The role of DE in reducing the impact of energy use – an ecological footprint perspective

The ‘ecological footprint’ is an accounting tool that makes sustainability measurable by comparing human demand on the biosphere to the biosphere’s ability to renew itself. It can help governments and businesses meet the sustainability challenge by supporting management and planning policies and identifying points of leverage for reducing ecological demand. Here, Alessandro Galli and Justin Kitzes consider the possible impact of using decentralized energy (DE) for energy footprint reduction.

Alessandro Galli & Justin Ktzes

The ecological footprint is a simple environmental accounting tool introduced in the early 1990s by Mathis Wackernagel and William Rees to account for demand on, and supply of, renewable natural capital. In short, a footprint adds up all of the biologically productive land and sea areas needed to support a population or an individual in a given year.

The method for measuring life-supporting natural capital reflects the fact that humanity is constrained by the Earth’s limits. The surface of the Earth is finite, therefore the available ecologically productive area as well as the annual amount of resources produced and wastes absorbed have to be finite as well. No society would be able to function without the support of healthy forests, clean waters, fertile soils and other types of ecological capital that provide resources for our use and absorb the wastes that we generate. Every drop of fresh water, bite of food and breath of air depends on the functioning of a healthy environment.

In the last half of the 20th century it became apparent that people have so degraded their environment that these ecosystems may no longer provide the life-support systems for humankind that they used to. Given that, the ecological footprint tries to measure human demand on nature by revisiting the concept of carrying capacity. Instead of looking at the number of individuals that could live on a given land or sea area without permanently damaging the ecosystem on which they depend (carrying capacity), the ecological footprint asks how many planets are necessary to support all of the people that actually were on the planet in a given year, under that year’s standard of living, biological production and technology. This is an accounting question that can be answered through the analysis of documented, historical data sets.

While the ecological footprint provides a measure of total ecological demand, at the same time, an ecological benchmark à‚— biocapacity à‚— accounts for the resource supply and waste disposal that can be sustained on a given territory or at the global scale. A population’s footprint can be compared to the biocapacity that is available to support that population, as expenses and incomes are compared in finance. This analysis allows us to answer important questions: Who is using how much? Do we all fit on one planet?

Both the ecological footprint and biocapacity are commonly expressed in the unit of ‘global hectares’ (gha): hectares of land or sea area normalized to the world average productivity of all biologically productive land and water area in a given year. Global hectares provide a useful representation of the ecological demand associated with the use of a product (for example timber or coal), as they measure how much of global ecological productivity, rather than just the size of a physical area, is required to produce a given flow.

As any other aggregate indicator, the ecological footprint has the ability to convey a large amount of information in a single number but, unlike other economics-based aggregate indices, such as GDP or GNP, it includes economic, ecological and social issues and looks at the interrelationships existing among them. Moreover, it does not assign arbitrary weights to individual components in order to add them together, but rather all different types of land and sea that are demanded are normalized to the common unit based on empirical data on the relative productivity of these different area types. This normalization is not arbitrary and is based on observable characteristics of the land and sea areas.

Do we fit on one planet?

In the year 2003, the most recent year for which data are available, humanity’s global ecological footprint exceeded globally available biocapacity by about 25%.

This state, where footprint exceeds biocapacity, is known as overshoot, the term ecologists use when a population’s demands exceed its environment’s ability to support those demands. In 2003, humanity demanded more than 1.2 times what the earth was able to regenerate in that year (humanity needed 1.2 earths).

This type of overshoot is possible because humans can use the biological capacity, or biocapacity, of the planet faster than it can regenerate, for example by cutting trees faster than they can re-grow (for example deforestation) or harvesting fish species faster than they can reproduce. In addition, human activity may also generate waste at a rate that exceeds the ability of the biosphere to act as a sink for the waste material, as is the case with CO2 generated from anthropogenic activities.

In other words, the earth needed 1.2à‚ years to renew itself following the demands humans placed on it in the year 2003. Unfortunately, global overshoot is not a one-year phenomenon, since human society has been in a state of overshoot since the mid-1980s, as shown in Figure 1.

Figure 1. Humanity’s ecological footprint by component, 1961 to 2003
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Figure 1 divides human demand on the biosphere into two areas. The first, and most commonly recognized, is area for providing resources (food, timber, etc.) for human use. The second, equally important demand is area needed as a sink for the waste products of the human economy. Significant among these waste products is CO2 from fossil fuel use. Considering the past 40 years, the vast majority of the increase in ecological footprint has been due to an increase in the bioproductive area that is required to sequester the carbon released into the atmosphere.

In 2003, the energy footprint was responsible for nearly 48% of human demand on ecosystems. In the case of the US, Canada and Europe, this demand is currently 60% or more of total national footprints. At the global level, then, human society is living beyond its ecological means.

What about at smaller scales, such as at the national level? Both footprint and biocapacity vary widely between nations. A country can have an ecological reserve (biocapacity sufficient to meet à‚— or even exceed à‚— domestic footprint) or an ecological deficit (footprint in excess of domestically available biocapacity). Countries with an ecological reserve may use their available biocapacity to satisfy their own domestic footprints, protect and preserve natural ecosystems or export ecological resources to other nations. An ecological deficit, on the other hand, indicates that a country must rely on biocapacity from outside of its own borders or draw down its own natural capital.

Developed countries and oil rich nations, often with high levels of infrastructure, carbon emissions and consumption, top the list of the 10 highest footprint nations (see Table 1).

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Table 1 shows high energy land footprint values for oil-producer countries such as the United Arab Emirates (76%) and Kuwait (87%) together with percentages ranging from 40% to 59% for most of the top 10 footprint nations. At the same time, among these top footprint nations, low energy land values characterize Swedenà‚ (17%), New Zealand (27%) and Norway (34%). Such relatively small contributions to the overall footprint are due to the low national carbon intensities for electricity and heat generation that are 60 g CO2 per kWh (Sweden), 182à‚ gà‚ CO2 per kWh (New Zealand) and 8.4 g CO2 per kWh (Norway), with the world-average carbon intensity equal to 507à‚ g CO2 per kWh.

What about developing countries? Figure 2 provides some information on footprint composition among countries of high, middle and low income. Note that the total average footprint value for high, middle and low income countries is 6.44, 1.89 and 0.79 global hectares per person respectively.

Figure 2. Percentage footprint composition by incomeà‚ group
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Again, it can be seen that human demand on the biosphere is mainly characterized by crop and energy land requirement. Crops demand constitutes 12% of the overall footprint value in the high-income countries and grows up to 25% and 43% in the middle and low-income countries respectively. The opposite pattern characterizes the energy land footprint, which represents 56% of the high-income countries’ footprint and goes down to 45% and 26% in the middle and low-income countries respectively.

Among the developing countries, an important role is played by China. It cannot be denied that China is among the world’s largest economies and is growing so quickly that its trends and practices will undoubtedly influence the nature of global economic and environmental changes in the 21st century. Even if China has not yet reached its maximum growth rate, the ecological footprint tells us an interesting story (see Figures 4a and 4b).

Figure 4a. Comparison of China’s footprint and biocapacity in 1961 by component
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Figure 4b. Comparison of China’s footprint and biocapacity in 2003 by component
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From 1961 to 2003, similar to the US, China has shifted from a reserve situation to a deficit situation, with local per capita biocapacity lower than the per capita footprint. In 2003, deficit characterizes most of the land types that, just a few decades before, where characterized by a surplus condition (for example pasture, fish). Moreover, focusing on the energy land footprint, it can be seen that per capita values increased an incredible 1100% over this time period.

Reducing human demand on energy land will be a very important challenge for both developed and developing countries. This could be accomplished through reducing consumption, producing energy and fuel in a more sustainable way by means of renewable resources (such as hydropower, wind power, thermal solar collectors, photovoltaic solar cells, biofuel, etc.), and increasing efficiency of energy technology (for example high-efficiency cogeneration, on-site renewable energy systems, energy recycling systems, etc.).

How does the footprint account for carbon?

For the sake of clarity, two types of related footprint measurements should be distinguished. The carbon footprint is a measure of the amount of biologically productive land required to act as a sink for human-driven emissions of CO2. At present, the only anthropogenic source of carbon included in ecological footprint accounts is CO2 emissions associated with the combustion of fossil fuels.

The energy land footprint is the sum of the footprints of all areas required to support the energy needs of human society, including not only the fossil fuel footprint, but also demands associated with other sources of energy (for example renewable biomass, wind power and hydropower). About 90% of the current global energy land footprint consists of the carbon footprint.

The basic method behind calculating the carbon footprint at any scale accounts for the amount of biologically productive land (expressed in global hectares) required to assimilate a given quantity of CO2. This is done by assessing the land area required to sequester, through photosynthesis, that part of the CO2 emissions from fossil fuel combustion which is not absorbed by the oceans and not sequestered away from the biosphere through other measures, such as carbon capture and storage.

In practice, the key calculation involves determining the carbon conversion factor (CCF), reported in global hectares per annual tonne, which converts a given emission of CO2 into a demand on biocapacity, or footprint.

The amount of CO2 absorbed by ocean surfaces and the amount of CO2 stored per year per unit area of forest land provides the conversion factor that is used to calculate the footprint of all the CO2 emissions. Many different land types and productive ecosystems have the capacity for long-term storage of CO2, such as crop land or grassland.

To avoid overestimating the land area required, however, the ecological footprint uses the bioproductive area that has the highest capacity for sequestering carbon: forest land. Note that all footprint and biocapacity accounts are performed in order to underestimate footprint values and overestimate biocapacity values.

The CCF is currently calculated as 0.27 global hectares per annual tonne of CO2.

Footprint reduction via renewable resources

As mentioned above, three pathways can be followed to reduce the total energy land footprint: reduction in consumption, use of renewable resources, and increased efficiency in energy use and production.

Figure 5 shows the range of footprints of different energy technologies, considering the current level of efficiency steady, in comparison with fossil fuels.

Figure 5. Range of footprints of various energy technologies compared with fossilà‚ fuels
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The high value of fossil fuel footprint points out the lack of ecological capacity for coping with excess CO2 and underlines the importance of reducing CO2 emissions. As Figure 5 shows, it may be possible to noticeably reduce the size of the energy land footprint by shifting our economy from a fossil fuel-based to a renewable fuel-based one. Increasing the use of photovoltaic solar cells, or wind turbines, in the electricity production could reduce CO2 emission and human footprint, helping reducing overshoot. Note that estimates of the footprint of biofuels vary widely depending on the amount of energy needed to convert the crop into fuel. Variations in the footprint of fossil fuels are also shown and are mainly due by the amount of natural gas rather than coal used: minimum footprint values are intended to represent a main use of natural gas, while maximum footprint values represent a main use of coal.

Footprint reduction via decentralized energy

When considering increases in efficiency, high efficiency cogeneration, energy recycling systems and on-site renewable energy systems all have a role to play. These technologies can be more broadly grouped under the name decentralized energy (DE) technologies and are aimed at producing electricity at or close to the point of consumption.

In 2002, the WADE Economic Model was designed to calculate the economic and environmental impacts of supplying incremental electric load growth with varying mixes of DE and central generation (CG). Starting with known generating capacity for year 0 and projections for retirement and load growth, the model builds user-specified capacity to meet future growth and retirement over a 20-year period.

Results from the WADE Economic Model show, among other data, the potential for a 47% reduction in CO2 emissions from electricity and heat production, over 20 years, if the global economy shifts from a 100% centralized system to a 100% decentralized energy system.

Considering that electricity and heat production accounts for about 36% of total energy land requirement, the ecological footprint has therefore been used to account for the environmental consequences of such a shift from centralized systems to decentralized energy systems. Figure 6 shows possible future paths for the world as a whole under four scenarios.

Figure 6. World footprint scenarios, 2003à‚—2023
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These four possible outcomes, which all refer to the year 2023, are derived from a joint interpretation of the scenarios proposed in the Living Planet Report 2006 (LPR 2006) and the WADE projections. Each bar in Figure 6 shows the ‘numbers of planets’ needed by humanity. A value greater than 1 in the ratio therefore shows an overshoot situation.

More precisely, the first scenario, business as usual (BAU), is directly drawn from the LPR 2006 and considers moderate rates of growth for both population and per capita footprint. Biocapacity is assumed to keep increasing at the same rate as the past 40 years. No specific decentralized energy plans are considered. The scenario doesn’t consider WADE projections at all.

The second scenario, BAU & 100% DE, has been calculated by integrating projections from the LPR 2006 business as usual scenario with the WADE projections. It therefore considers all the same trends of the previous one and a 100% shift from central generation to decentralized energy (DE).

The third scenario, rapid reduction (RR), is directly drawn from the LPR 2006 and depicts an aggressive effort of the global community to move humanity out of the overshoot by 2050. Under this scenario, a 50% reduction in CO2 emissions is achieved by that time. Despite the pressures of a growing population, the absolute consumption of crop land and grazing land rise by only 15% by 2100.

Under median population projections, this requires a 25% decrease in per person demand on crop land and grazing land that can be achieved by decreasing calories or weight of food products consumed, increasing yields and reducing the proportion of global crop production that is grown for animal feed. The consumption of forest products increases by 50% by 2100, and urban land does not increase in extent. Biocapacity is assumed to keep increasing at the same rate as the past 42 years. No changes in the energy production mixes are considered and no WADE projections are involved.

The fourth scenario, RR & 100% DE, has been calculated by integrating projections from the LPR 2006 rapid reduction scenario with the WADE projections. It therefore considers all the same trends as the previous one and a 100% shift from central generation to decentralized energy (DE) in energy production.

The BAU scenario reports an increased overshoot in 20 years with an increased demand on crop land, fishing ground and energy land. That would cause a demand by human activities equal to about 1.6 earths. A 100% shift from central generation to decentralized energy in energy (and heat) production mixes (BAU & 100% DE scenario) would result in a slightly lower global overshoot, even though population will increase. A higher number of people with a 10% lower average per capita footprint will unsustainably live on the planet.

Looking at more optimistic future paths, the RR scenario shows a slightà‚ reduction in overall overshoot with a greater reduction obtained by completely shifting to decentralized energy technologies. The RR & 100% DE scenario in fact reports a close toà‚ overshoot-ending situation with mankind using about 1.1 earths, even if energy land remains the most relevant contribution.

This outcome underlines the important role that DE could play in footprint reduction in the coming years. If the global economy will be able to follow the Rapid Reduction pathway, humanity will move out of the overshoot by 2050, but if this pathway is followed together with a shift from central to DE systems (RR & 100% DE scenario), overshoot ending will be anticipated by nearly 15 years.

On the other side, ecological footprint accounting suggests that a switch from central generation to DE may not always result in a reduction of total human demand on the biosphere but instead simply substitute an increase in built-up footprint (required for decentralizing the power plants) for the decrease in energy land footprint.

The dotted boxes of Figure 6 on the BAU & 100% DE and RR & 100% DE scenarios give a very rough approximation of the increase in built-up land footprint. Since no data were available regarding the increase on built-up area connected with decentralizing energy, a high-end estimate of the possible increase has been considered.

Despite this increase, the additional built-up land footprint should be relatively small if compared with theà‚ footprint saving showed in the energy land demand. A full analysis of the totalà‚ ecological capacity required to support the use of both traditional and DEà‚ technologies would, however, help ensure that discussions of energy sustainability are grounded in ecological reality.


The ecological footprint can be used to compare human demand on and nature’s supply of natural capital, similar to how economic expenses and incomes are compared in classical economic balances. The year 2003 is characterized by a negative balance, with demand exceeding supply by about 25%.

Looking at the reasons for this deficit, in 2003, the energy footprint was responsible for nearly 48% of human demand on ecosystems and has shown a 500% increase from 1961 to today. Focusing on possible ways to reduce energy land footprint, global society might consider consumptions reduction, use of renewable resources, and increased efficiency in energy use and production, including DE technologies.

Significant improvements can be made by working on electricity production systems, considering both renewable energy sources and DE technologies. Changes in the production systems can reduce overshoot as well as negative effects of climate change.

As a first approximation, four possible scenarios can be considered forà‚ the year 2023 to see the effects of implementing DE technology. One of these scenarios, the RR & 100% DE scenario, shows the possibility of appreciably reducing overshoot by 2023, withà‚ mankind using about 1.1 earths in that year.

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Alessandro Galli is Research Fellow at Global Footprint Network, Oakland, California, US, and PhD Student at the Department of Chemical and Biosystems Sciences at the University of Siena, Italy.
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Justin Kitzes is Senior Manager of the National Accounts Program at Global Footprint Network, Oakland, California, US.

Ecological footprint of the US

The US has one of the highest per capita footprints in the world, averaging around 9.7 global hectares per person. Figure 3a reflects an increase of 82% in the past 40 years, mainly due to an increase in energy land requirement. Figure 3b shows per capita footprint, biocapacity and GDP for the same time period.

Figure 3a. Ecological footprint of the US by component from 1961 to 2003
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Figure 3b. Ecological footprint, biocapacity and GDP of the US from 1961 to 2003
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Two main trends can be observed in these figures. First, in 1961, the US’s total biocapacity was greater than its footprint, a situation of ecological reserve. Less than 10 years later, the US’s ecological footprint value passed its national biocapacity, leading the US into an ecological deficit situation that is still continuing. This deficit can only be met in two ways: by ‘borrowing’ biological capacity from other nations or by drawing down the actual natural capital base within the US.

In the latter case, it could be stated that during the past 30 years, the US has drawn on principal rather than just annual interest. Looking at the individual components, the demand for energy land, or the demand for carbon sequestration capacity, accounted for the largest portion of the US footprint growth.

A 300% increase in energy land footprint was seen between 1961 and 1974, followed by a lower 30% increase in the past 30 years. We might suppose that this trend reflects an increase in oil consumption during the years of the ‘American Dream’, with this increase being moderated by the oil crisis in the early 1970s.

The trend of the US’s per capita GDP closely follows that of its footprint, particularly that of energy land (see Figures 3a and 3b together). Overall, the large increase in the national economy (represented by the GDP curve) has thus been financed at least partially through a constantly increasing demand on nature (represented by the footprint curve).

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