating space for anthropogenic greenhouse gas emissions".

But that's not all.

Agriculture hugely increases the amount of reactive nitrogen in the environment - and the use of fertilisers implicit in this meat doubling picture entails, they calculate, that the "sustainability boundary condition" for reactive nitrogen would be surpassed by 117%.

Meanwhile, livestock production would occupy 72% of the "safe operating space" for human "appropriation of biomass" - a measure of how much of the world's is being consumed by us, with less left over for everything else growing on Earth.

The notion of "planetary boundaries" was expounded at length in a paper published in Nature last year. That paper contains one of the sets of suggestions for boundaries that are out there in the scientific literature, and which the Dalhousie team uses in its analysis.

The Gulf of Mexico often shows a "dead zone" - here imaged in red by satellite

When you get down to the specifics, all of the proposed boundaries are of course estimates.

And there's unlikely to be any single limit beyond which things suddenly become unbearable the world over. After all, the most spectacular impacts of excessive fertiliser use - - are exactly that - dead zones, not a dead totality.

So any and all of the numbers can easily be challenged.Ìý

But the exact numbers are not the point.

Rather, it's the conceptualisation - the notion that here is humanity travelling in some kind of vehicle and heading rapidly towards a giant elastic band, and if we plough into it full-tilt, we will be flung backwards at quite a rate of knots.

The task is to deduce exactly where the elastic band lies, and what scale of impact is bearable; when it is wise to withdraw

The concept has been fully developed, with big reports and loads of numbers, in the field of climate change, including the . It's less well developed in terms of other proposed "planetary boundaries" - but as this paper shows, it's beginning to be more developed, and integrated across the various domains of humanity's interaction with nature.

So I think we can expect more of the same - elastic bands with progressively more sophistication.

What we do with the projections, however, is less clear.

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