Abbreviated to eco-footprint or just footprint. Defined by the WWF, "an ecological footprint compares countries' consumption of natural resources with the Earth's biological capacity to regenerate them," or "a measure of humanity's use of renewable resources."

The biologically productive space available to each person on the planet. Divided into equal shares (i.e. divided by world population) it was 5 - 6 hectares per person in 1900 and decreased to 1.5 hectares per person by 2000. Ecological space can expand or shrink depending on resource consumption, technological innovation, population growth and other factors.

The amount of energy that a substance contains and therefore can produce. For liquid fuels this is usually measured in joules per litre. Different types of have different energy content - for example, ethanol has about two-thirds the energy content of petroleum

Ethanol is a liquid fuel produced by converting the starch content of biomass feedstocks (e.g. corn, potatoes, beets, sugarcane, wheat) into alcohol by variations on a fermentation process. Ethanol has different chemical properties to petrol (gasoline). For example, ethanol evaporates (changes from a liquid to a gas) less readily than petrol does. It can be blended with petrol, though catalytic converters may be needed to control harmful emissions, as with other types of petrol.

A constituent or component category of eco-footprinting, used to calculate a regional, national or total (Earth) footprint. The constituent categories most often used are: cropland, grazing land, forest, fishing grounds and energy (C02 from fossil fuels, fuelwood, nuclear and hydro). The results of these constituent footprints for one country are aggregated (sometimes balanced by equivalence factors) to make a national footprint.

A country's eco-footprint is the total area needed (inside or outside its own territory) to produce the renewable natural resources it consumes, including those used to sustain energy, and those used for infrastructure. This area is expressed in a theoretical measurement called global hectares of productivity. For example, the UK [LPR 2002] has a total footprint of 5.35 global hectares per person, while Bulgaria's footprint is only 1.84 gha per person. The total footprint is made up of five constituent footprints: for cropland, grazing land, forest, fishing grounds and energy . When these constituents have been taken into account, extra is added for water withdrawals, infrastructure and, in OPT's case, another 12 per cent land allocation is made to allow for wilderness (all other territory being already used to fill one of the five constituent footprint purposes - such as forest for forest products).

"The area required to produce all the crops which a country consumes, including cereals, fruits and vegetables, roots and tubers, pulses, nuts, tea, coffee, sugar and vegetable oils, as well as tobacco, cotton, jute and rubber. It also includes crops grown to feed animals whose meat, milk, or eggs are consumed in that country (meat from free-ranging animals is accounted for in the grazing-land footprint). Within each country, the footprint accounts distinguish between two types of cropland: marginal cropland includes lower quality land used for growing sorghum, millet, olives and fodder grass, and standard cropland includes all other crops. Cropland that is unharvested, temporarily grazed, or fallow land is also included as standard cropland. The cropland footprint does not take account of land lost each year to erosion, salination or degradation." [LPR 2002, WWF]

A prefix meaning 1,000 million (one billion or 1,000,000,000) times. Examples: 1 Gigawatt (Gw) is one billion or 1,000,000,000 watts; 1 Gigatonne (Gt) is one billion or 1,000,000,000 tonnes.

One hectare (ha) is 10,000 square metres (m²), e.g. an area measuring 100m x 100m. (One hectare = 2.47 acres.) One square kilometre is a square measuring 1 kilometre on each side = 100 hectares i.e. an area measuring 1 million square metres.

Eco-footprinting uses as its basis the biologically productive area of the planet. Earth is believed [LPR 2002] to have 11.4 billion hectares of biologically productive space, made up of productive land and fishing grounds. To work out the structure of eco-footprinting, see definitions in the following order: local hectare, yield factor, worldwide hectare, equivalance factor, global hectare.

In eco-footprinting, 1 hectare of biologically productive space with world-average productivity. In 2002 the biosphere had 11.4 billion hectares of biologically productive space corresponding to roughly one quarter of the planet's surface. These 11.4 billion hectares include 2 billion hectares of cropland, 3.5 billion hectares of grazing land, 3.8 billion hectares of forest land, 0.3 billion hectares of inland waters and 0.3 hectares of built-up land. One global hectare is therefore a hectare representing the average capacity of one of these 11.4 billion hectares. Thus a hectare of highly productive land represents more 'global hectares' than the same surface of less productive land. Global hectares allow the meaningful comparison of the ecological footprints of different countries, which use different qualities and mixes of cropland, grazing land, and forest. [LPR 2002]

A basic building block in eco-footprinting, this shows the biological productivity (biocapacity) of one real hectare within a specified region or country, in one year.
Example: Take four potato-growing countries, A, B, C and D, and assume that potatoes are one of the crops counted as components in the total cropland hectares used in eco-footprinting. Total potato-producing hectarage in the world is 100 hectares (ha), of which Country A has 40 (local) hectares (lha); Country B - 25 lha; Country C - 20 lha; and Country D - 15 lha. The yield of potatoes per hectare is measured in tonnes of produce by dry weight per annum. But because of differences in soil, climate and farming methods, there are differences in yield between each country.


Country A (40 lha) grows 2.0 tonnes per hectare = 80 tonnes
Country B (25 lha) grows 2.5 tonnes per hectare = 62.5 tonnes
Country C (20 lha) grows 1.5 tonnes per hectare = 30 tonnes
Country D (15 lha) grows 3.0 tonnes per hectare = 45 tonnes

Total world potato production is therefore 217.5 tonnes a year, and the average yield - tonnes produced per hectare - is 2.175 (217.5/100 hectares). It is now possible to calculate a measure of the share of worldwide potato productivity that each country has - which is called a worldwide hectare (wwha). At average worldwide productivity:


Country A would produce (40 lha x 2.175) = 87 tonnes of potatoes
Country B would produce (25 lha x 2.175) = 54.3475 tonnes of potatoes

But they don't. Country A produces 80 tonnes, 7 tonnes below the worldwide average, so its yield factor (yf) is 80/87 = 0.92. Country B produces 62.5 tonnes, 8.1525 tonnes above the worldwide average, so its yield factor would be 62.5/54.3475 = 1.15. To arrive at the relative shares of potato production by each country expressed as notional worldwide hectares (wwha), which are units of account in an ecological footprinting balance sheet, each country's local potato hectarage is multiplied by its yield factor. So:


Country A has (40 lha x 0.92 yf) 36.80 wwha
Country B has (25 lha x 1.15 yf) 28.75 wwha

If all the wwha are added together they should make 100, which is the actual total potato-producing hectarage (rounding fractions up or down can cause errors to compound).

The Earth has abundant supplies of the odourless gas hydrogen, which is also present in water (H₂0), from which it can be made by electrolysis. It can be stored as a liquid or as metal hydrides.

Fuel cells operate without combustion. Hydrogen fuel cells operate by converting energy, which is stored in hydrogen atoms, directly into electricity. This is achieved by bringing together hydrogen and oxygen in an electrochemical reaction. The only by-product of the reaction is water. A polymer membrane separates the oxygen from the hydrogen.[BOC]

It is an energy carrier which can be converted efficiently in a fuel cell. The energy in in the hydrogen can be converted into heat or electrical energy, but about 40% of the energy in the hydrogen is lost in conversion to electricity. There are also losses in converting the original energy into hydrogen. How big those losses are depends in part on the amount of compression required (or perhaps liquefaction). In producing liquid hydrogen from electricity, about 60% of the energy in the electricity is retained in the hydrogen. Thus, allowing also for the double conversion of electricity to hydrogen and hydrogen to electricity, the energy recovered as final electricity is about 0.60 x 0.60 = 36%. That is the brief reason why making hydrogen from wind turbines is unlikely, at current technology and price, to be viable: 1/0.36 = 2.8 times as much electrical energy to start with as is delivered after its transformation into hydrogen and back again. There is more informatino on fuel cells on the OPT Journal website. [Andrew Ferguson].

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