The different ways of exploiting nature—taking resources (with low entropy) from its sources and stocks, returning waste and pollutants (with high entropy) to its sinks (throughput of resources), and using its space for human infrastructure—can only be reduced sufficiently and distributed in a just manner [1,2], if they are converted to one unit by means of a single indicator of sustainability. This would enable all of these manners of natural consumption to be quantified, compared with each other, added, and expressed in terms of the yield of natural capital required to maintain them permanently. Such an indicator would quantify the total exploitation of mankind, of a group of people, or of a single human or their activities. It would enable the comparison of this total exploitation with the exploitation of a sustainable state, which meets the four criteria of sustainability [3,4]:

Anthropogenic material flows must not exceed the local assimilation capacity and should be smaller than natural fluctuations in geogenic flows.

The load on Nature must everywhere and in every instance remain below its maximum permissible value. Sustainability means for instance, that the harvested amount of biomass per area on a spot does not cause irreversible reductions of biomass yield there in the following years. This sustainable state differs strongly from the present one, as the fishery example shows. Experts debate how large quantitatively a sustainable yield is, but there is little agreement, and in the meantime, the overfishing continues and the fisheries are in severe decline. Nevertheless, in order to achieve sustainability, its state is defined here first of all qualitatively with the above criteria. This enables the deduction of quantitative limits of sustainable yields. The sustainable state, defined this way, is within the safe operating space for humanity, as defined by nine planetary boundaries that must not be transgressed, if disastrous environmental changes are to be avoided [5]: climate change; rate of biodiversity loss (terrestrial and marine); interference with the nitrogen and phosphorus cycles; stratospheric ozone depletion; ocean acidification; global freshwater use; chemical pollution and atmospheric aerosol loading. Three of these boundaries (biodiversity, nitrogen cycle and climate) have already been overstepped. The current extinction rate of 10⁸ species per annum exceeds these boundaries by a factor of ten and the rate of removal of N₂ from the atmosphere by a factor of three. The current atmospheric carbon dioxide (CO₂) concentration of 387 ppm exceeds the upper boundary of 350 ppm. These planetary boundaries are not independent but strongly interlinked. If one boundary is transgressed, then other boundaries are also under serious risk [5].

According to the above four sustainability criteria, these boundaries are overstepped if more of nature is exploited than the permanent yield of its natural capital. For example, as long as more carbon is removed from the lithosphere and emitted into the biosphere than is assimilated again by the lithosphere, the carbon concentration in the biosphere (and with it in the hydrosphere and the atmosphere) will grow. A limitation of the acidification of the oceans or a reduction of atmospheric CO₂ concentration below 350 ppm becomes impossible. In this context, the term “natural capital” refers to biologically productive soil and water areas. In future, the yield of biomass or of directly used exergy of these areas will be the only source of natural resources available for human beings. Therefore, every human interference into nature which results in the exploitation of this yield (e.g., removing resources from nature, emitting pollutants like greenhouse gases, degrading soil, endangering biodiversity) has to be converted into a share of the Earth's limited surface—i.e., into an evaluation of the yield used up by it, applying a suitable indicator. This conversion is necessary, if the total area demand of humankind is to be reduced to the total area supply of one planet Earth. Decreasing the total exploitation of nature to the permanent yield of its capital this way, this resource yield can be preserved for all future generations, so that the limited resources are shared in a just way between the generations.

Although this would solve the ecological resource distribution conflict between the generations, it would also aggravate the social resource distribution conflict between humans of the same generation. A solution to these two conflicts, a sufficient reduction of total area demand and a just distribution of this reduced demand among the consumers (for instance with the help of resource certificates) necessitates the quantification of the area demand of a single consumer [1,2]. The area demand of a consumer is the sum of the demands caused by his/her purchases of commodities and services. It only can be found out, if the area is known, which is claimed by a commodity or a service. But is it possible to imagine an incredibly complex system of bookkeeping that would keep track of all of the energy and resources that go into producing products and services? It is shown here, how such a lavish bookkeeping can be avoided, by ecolabeling the natural resources with their area requirements (i.e., with the fraction of the Earth's surface, which is claimed by extracting them from nature) directly on the spot, where their extraction happens. The manufacturer of a product composes it of resources and other products. He is able to add up the area requirements of the resources from which a product is assembled, with a computer program, in order to find the area requirements of the product and to ecolabel it with these requirements—in addition to price labeling. Details of the political enforcement can be found in [2].

But is it possible to derive the resources, which are needed to maintain and use a product through its lifetime? For example, how much energy is needed to drive a particular automobile, taking into account how far and with which energy intensity it is driven? These resources, in particular the fuel energy, are additional products. If an automobile is labeled (besides its price) with the sum of the areas of the resources, which have been necessary to produce it, then in addition another product, the fuel necessary for car driving, is labeled with the sum of the areas, which it occupied during mining and refining it, as well as throughout the air pollution its burning caused. Moreover the driver uses the service of the infrastructure, which above all is labeled with the road area. In this way, not only the resources can be found which are necessary to produce a commodity, but also those which are required to use it. For example, the sum of the area demands of all the commodities and services purchased within a nation in relation to its area is an indicator for the sustainability of the nation in the same way, as the sum of all human area demands in relation to Earth's surface for mankind's sustainability [6].

What would the average consumer do with the knowledge of the resource content of a commodity or service, which he purchases? In his classical paper of ecological economics, Daly shows that the ecological goal of “scale” (limiting the total area demand of all consumers) can only be achieved with certificates, which permit the consumption of area [1]. The second social goal of “distribution” (a “just” sharing of the area between the consumers, whatever the goal of justice is) can be achieved by an appropriate initial distribution of these certificates among the consumers and the third economic goal of “allocation” (an optimal division of the resource flow among alternative product uses) by trading these certificates [1]. Daly's suggestions became the backbone of the widely used “cap-and-trade” policies. A further development of his proposals was applied to the “point of sale” of products [2]. Purchasing a product, the payment is made with money according to its price label and with certificates according to its area demand label, this way completely internalizing external costs within the market prices. This has an ecological and a social effect: First, the payment necessary for the purchase of a commodity with a large resource input (and a small labor input) goes up, that for a commodity with a small resource input (and a large labor input) goes down, leaving the price average unchanged. Second, those which consume many resources subsidize directly those which consume few resources. No other mechanism ensures that the economy takes into account the immutable natural limits, enabling a sufficiently rapid as well as a just reduction of nature exploitation without an undue loss of GNP [2].

But why do we want to get a high GNP? After all, is it not the global striving to increase GNP that is responsible for many of the problems the world economy is facing today with resources and energy? There is nothing wrong with a large GDP, as long as the exploitation of nature is sufficiently small on an absolute scale, as it is suggested here, or as long as the resource productivity (the ratio out of the GDP and the exploitation) is adequately large.

Moreover, by including resources, which are ecolabeled with their area demand into an input–output analysis, the demand of nations, regions, economic sectors, or of socioeconomic groups can be assessed in a way that is analogous to the inclusion of the ecological footprint into input–output analysis [7]. Therefore, a single indicator, which converts every human disturbance of nature into a proportion of the Earth's surface it indirectly consumes, becomes an essential economic and political tool.

The EF meets only some of these challenges, in spite of being one of the most successful indicators for communicating the Earth's physical limits. It indicates the demand for biologically productive land and water area with world-average productivity (in units of “global hectares”) by individual people, groups of people (such as a nation), or activities (such as manufacturing a product), delivering all of the biological materials consumed by these individuals or groups and absorbing all biological wastes generated by them, in a given year. In addition to the areas necessary for producing biological materials, such as cropland (for crops), grazing land (for animal products of pasture-fed animals), fishing grounds (for fish), forest land (for forest products), cropland is also taken into account as a site for building infrastructure (land take) and forest areas for the sequestration of emissions of carbon dioxide (CO₂), or for the production of fuel wood. Nuclear energy was considered as if it were fossil energy and is not taken into account at all now [8,9]. The EF can be compared with the “biocapacity” of the Earth, indicating the supply of the existing biologically productive area on Earth [10-13]. Nevertheless the EF has some serious deficiencies.

The area absorbing the CO₂, which is emitted during combustion of fossil energy carriers, constitutes the largest share of the EF for most countries [8]. Nevertheless, this area is too small, because the forest areas assumed for carbon sequestration in the EF cannot store the CO₂ permanently and finally emit it into the atmosphere again [13]. Moreover the EF ignores the fact that 1.6 Gt of carbon (C) per annum is emitted into the atmosphere due to deforestation, land-use change and soil cultivation [14]. This underestimation of the carbon sequestration area in the calculations of the EF gives the impression that biocapacity has grown as a result of industrialization [15]. In Section 3.1, it is suggested therefore that CO₂ emissions should be limited to the absorption of CO₂ by sustainable sinks, such as the lithosphere [16]. Thus, carbon must not be burned at a higher rate than it is absorbed by lithosphere.

The renewable biomass output of a square meter of bioproductive area, used in the EF, is not an output of this area alone. It includes the input of non-renewable resources, e.g., of fossil energy or of abiotic resources, which do not come from this area, and are added to it from lithosphere stocks, e.g., the ratio of the food-energy output per unit of energy input may equal approximately 20 for low intensity agriculture, 2 for intensive field crops, 0.2 for livestock production and 0.02 for greenhouse production. It has been calculated that for every calorie of food that we eat at our table it has taken 5 calories of energy to get in onto our plates—this is with a western standard of living. Most of these calories are inputs after the products have left the farm gate and include transport, packaging, retailing and cooking [17]. In Spain, this ratio dropped from 6.1 in the year 1950/51 to 1.27 in 1999 [18]. This input (which may be a large part of total output) is not subtracted from the output of intensive land use, resulting in an EF value which is far too small [16]. Taking into account the indirect upstream fossil fuel use of agriculture by considering all human CO₂ emissions does not compensate for this neglect of subtraction, since the CO₂ absorption areas evaluated in the EF are too small, because they do not store carbon permanently. Moreover, the large non-sustainable industrial yield made possible in the EF by using non-renewable inputs might over-exploit the soil and water areas and reduce their future sustainable yield—which will, by definition, be possible only without any inputs [19-21]. Section 3.2 suggests a local sustainable yield as the basis of an indicator of sustainability instead.

The EF also does not take into account future yield reductions [19,22,23], or yield changes due to changes in land use. This is dealt with in Section 3.3. Furthermore, abiotic resources and resource depletion are ignored by the EF. They are considered in Sections 3.4 and 3.5.

Due to its shortcomings, the EF is not accepted as a stand-alone indicator. In a careful analysis, the potential of the EF to monitor the environmental impacts of resource use was compared with that of thirteen other indicators [24]. As a result, it was decided in this paper to combine the EF with three other indicators in a basket: “Human Appropriation of Net Primary Production (HANPP)” [25,26], “Environmentally-weighted Material Consumption” (EMC) [27] and the “Land and Ecosystem Accounts (LEAC)” [28]. The HANPP calculates the contribution of human intervention into nature to the reduction of land and water area yield. The EMC is a weighted indicator of material consumption based on environmental impacts. LEAC is a method developed to account for the interactions between nature and society on the basis of a detailed grid (1 km × 1 km) for land use and land cover changes.

The total human exploitation of nature only can be reduced if it can first be established. This is only possible if all the different contributing factors (of resource throughput and direct use of area) are converted into a single unit, so that they can be quantified and added together. This means that one single indicator can be applied, rather than a basket of indicators.

Furthermore, decision-making is facilitated by the comparison of different contributions to the exploitation of nature made by individual people, groups of people, or of different human activities. In addition, it takes more effort to calculate numerous indicators than one single indicator uniting their benefits. For all these reasons, a combination of all the necessary qualities of a successful single indicator of sustainability imposed on a modified version of the EF is proposed here.

The EF converts the consumption of biotic resources in a very straightforward way into the share of biologically productive area existing on planet Earth. Therefore, it is very successful in raising public awareness. However, this simplicity is achieved at the cost of producing an incomplete picture, which results in three major weaknesses.

First, the EF is not an indicator of sustainability or of intergenerational justice, because it allows excessively large non-sustainable biomass area yields, and ignores their future reduction due to overexploitation. Furthermore, it underestimates the absorption areas for CO₂, excludes the area demand for throughput of abiotic resources, and does not consider depletion.

Secondly, the EF is not an indicator of intragenerational justice, because it undervalues the environmental burden of non-renewable resources relative to those of renewable ones, and operates with non-local average yields.

Thirdly, with the EF's global yield averaging, spatial information is lost regarding where resources are taken from nature and where waste and pollutants are returned to it. Therefore, the environmental burden of resource throughput of a product, a service, a human being, or a nation cannot be compared with that of another product, service, human being, or nation. For this reason, the EF is not suitable for informing about all the costs of a purchase decision, and only incompletely successful in enabling sensible decisions between life-styles or economic policies.

The EF can be improved if the suggestions of critics and authors of competing indicators are adopted in order to modify it. The main idea of a modified EF is the derivation of the size of the soil and water area demanded by a unit resource or waste of every kind, which is taken from it, or given to it, in a sustainable way, just as in the cases of SPI and DAI. Its derivation is based on the local sustainable area yield, proposed also by Ferng [22,34]. It specifies which share of the biomass area yield can be harvested without reducing future area yield. By use of the HANPP, how much of this yield is used up by resource extraction, waste absorption or build-up of human infrastructure, can be calculated. As in the case of the EMC, SPI and DAI, also the area demand of abiotic materials can be derived. In addition to this, the area demand for harvesting solar exergy is considered, which is necessary to compensate for exergy loss of non-renewable resource use.

In a sustainable state, the sum of areas ( A MEF int sum + A DEP sum )calculated in this way (the areas A_MEFint for extracting materials with low entropy from nature, for their return to it with high entropy, for built structures and A_DEP for compensating the exergy loss) are not to exceed the Earth's surface: A MEF int sum + A DEP sum < A EARTHBesides this, the CO₂ emissions are limited to the absorption of CO₂ by sustainable sinks: A C O 2 sum < A EARTH. As an approximation, the two limits can be combined: A MEF int sum + A C O 2 sum + A DEP sum < 2. A EARTH.

In addition to the demand of an area by unit of resource throughput, the demand of resource throughput by unit of consumption must be found out. Usually, “Life Cycle Assessment” [54,55] or “Material Input Per unit of Service (MIPS)” [56,57] is applied for the calculation of the resource demand for the lifecycle of a product and for the environmental impact of its consumption as well, and “Input Output Analysis” is applied for resource demand of consumption within nations, regions, economic sectors, or of socioeconomic groups [7,55,58]. These studies can be simplified, by ecolabeling a resource with its demand of Earth's area on the spot where it is extracted from nature. The area demand of a commodity can then be composed of those of the labeled resources needed for its production in a simple manner [2]. Thus the commodities can also be ecolabeled with their area demand, in addition to labeling them with a price. This is a precondition for integrating natural limits via the prices (containing all positive and negative costs, externalized so far) into economic selfcontrolling mechanisms reducing this way the demand of resource areas ( A MEF int sum + A C O 2 sum + A DEP sum )to their supply (2.A_EARTH). Thereby not necessarily the usual “homo economicus” has to be assumed, who makes exclusively rational decisions during purchasing a product. Because the sales of products not only rise (drop), since their prices drop (rise) but also because their purchase is ecologically and socially fair (unfair), due to the perfect internalization of all external costs…” In summary, an indicator of just resource distribution between and within generations, and a benchmark for decision-making between alternative types of consumption, life-styles and economic policies, results.