The Nation



Admit it: we can't measure our ecological footprint

Have you heard the one about the 1.5 Earths? Chances are you have: it is one of the most widely quoted ecological statistics of all, an easy-to-understand measure of our impact on the planet. Each year we consume 50 per cent more resources than Earth can regenerate. Scary.

The number has gained widespread currency, cited by governments, news media and in reports from United Nations agencies. But a recent analysis suggests that it is nonsense.

The idea first came to prominence in a 1996 book called "Our Ecological Footprint" by William Rees, an ecologist at the University of British Columbia, and his student Mathis Wackernagel. It is now owned and promoted by the Global Footprint Network (GFN), a small non-profit organisation of which Wackernagel is founder and president, in partnership with WWF, the world's biggest environment group.

Every year they declare Earth Overshoot Day, the date "when humanity exhausted nature's budget for the year" and began "drawing down local resource stocks and accumulating carbon dioxide in the atmosphere". This year it was on August 20, the earliest date yet.

It's a nice publicity stunt. Unfortunately it's piffle, says the Breakthrough Institute, another independent research group based a few blocks away from GFN in Oakland, California, with a well-earned reputation for environmental iconoclasm. The measurements behind the 1.5 Earths claim are "so misleading as to preclude their use in any serious science or policy context," it says in a paper in PLoS Biology.

I fear the criticism is right. The footprint analysis does not really measure our overuse of the planet's resources at all. If anything, it underestimates it.

The analysis tries to compare our demand for biological resources such as food, timber and fibres with the planet's "biocapacity" - its ability to renew those resources. It does this by measuring the productive land and sea area available - cropland and pasture, forests and fishing grounds - and matching that against the demands placed on them. This biological accountancy system concludes that planet Earth has a biocapacity of 12 billion hectares, and a human demand equivalent to 18 billion hectares. Hence the 1.5 Earths figure. Our footprint is 50 per cent too big.

But dig into the numbers and problems quickly emerge. They begin with cropland and pasture, the two most important land categories for supplying food. GFN's analysis finds that demand and biocapacity are in balance. Uncannily so, until you realise that it could not be otherwise. Cropland and pasture simply fill the land available.

I had assumed that the analysis assessed the damaging environmental consequences of how we use the land - things like soil erosion and the overuse of water reserves. But no. It only measures land area.

Wackernagel explains this in a response published alongside the Breakthrough analysis. "Local ecosystem abuse is a significant problem [but] to make reliable adjustments would require data sets that do not exist," he writes. Or, as he told me: "Our current accounts cannot include soil erosion. Hence cropland use equals cropland availability."

That is a problem. Certainly it is hard to see how the calculation measures the extent to which we are "drawing down local resource stocks", as GFN claims. It is true that if we destroy soils, we will eventually have to abandon the land and chop down more forest. But until then, the analysis doesn't account for our mismanagement. It looks like Breakthrough is right to argue that the footprints for croplands and pastures "say absolutely nothing about the sustainability of agriculture".

The analysis does a bit better for forests. It uses UN statistics to compare the timber we harvest against annual growth. The conclusion is that, while we are deforesting some areas, growth elsewhere more than makes up for the loss. This is reflected in a surplus in the accounts.

Similarly for fisheries, the analysis compares UN data on catches with an estimate of the natural regeneration of fish stocks. If the calculation is to be believed, while some fish stocks are being over-exploited, a greater number are under-exploited and overall fish biomass is increasing. That's another surplus - though Wackernagel told me he thought the surplus might be more a reflection of poor UN fisheries data than healthy fisheries.

So if our forests and fisheries are in surplus, and croplands and pasture are in equilibrium, where does the headline figure, about us consuming half a planet more than we have, come from?

The answer is a final category: the land that we use to soak up surplus carbon emissions. Or rather - since we know most of our carbon emissions are not being soaked up but are accumulating in the atmosphere - the land we would use if we had it. This category accounts for 8 billion hectares of our total footprint of 18 billion. In fact, as Breakthrough points out, "the entire overshoot is human emissions of carbon", converted into a proxy measure of land area.

Isn't this a rather strange exercise? Why bother with all that land stuff if the footprint analysis ends up mostly measuring excess carbon emissions?

I put this to Wackernagel. He argues that land we need but don't have is a genuine measure of our overuse of land as a whole. "Overshoot is the combination of all our demands meeting limited biocapacity."

That is a fair point. But I for one imagined that the footprint analysis was a bit smarter, that it had some handle on how we are overusing our soils and water reserves. Sadly, it does not measure the things that most of us assumed it does - and the things we really need to know.

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