Also known as 'environmental footprint'
"The ecological footprint is a measure of the load imposed by a given population on nature. It represents the area of the Earth's surface necessary to sustain levels of resource consumption and waste discharge by that population." 
The concept is excellent – actually, it is invaluable – but it needs to be used in the light of six limitations.
Limitation 1. Unfortunately, "ecological footprint" is too benign a term. The need for the concept arose only because humans are damaging the planet's ecology and using a word deriving from "ecology" can bring with it a positive value which it does not have. I would suggest "monopolized destruction area" or "damage trail" as more accurately encompassing the notion. 
Limitation 2. The term ecological footprint also lacks a temporal dimension. For example, safe custody, monitoring and storage of high level nuclear waste will tie up people, corporations and land for over 100,000 years. For it to be successful will require sustained political will, social stability  and unwavering purpose through those millennia. This, too, will impose its load on the planet, both directly and in terms of the opportunity cost.
Limitation 3. (Building on Limitation 2) The definition of the ecological footprint carries with it an assumption that human-caused damage to the environment is repairable over a time that makes sense in terms of human history. An example where this is not possible is the depletion of concentrated phosphorus reserves.  A second example is the dissipation of phophorus as fertilizers at highly diluted concentrations which will take millions of years of a healthy, Pleistocene quality biosphere to restore through natural processes. For further examples, see note 3.
Limitation 4. The "given population" in the definition above needs to be specified: is it the human population? The population of all animals? The population of all life?
Limitation 5. Recent work has indicated that the Wankernagel/Rees approach (which shows that our ecological footprint is now equivalent to ~1.3 Earths) is a gross underestimation if we take the more realistic approach of incorporating into the ecological footprint calculation our use of fossil energy. If we had to source our total energy consumption from current biomass, we would need 24 - 40 planet Earths .
Limitation 6 . Once we humans crossed the threshold of a footprint score greater than 1.00 (in the 1970s ), this changed fundamentally the implications for almost all our behaviours on the planet.  Those who promote the use of the ecological footprint are generally too timid or narrowly-focused to point out what it means to be destroying Earth's ecosystem services faster than they can be replenished. This is not so much a limitation of the concept as it is a problem for human civilization that we fail to act urgently and effectively on the information provided in the footprint score to change our behaviour so that we cease destroying the basis on life on Earth as we know it.
On 18 August 2003, the BBC News Online published an article by Megan Lane 'Can the world go on as it is?'. The report was based around a Fabian Society report by Roger Levett 'A better choice of choice' published on 17 August. The BBC adds its own value to Levett's report by quoting Crispin Tickell of Oxford University's Green Centre for Environmental Policy and Understanding saying that our consumption has created a demand which since the late 1970s  has exceeded the Earth's ability to provide. "The ecological overshoot could have been as much as 20% of supply by the beginning of this century" says Tickell. Pursuing these metaphors, we could say that since the late 1970s the ecological footprint has extended beyond planet earth. Or we could say that our footprints have other footprints trampled over them. Or we could say that the overshoot has been advancing rapidly into the future since the 1970s, if we consider the time it will take to recover from the damage.
This approach to the 'ecological footprint' concept provides a useful corrective to the benign image of a bare footprint in moist, soft earth in a pristine rainforest. The metaphor of the 'overshoot' is also valuable in illustrating how the increasing human population is catapulting us ever further beyond the date by which the planet's life processes can undo the damage we have done since the 1970s.
1. See the 2003 paper by Jeffrey Dukes Buried Sunshine: Human consumption of ancient solar energy, Climatic Change, 61(1-2): 31-44. 'The fossil fuels burned in 1997 were created from organic matter containing 44 × 10^18 gC,which is >400 times the net primary productivity (NPP) of the planet’s current biota. As stores of ancient solar energy decline, humans are likely to use an increasing share of modern solar resources. I conservatively estimate that replacing the energy humans derive from fossil fuels with energy from modern biomass would require 22% of terrestrial NPP, increasing the human appropriation of this resource by ∼50% For a discussion of the impications of humanity's use of fossil energy, see this paper by Joseph Tainter.' Back to text
2. Tony McMichael, in his Human Frontiers, quotes the same date - the 1970s - as the threshold; however, nothing special happened in the 1970s, we just failed at that time to halt a long-term trend. Back to text
3. Two further examples of the absence of a realistic temporal dimension:
(a) Topsoil loss - The sweeping away within years by wind, soil and water erosion of topsoils which took many thousands of years to form, accumulate and contribute fully to Earth's life systems is not accommodated in the ecological footprint model. Any model which posits the land area required to replenish these topsoils at the rate they are being lost to both agriculture and ecosystem services would need to draw on the soil-creation capacity of a number of Earths – not the 1.3 Earths calculated by the standard model. Julian Cribb wrote in the Canberra Times (28 September 2009, not on-line) that "... in 1990 a world study called GLASOD found 15 per cent of the worlds' usable land to be seriously degraded. In Asia, Africa and South America, soil losses were 30-40 times greater than the rate at which soil naturally forms. slopes and badly-degraded rangelands were losing up to 100 tonnes of soil per hectare every year. In 2008 a world satellite survey found the degraded area had risen to 24 per cent - and it mostly appeared to be in new areas. Current rates of soil loss are estimated to be about 1% per year, which doesn't sound much, but add 1 per cent a year to 24 per cent, and then see how much food-producing land is left in 2050 to meet human demand for a doubling in the world's food supply". 
(b) Destruction of ocean life (oceanic biomass exceeds land biomass ) and has reached a point where some 'ocean deadzones' may be irreversible (for political, not biological reasons). The collapse of the west Atlantic cod fishery (see Wikipedia and Mark Kurlansky's book Cod) is another example. How many Earths – functioning over what period of time – would it take to restore those natural phenomena? Or does the ecological footprint model seek only to "sustain" our existing, fracturing ecological processes? No number of additional Earths could bring back extinct species such as the Dodo or the Passenger Pigeon and the network of ecosystem services  of which they were once a part. The ecological footprint methodology fails to account for irreversible destruction, yet it is irreversible damage that we have to be most concerned about. Back to text
4. The creation of complementary metaphors such as Earth Overshoot Day has added to the confusion. Earth Overshoot Day purports to mark the day each year by which 'humanity will have used all the resources nature will generate that year' (in 2008 this was calculated to occur on 23 September), but it does not point out clearly that on 1 January we start not afresh, but with the accumulated un-repaired damage of past years. Back to text
5. Wackernagel and Rees, Our ecological footprint; New Society Publications, 1996. Reference to the Earth's surface includes land and sea. However, as we know because of concern about climate change, the atmosphere is also relevant. Back to text
6. Reported in a mid-2009 BBC World Service program on the oceans (no transcription available) Back to text
7. It is often forgotten that that the American Civil war was fought less than 150 years ago and World Wars 1 and 2 occurred within living memory (it happens that the last surviving British serviceman from WW1, Henry Allingham, died this weekend – 18 July 2009). The bitterness and desperation of these conflicts were such that, were such conflicts to arise in the future, some people in power on either side would be tempted to damage nuclear waste stockpiles as an act of war. Back to text
8. Google "peak phosphorus" for more information. Back to text
9. National Geographic magazine's September 2009 edition contained on pages 134-135 a Muir web depicting the ecology of a beaver. The web included ten ecosystem services on which the beaver depends (freshwater streams, birch trees for food) and 56 impacts the beaver has on the environment (for example, the beaver lodges provide shelter for Eastern mud turtles which are eaten by snapping turtles. Another example: beavers' blood feeds mosquitoes which are eaten by flies which are eaten by Virginia rails and green frogs). The wonderful thing made clear by the Muir web is that any of the 66 identities could itself be the centre of its own Muir web in which the beaver would also appear. Back to text
10. See note 15 on the maximum sustainable human population page. Back to text
11. It also has implications for human ethics - overturning the ethical principles developed over millennia. Previously ethics set the principles by which we judged the rightness of our behaviour towards others. The environment was assumed to be infinitely resilient and, therefore, irrelevant to our choice of behaviour. Since the 1970s we know that the environment is likely to suffer - directly or indirectly - as a result of our behaviours if we do not consciously act to protect the affected ecosystem services. If we do not make protection of the environment our first priority in choosing between different actions and behaviours, we risk damaging the planet's ecosystem services and so affecting not only those services, but the humans who would otherwise have been supported by them. Back to text
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