How much of the earth do you tread on?

A good metaphor is worth a thousand words, and whoever coined the phrase "ecological footprint" has done us all a favour. At last we have a way of expressing - and measuring - our impact on the natural world. At the latest computation, the "footprint" of the average Irish person covered 5.9 hectares of the planet's biologically productive area, or more than three times our fair share of the globe's resources.

Since the world's population started soaring towards 10 billion (a figure, at present trends, only a little more than 30 years away), the application of an ecological "carrying capacity" to the human global habitat has grown more and more compelling. It underpins the whole notion of sustainable development agreed at the Earth Summit in Rio in 1992. And from it has evolved a measure of the area of land and sea that are needed to produce the food and resources we use, and to absorb the wastes we create.

The planet's total productive area works out at roughly two hectares per person. Allowing 12 per cent to protect the rest of the species in the world (a suspiciously modest figure, emanating from the World Commission on Environment and Development), we are left with 1.7 hectares per head available for human use.

By 1997, the average human "footprint" was already 35 per cent bigger than nature could regenerate on a continuous basis, and growing fast. In that year, in the Sustainable Development Strategy for Ireland prepared for the Government, UCD's Environmental Institute used four measures of domestic consumption (fossil fuels, built-up land, food and forestry) to arrive at an Irish ecological footprint - 2.38 hectares per person, or a total of 86,325 square kilometres - about one-and-a-quarter times the size of the State.

It was, as the Strategy document admitted, "a conservative illustration". New factors and calculations were already at work to approach the true size of national footprints. The research, developed mainly by Dr Mathis Wackernagel and his team at the Centre for Sustainability Studies in Mexico, now analyses consumption of m ore than 20 major resources (such as cereals, timber, fishmeal, coal and cotton ) in 52 countries, along with the energy balance of their traded goods. It counts in national demands on productive sea space and the area of forest needed for CO2 absorption. The result is a spreadsheet for each country that uses 100 lines and 12 columns - an ecological current account that leaves most societies heavily overdrawn.

India, Pakistan, China and Egypt are among the few countries which consume at a level well within the planet's life-support capacity. The US, predictably, has a monstrous footprint - 10.3 ha per person, or almost twice the area needed by the average west European. Ireland comes in at 5.9 ha, on a level with Sweden and more than half a hectare ahead of the UK.

In its Living Planet 2000 Report, the World-Wide Fund for Nature (WWF) urges the use of the Ecological Footprint when EU leaders meet to draw up their Sustainability Strategy at the Gotenberg Summit in 2001, and when world leaders meet in the following year, Rio 10, to take stock of their progress in curbing the human pressures on nature. The rapid expansion in Ireland's footprint should surprise nobody. It is part of a global trend, but also, of course, special to the velvet spoor of the Celtic tiger, ravenous for imported resources. What has kept our footprint relatively small in the past has been our low population density and high percentage of productive agricultural land - a ratio now in rapid change.

The sprawl of buildings and roads across farmland, the soaring importation of building materials and fuel, the demand for new machinery, cars and computers, acquisitions such as the biggest trawler in the world - all are adding to the costs in energy and pollution that we pile up in other countries as well as our own. Even our productive agriculture is heavily subsidised ecologically in fossil fuels, fertilizers and remedies for pollution.

The new forestry targets, the (imported) wind-turbines and plans for wave-power, even the new natural gas supply could help eventually to shrink our national footprint by a toe or two. But habits of affluent consumption, in an island of such narrow manufacturing resources, could have us in hot pursuit of the 7 ha footprints of places such as Iceland and Singapore.

My drawing of an old-fashioned orchard may seem a little off the point. But when I think of this island's share in the crazy waste of world resources, it's apples that come to my mind - the way we ship in apples from the other side of the world, burning oil all the way, rather than grow them ourselves. That goes, of course, for any number of fresh foods we seem to think we should find in the supermarket every day of the year, regardless of season.

To feel some consciousness of the Ecological Footprint, we need a closer link to nature and its limits. One of the more persuasive green ideas is bio-regionalism - working to increase an economic self-reliance within a natural boundary. Irony might insist that Ireland tried this once and called it the "frugal comfort" of the Free State. Today, alternative economists such as Richard Douthwaite are proposing independent local economies on a more intimate human scale.

In his scenario (spelled out in his book, Short Circuit, in 1996): "The district must produce at least enough food and raw materials to enable its members to live simple, comfortable lives while staying within the limits of their environment and not exploiting other parts of the world".

If this is one extreme, then free-trade and globalization, with its hidden slaves and destructive lurches of capital, and its frenetic transportations of food, is certainly at the other. We shall be forced, eventually, to choose communities that tread more lightly on the world.

Website: www.ecouncil.ac.cr/rio/focus/report/english/footprint/