Common Criticisms

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Ecological Footprint: Popular and Policy Relevant — a Response to van den Bergh and Grazi (2014)

In the February 2014 issue of Journal of Industrial Ecology, Jeroen van den Bergh and Fabio Grazi published “Ecological Footprint Policy? Land Use as an Environmental Indicator,” an evaluation of the Ecological Footprint as an accounting method and aggregate indicator.  After discussing eight criticisms of the Footprint methodology, interpretations and applications, the authors conclude that the Ecological Footprint, despite its increasing use by scientists, environmental organizations and the popular media, “does not offer any meaningful information for public policy.”

Mathis Wackernagel, co-creator of the Ecological Footprint, published a response to van den Bergh and Grazi in the same issue of the journal. Wackernagel pointed out that the Footprint answers a different research question than that which van den Bergh and Grazi imply it addresses, thus making most of their criticisms irrelevant. More specifically, van den Bergh and Grazi seem to assume that the Footprint is intended to be “an overall indicator for environmental pressure or impact.” Instead, as is made clear in numerous publications, the Ecological Footprint is not an impact indicator, nor it is meant to be a complete measure of environmental pressure. It simply measures one key dimension that contributes to the sustainability or unsustainability of human activities: the extent to which the Earth’s productive ecosystems have sufficient regenerative capacity to keep up with society’s metabolic demands. It measures people’s competing demands for biologically productive surfaces, including the demands for renewable resources and the capacity to assimilate carbon dioxide waste from fossil fuel use.

In their response to Wackernagel, van den Bergh and Grazi again did not clearly delineate between their version of the research question and that which actually underlies Ecological Footprint accounting. They also complained that the eight main points in their original article were not individually addressed by Wackernagel, who adopted a broader logical framework in assessing the validity of these points. To further understanding of the Ecological Footprint methodology and its policy relevance, a brief summary of each of van den Bergh and Grazi’s eight points are provided here, along with Global Footprint Network’s response.

1. The Ecological Footprint method represents “false concreteness”

Van den Bergh and Grazi claim that Footprint accounting compares “real” land area with “hypothetical” land area, and this represents a “false concreteness” of the method.  The core of their claim is based on a common misconception about the Footprint methodology, in their words “that the [Ecological Footprint] method calculates the land area used by a human activity or economy as if they were sustainable, meaning that certain types of environmental, negative externality (e.g., pollution) have to be neutralized. This neutralization then translates into land area (used). Shifting from an unsustainable situation to a sustainable one requires a set of assumptions, which render the result, in terms of land use, hypothetical in nature.”

In fact, Footprint methodology makes no assumptions about the sustainability of human demands on a land area, nor does it “neutralize” negative externalities in the calculations. If it did, it would indeed be a hypothetical, predictive measure. Instead, it is simply descriptive, with both Footprint and biocapacity results based on actual measured values of current ecosystem productivity, including both resource production and carbon sequestration, without assessing whether other factors may make this level of productivity unsustainable in the future.

In addition, they argue that because “Obviously, real land use could never exceed available land area,” the fact that the Footprint can overshoot biocapacity reveals the “unrealness” of this measurement. This is a perplexing comment. Ranchers, for example, who must ensure that there is sufficient feed for their herds, calculate the ability of available land to support cattle in “cow-calf acres” or “animal units,” taking into account both the number of hectares on their ranch and the average productivity per hectare. This rangeland management methodology parallels that of the Ecological Footprint, with the latter taking into account human demand on cropland, forest and fishing grounds in addition to that on grazing land, and expressing this in a measure reflecting the average productivity per hectare of all these areas combined. Just as a ranch may not produce a sufficient flow of grass to keep up with the demands of a herd, so too a biologically productive area (a nation or the whole planet, for example), may not produce a sufficient flow of resources to keep up with human demand. This does not make overshoot hypothetical, however, it just means that demand will be met by dipping into prior years' accumulated flows (stocks of timber or fish, for example), or by allowing carbon emissions to go unsequestered and thus accumulate in the atmosphere. 

There is nothing hypothetical about these stocks or carbon accumulation, even if expressed in global hectares, a proxy representing the area of land required to produce these stocks or sequester the accumulated carbon. One would hardly make the argument that because distance is converted into abstract units called “meters” that this measurement is hypothetical rather than real. For any measurement, “the map is not the territory;” if van den Bergh and Grazi feel the Footprint accounting has an issue with false concreteness simply because it attributes a biocapacity value to surface areas, then they must have a gripe with any metric.

2. The use of global hectares contributes to the hypothetical character of the Ecological Footprint

In addition to their qualms discussed above, van den Bergh and Grazi argue that the fact that a global hectare may represent a varying amount of productivity each year contributes to its hypothetical nature. This is a bit like saying that because a dollar may vary in purchasing power each year, it is a hypothetical rather than a real measure of financial wealth. 

A global hectare is a hectare of productive land with the average resource flow of all productive area on the planet. Because this average flow varies somewhat from year to year, so too does the value of a global hectare in terms of the actual resource productivity it represents. But because both demand (Ecological Footprint) and the capacity to meet this demand (biocapacity) are measured in this same unit, when calculating the degree of overshoot the effects of any annual variation will cancel. To allow direct comparison of actual productivity or demand across years, more recent Footprint accounts have been expressed in “constant global hectares” based on a given reference year, much in the same way that “constant US dollars” are used to hold purchasing power constant in comparisons across years.

3. Footprint accounting implies that land use is the all-important sustainability variable

Any indicator in widespread popular use is invariably subject to distortion or misinterpretation— this is hardly unique to the Footprint. While it is not clear (since they provide no references) where van den Bergh and Grazi are seeing an “[Ecological Footprint] assumption that all (important) environmental impacts of humans can be captured or approximated by land use” it is surely possible that this erroneous claim exists somewhere in the universe of print and online media. This does not mean, however, that it represents the position of Global Footprint Network, which has repeatedly emphasized that Footprint accounting measures only a single dimension of human pressure on the biosphere— human demand on the regenerative capacity of the Earth’s productive areas— while stressing the importance of including other variables, such as those impacting the fragility of biocapacity and those such as the HDI that track important human development variables, in an attempt to provide a more complete and nuanced assessment of the sustainability of human activities.

4. The Footprint does not capture all relevant environmental pressures

Aside from the question of how completely Footprint accounting captures demand on biocapacity, van den Bergh and Grazi complain that “the common interpretation of the [Ecological Footprint], in most of the broader academic and popular literature (and media), is as an aggregate environmental indicator.” Without citing any references to support the validity of this claim, they go on to offer an incomplete but accurate list of environmental pressures and impacts not included in Footprint accounting: water pollution, toxic emissions, noise pollution, depletion of the ozone layer, acid rain, ecosystem fragmentation and biodiversity loss. From all this they somehow conclude that the Footprint is problematic.

Their logic seems to be as follows: That regardless of what an indicator is designed to measure, if its purpose is misinterpreted by some hypothetical public, and the indicator then does not effectively support this misinterpreted purpose, it is a bad indicator. We suggest instead that this is bad logic, and that a far more useful way of evaluating an indicator would be to assess how well it measures what it is intended to measure—which for Footprint accounting is not a somewhat random litany of environmental ills, but instead, competing demands on biocapacity, and the availability of biocapacity.

5. The Footprint aggregates distinct environmental problems using arbitrary weights

Van den Bergh and Grazi conflate two different issues in this criticism. The first is the question of whether Footprint accounting is aggregating fundamentally different types of environmental problems, and the second is whether the weightings used in this aggregation are arbitrary and unfounded. 

The answer to the first question is simple: Footprint accounting focuses solely on one environmental problem, competition for available biocapacity. Aggregation of these demands on biocapacity is based on the percentage of global biocapacity each type of demand— for food, fiber, timber or carbon sequestration—occupies. At its most basic, each of these demands requires surface area on the planet where plants can grow to provide resources that are useful to society; these resources can then either be harvested, or left standing in order to absorb carbon emissions. Since there is limited area available on the planet to grow these resources, Footprint accounting simply asks if this area is sufficient to keep up with all the competition on it, and if not, how much additional area of the same kind is needed to provide for this excess demand.  Only by aggregating all of the demands on biocapacity can Footprint accounting effectively shed light on the status of this critical environmental problem. 

The second question is more complex, in that it is asking just how the productivity of different types of ecosystems, and of the demands on them, is aggregated. While this is explained in detail in publications made freely available by Global Footprint Network, the short answer is that the aggregation is based on clearly defined principles and on measurements of actual productivity, and not on arbitrary weights. In particular, all Footprint and biocapacity components are expressed in a common unit, the global hectare. In current National Footprint Accounts, global hectares are calculated using yield factors that adjust for the differences between local and global average productivity of each type of ecosystem, and using equivalence factors that adjust for inherent differences among forest, cropland, grazing land and fishing ground ecosystems in their capacities to generate resources. 

6. Calculation of the carbon Footprint component is based on an arbitrary “sustainable energy scenario”

Using a logic similar to that adopted by the UN Reducing Emissions from Deforestation and forest Degradation (REDD) Programme, calculation the carbon component of the Ecological Footprint is based on the average rate per hectare at which global forests can sequester carbon. But contrary to claims by van den Bergh and Grazi, the choice of this approach is not arbitrary, and the calculation of the carbon Footprint is based on empirical measurements of average forest productivities, not on hypothetical values. 

At the risk of sounding repetitive, Ecological Footprint accounting tracks competing demands on biocapacity, and compares that with available biocapacity. Anthropogenic carbon dioxide emissions not captured by the oceans and that are not otherwise sequestered (for example, through carbon capture and storage) compete for available biocapacity in that if they are not absorbed into terrestrial ecosystems, they will accumulate in the atmosphere. While the biocapacity demanded could be calculated in a number of ways, the Ecological Footprint methodology adopts the strategy commonly used in carbon trading and similar protocols of basing this calculation on forest sequestration capacity. The latter is a directly measured property of forests, and global average values can be obtained from the UN Food and Agriculture Organization (FAO). Thus neither the overall approach nor the specific sequestration values used in calculating the carbon Footprint component are arbitrary, but are instead based on what is currently established best practice.  

7. Footprint applications focus on countries rather than on “bioregions”

Global Footprint Network’s National Footprint Accounts measure the Ecological Footprint at the scale of nations, but the metric can be applied at any scale desired— to a single activity, an individual’s lifestyle, a city, a bioregion, a nation, or humanity as a whole. 

The Accounts focus on the national level because the best quality and most complete data on production, consumption and emissions exist at this level, plus many other complementary indicators that one might want to look at in conjunction with the Footprint track aspects of national performance (e.g., human development, economic, population). In addition, many of the key policy decisions are set and enforced at the national level, because typically countries enter into international agreements on environmental concerns (e.g., ozone depletion, biodiversity, carbon emissions). It hardly seems a weakness of an indicator that it can provide results for the same geographic areas that are of primary concern to policymakers.  

Nothing, in principle, limits researchers who prefer to analyze Footprint or biocapacity for bioregions. The challenge, however, may be in finding sufficient data to carry out the analysis.

8. Measurement of national ecological deficits supports antitrade sentiments

If a country’s total Ecological Footprint exceeds its total biocapacity, it is described as running an ecological deficit. This means it is harvesting resources from its domestic ecosystems faster than these resources are being regenerated,  and/or emitting more carbon dioxide than its ecosystems are sequestering, and/or that the embedded biocapacity in its imports is higher than in its exports. 

Van den Bergh and Grazi seem to assume that an ecological deficit only reflects the latter factor of trade balances in embedded biocapacity, which is incorrect. Based on this assumption, they argue that in calculating ecological deficits, “the [Ecological Footprint] approach suggests that trade should be restricted to remain within the national ecological capacity. That is, trade is limited by the [sic] own land area, not by the population size.” Continuing this line of reasoning, they then go further afield, even to the point of suggesting that the Footprint makes “a strong plea against cities.”

But there are no “shoulds” in Ecological Footprint accounting; it is simply a descriptive measure, and does not offer any policy prescriptions. In short, the Footprint documents, but it does not judge. Blaming Footprint accounting for the fact that the results are sometimes misinterpreted and used to promote policies that one finds undesirable is a bit like blaming your bank statement for the fact that your expenses exceed your income. In both cases, the accounts provide information that you may want to consider in planning future strategy, but on their own do not tell you the best way to proceed. 

Regarding trade specifically, it would seem helpful to know how much biocapacity is embodied in international trade flows, and how much of a country’s consumption can be met by local production capacity as opposed to having to rely on imported goods. For example, if imported goods were to become unavailable, and a country that needed these goods was forced to rely on its own ecosystem productivity to replace them, how much biocapacity would be required? 

Footprint accounting on its own does not tell what to do, but it can help drive policies designed to build resilience in a world that will have to adapt to the consequences of climate change and other ecological disturbances. This is true whether these policies are designed to address trade, investment in societal infrastructure, or many of the other factors that contribute to the development of a sustainable society.

 

Misconceptions in the PLOS Biology paper

Mathis Wackernagel and Bill Rees' response to
Does the Shoe Fit? Real versus Imagined Ecological Footprints
by Linus Blomqvist, Barry W. Brook, Erle C. Ellis, Peter M. Kareiva,
Ted Nordhaus, and Michael Shellenberger
in PLOS Biology, November 2013

Linus Blomqvist et al.’s criticism of the Ecological Footprint method, Does the Shoe Fit? Real versus Imagined Ecological Footprints, appeared in the November 2013 issue of PLOS Biology. Three of the authors are from the Breakthrough Institute. William Rees and Mathis Wackernagel, co-creators of the Ecological Footprint, published a response in the same issue, which Blomqvist et al. rebutted (also below). Because PLOS Biology limits such exchanges, Rees and Wackernagel offer their final response here after the Blomqvist et al. rebuttal.

Blomqvist et al. rebutted:

"Having read Rees and Wackernagel's response, we worry this exchange will only confuse readers. For that reason it is worth emphasizing a few key points that we do not think Rees and Wackernagel could dispute. First, the entire global ecological overshoot (footprint of consumption in excess of biocapacity) results from carbon dioxide emissions reframed as the hypothetical forest area needed to offset these emissions. Plantations of fast-growing trees would, by-the-numbers, eliminate the global overshoot. Second, the [Footprint or] EF's assessments for cropland, grazing land, and built-up land are unable to capture degradation or unsustainable use of any kind. We conclude from the above and other arguments in our original paper that we would be better off discussing greenhouse gas emissions directly in terms of tons of CO2-equivalent (and thus focus on solutions to emissions), and developing a more ecological and ecosystem process framework to capture the impacts humans currently have on the planet's natural systems. The appropriate scale for these indicators will in many cases be local and regional. At this level, the EF is a measure of net exports or imports of biomass and carbon-absorptive capacity [1]. Any city, for example, would show a deficit, as it relies on food and materials from outside. That in itself, as Robert Costanza has noted, 'tells us little if anything about the sustainability of this input [from outside the region] over time' [2].

[1] van den Bergh J.C.J.M., Verbruggen H (1999) Spatial sustainability, trade and indicators: an evaluation of the 'ecological footprint'. Ecol Econ 29: 61-72.
[2] Costanza R (2000) The dynamics of the ecological footprint concept. Ecol Econ 32: 341-345."

Rees and Wackernagel point out six misconceptions in the above argument:

1) Blomqvist et al.: "...First, the entire global ecological overshoot (footprint of consumption in excess of biocapacity) results from carbon dioxide emissions..."

Rees and Wackernagel: This statement is incorrect. The total Footprint is made up of the sum of all demands. If humanity demanded less food and timber, more land can be dedicated to carbon sequestration. Current carbon emissions alone would not lead to global overshoot.

2) Blomqvist et al.: "...Plantations of fast-growing trees would, by-the-numbers, eliminate the global overshoot."

Rees and Wackernagel: This argument does not apply, since Footprint accounts document what is. They are not a speculation about what could be.

3) Blomqvist et al.: "...We conclude from the above and other arguments in our original paper that we would be better off discussing greenhouse gas emissions directly in terms of tons of CO2-equivalent (and thus focus on solutions to emissions)..."

Rees and Wackernagel: Footprint research does not preclude the use of tons of carbon as a measurement unit. However, Blomqvist et al. themselves use the land-based argument that plantations could combat CO2 accumulation in the atmosphere, thereby admitting that CO2 sequestration is one of several competing demand on biocapacity. Hence they contradict their own argument.

4) Blomqvist et al.: "....and developing a more ecological and ecosystem process framework to capture the impacts humans currently have on the planet's natural systems..."

Rees and Wackernagel: Better frameworks may indeed be possible. But they do not currently exist.

5) Blomqvist et al.: "The appropriate scale for these indicators will in many cases be local and regional."

Rees and Wackernagel: Precisely. We agree, which is why we point out that many of the most policy-relevant Footprint applications are at the local or national scale.

6) Blomqvist et al.: "Any city, for example, would show a deficit, as it relies on food and materials from outside. That in itself, as Robert Costanza has noted, 'tells us little if anything about the sustainability of this input [from outside the region] over time."

Rees and Wackernagel: Obviously, large cities cannot sustain themselves from their own biocapacity. But our argument is a different one: it is that cities are running out of hinterland. We point out that not all countries can run biocapacity deficits if the world as a whole should not be in overshoot. Current economic strategies of the vast majority of countries ignore this simple fact.

Additional Information:

For more information about the Footprint accounting method, click here.

Frequently asked questions are answered here.

For Blomqvist et al.’s original article in PLOS Biology, click here.

For Rees and Wackernagel’s response to Blomqvist et al. in PLOS Biology, click here.





Is the Ecological Footprint misleading?

Global Footprint Network response to
“L’empreinte écologique: un indicateur ambigu”
by Frédéric Paul Piguet, Isabelle Blanc, Tourane Corbière-Nicollier, and Suren Erkman from the University of Lausanne and École des Mines
in Futuribles, October 2007

Introduction

None should be asked to “believe in the Ecological Footprint” as an article of faith. Rather, as a scientific tool, Ecological Footprint accounts address, through empirical analysis and with ever increasing accuracy, one particular research question: how much of the planet’s productive capacity is demanded to support human activities? We believe that this is perhaps the single most important research question for the twenty-first century, one that humanity cannot afford to ignore. Failing to live within the budget of nature will eventually lead to ecological bankruptcy and collapse. There may thus be no single research endeavor more important than improving our knowledge of humanity’s demand on the biosphere through an open, transparent, scientific process. 

Frédéric Paul Piguet and his colleagues claim in their Futuribles article that the Ecological Footprint provides a poor and even misleading answer to this research question of how much of the planet we are using. This is a serious charge, and one that should not be taken lightly.  

Criticism is welcome

As stewards of the most widely used Ecological Footprints accounts and methodology in use today, we are the first to acknowledge that Footprint accounts can be improved. This is not unique to the Ecological Footprint, but a fundamental characteristic of all true scientific endeavors. As scientists, we are always eager to hear suggestions from others about how our methods can improve and what is not working.  

Many of Piguet et al.’s critiques, however, are based on misunderstandings of Ecological Footprint accounting methodology. Several also reconsider issues that have already been discussed at length by the Footprint community, though perhaps not as thoroughly in the French language. We regret that Piguet et al. chose to release their article without peer-review from other academics in the Footprint community. As a result, their article contains many misconceptions that we hope to clarify. 

Our response is divided into two parts. The first part explains what the Ecological Footprint actually attempts to measure and provides general background on our research process at Global Footprint Network. The second part responds to some of the main criticisms and misconceptions in Piguet et al.’s piece. A detailed, point-by-point response is downloadable here. Our first response was published in La Revue Durable (http://www.larevuedurable.com/).  

What is the Ecological Footprint, and how is it being improved?

Sustainable development implies a commitment giving all people the opportunity to lead fulfilling lives within the means of our one planet Earth. This concept continues to receive a great deal of attention in the public and political arenas. Yet when it comes to actual environmental strategies and policies, are our policy makers asking the right questions to lead us towards this goal? 

We know that when we catch more fish than can regenerate, fisheries eventually collapse; when we harvest more timber than forests can regrow, we advance deforestation; when we pump more water out of the ground than gets recharged, water tables drop; when we emit more CO2 than the biosphere can absorb, CO2 accumulates in the atmosphere and contributes to global warming. This overuse of resources is called ecological overshoot. Global ecological overshoot can be thought of as humanity withdrawing more from Earth’s biological bank account than is being deposited, which, as in the financial analogy, eventually leads to bankruptcy. To achieve sustainable development, it is critical to have information about both humanity’s demand on our planet and what our planet is actually able to provide. 

This is the research question answered by Ecological Footprint accounting. Footprint accounts measure the actual amount of biological resources produced and wastes absorbed by the planet in a given year, and compares this with how many resources humans extract and how much waste we generate. Our most recent accounts – the 2006 edition - show that, as of 2003, humanity was in overshoot, demanding more than 25% more than what the planet provided in that year.  

This graph and its supporting accounts provide a clear answer to the question of whether we are currently achieving sustainable development. The answer is a clear no – currently, global society is not living within the means of our planet, putting our society and economy at ever increasing risk. 

We encourage interested readers to learn more about our Ecological Footprint data and results by reading WWF’s Living Planet Report 2006, available for download – in 11 languages - at WWF or directly from us. Our calculation methodology is described in several academic papers, including Kitzes et al. (2007) or Kitzes et al. (2004). Also our data and methods page.

As with all rigorous scientific processes, our process of calculating Ecological Footprint accounts does not apply a single, fixed methodology, but rather continuously changes in response to new scientific information. We have several processes underway at Global Footprint Network to ensure that our calculations remain up to date and transparent for users. 

First, we engage in ongoing internal and community reviews of our methodology through the activities of our National Accounts Committee. This committee, composed of representatives from Global Footprint Network partner organizations, is responsible for suggesting methodological changes to our core National Footprint Accounts and considering any changes suggested from external academics and reviewers. All changes to our calculation methodology are open for public comment before implementation, in accordance with the Committee’s charter, and external parties are encouraged to submit recommended changes to the accounts directly to Global Footprint Network for consideration by the Committee. 

Second, we also engage in collaborative research projects with national governments and external consultancies to review our accounting methodology for specific nations. The governments of Switzerland (http://www.bfs.admin.ch/bfs/portal/de/index/themen/21/03/blank/blank/01.html), Japan, and the United Arab Emirates have all engaged with us to review the methodology and data for their nations, and we expect soon to begin similar reviews with Belgium, France, Ecuador, and other nations. The results of these reviews are published publicly and have been used in the past to improve the quality of our Ecological Footprint accounts for all nations. 

With this background in our Footprint accounts and our methodological review process, let us now turn to a selection of the criticisms presented by Piguet et al.   

Are Ecological Footprint results insufficiently accurate for tracking overshoot?

As described above, current Ecological Footprint accounts show the existence of global overshoot, a fact also recognized by Piguet et al. Piguet et al.’s main criticism in fact is that global overshoot is significantly larger than what we calculate, concluding that our results lack sufficient accuracy for policy recommendations. 

We agree with Piguet et al. in recognizing that the current calculations can and are being improved, and that our results today represent only the best available data today. We strongly disagree, however, that our current state of knowledge is insufficient for taking policy action. Whether global overshoot is 25% or perhaps larger, the fact remains that global overshoot exists, that it continues to increase, and that globally the majority of consumption is taking place in North America, Western Europe, and eastern Asia. We believe that this recognition alone provides sufficient direction for a great deal of policy action. 

We also disagree with the Piguet et al.’s fundamental argument that the accounts’ carbon sequestration factor has very high uncertainty. Piguet et al. recalculates the National Footprint Accounts’ carbon Footprint using different sequestration rates, including cultivated forest sequestration rates (higher than ours) as well as a rate based on the global forests' surface area and the effective carbon sequestered by terrestrial land area (lower than ours). They claim that the wide variation that these re-estimates produce in global overshoot means that current Footprint accounts are insufficiently precise for decision making. 

While at first glance, this exercise appears to be a valid form of sensitivity analysis, we believe that the high and low values chosen by Piguet et al. reflect a fundamental misunderstanding of the accounting framework, and are not valid for criticizing the precision of the existing carbon sequestration factor. To begin, the high sequestration rate based on cultivated forest is inconsistent with the underlying research question, which calculates Ecological Footprint and biocapacity in any given year based on the planet’s actual resource production and waste absorption in that year. Using a cultivated forest rate, and applying it to all forest area as potential sequestration, would be equivalent to saying that because aquaculture ponds placed in deserts could potentially provide more edible fish than we are currently catching, there is no overshoot in fisheries today. This is clearly a fallacious argument that confuses what is actually occurring today, which is what Ecological Footprint accounts measure, with what may or may not be true at some point in the future. 

On the other hand, the low sequestration value, based on the actual effective carbon sequestered by terrestrial land area, is similarly misguided. The current understanding is that the actual capacity or utility of each land type should be calculated based on its current use. For example, although many areas of the world could potentially grow potatoes, at varying yields (tonnes per hectare), we calculate the Footprint of a potato using the yield values for only that land which currently grows potatoes. 

Let us take an example of a 10 hectare potato field, producing 150 tonnes of potatoes, adjacent to a 10 hectare tract of forest that produces a single wild potato but could potentially be planted fully with potatoes if cleared. In calculating the Footprint of a potato eaten by a consumer, we use a yield of 15 tonnes per hectare, based on the amount of potato produced per hectare of cropland. Even though there is a single wild potato produced in the forest area, we do not calculate the yield of potatoes as 7.5 tonne per hectare (150 tonnes divided by 10 hectares of potato field plus 10 hectares of forest), as the forest is not managed for producing potatoes. Even making allowances for multiple use, in which the 10 hectares of forest would be divided conceptually into two portions, perhaps 9.9 hectares devoted to timber and 0.1 hectare devoted to potatoes, the yield for potatoes in our example will still predominantly reflect the yield of the area devoted to potato production. Dividing total actual potato production by the sum of the areas of actual and potential potato field gives a nonsensical value for yield that is strongly biased downward. 

This is analogous to the error made by Piguet et al. in choosing to divide total carbon sequestration by all forest area, even that forest which is not devoted to or managed for carbon sequestration. Taking this approach gives a meaningless value for carbon sequestration. The most accurate method for approaching a carbon sequestration factor would be to calculate the actual carbon sequestration rate in a given year (not the potential, as suggested by Piguet et al.’s high estimate) on land that is actually devoted to carbon sequestration (not all land potentially available, as suggested by Piguet et al.’s low estimate). Thus we would ideally like to estimate the actual carbon sequestered by all land which is currently actively managed for carbon sequestration. As currently no global sequestration estimate exists just for the land set aside for carbon sequestration, the accounts use the actual average of all forest sequestration, managed and unmanaged, as a proxy for this, recognizing that the areas actually set aside for the purpose of carbon sequestration today are likely to be forests and that current data sets do not divulge whether these areas would be growing at natural rates or managed in some way. 

This calculation of the actual carbon sequestration rate on land set aside currently for the purposes of carbon sequestration would be an excellent research question, as suggested by Piguet et al. The current answers they provide, however, do not address this specific question and hence are clearly not applicable in a Footprint accounting framework as currently practiced. 

In addition to this point, we have noted several other common misunderstandings through the Piguet et al. paper. A short table provides a selection of the most important, several of which are also related to the carbon Footprint calculation methodology. A full list of 53 comments is available here.   

Conclusion

We welcome Piguet et al.’s contribution to the debate surrounding national Ecological Footprint accounting. In spite of the misunderstandings that we have clarified here, the article raises many important issues, and we look forward to continued dialogue with the authors and others in the community about how to continue strengthening and improving our ecological accounting systems to support a sustainable future.