UK Government Report Calls for “Strategic Metals” Plan.
Not only are supplies of oil and natural gas under imminent threat of failing to meet demand for them, but so is a whole range of precious metals, along with indium, gallium and germanium and other vital elements such as phosphorus and helium, as is discussed throughout this Commentary. A report1 from the Science and Technology Committee, advised by the Royal Society of Chemistry, warns that if the U.K. does not secure supplies of strategic metals, its economic growth will be severely jeopardized. Of particular concern are indium, used in touch screens and liquid crystal displays, and rare earth elements (REEs) particularly neodymium and dysprosium, used to fabricate highly efficient magnets for electric cars and wind turbines. Platinum group metals are an issue too, used in catalytic converters and fuel cells. As is true of oil and gas, and indeed world population, such resources are not evenly distributed around the globe, and for example 80% of available new platinum is extracted from just two mines in South Africa. 92% of the niobium used in the world (for superconducting magnets and highly heat-resisting superalloys e.g. in jet-engines and rocket subassemblies) is exported from Brazil, and 97% of REEs are presently supplied from China. In developing a low-carbon transport infrastructure, it is proposed that biofuels should be used principally for aviation where there is no practical alternative to liquid fuels. Thus, it is ventured, electric cars will become increasingly important in providing personalised transport while avoiding the use of petroleum or natural-gas based fuels. The knock-on effect is that new sources of lithium must be found along with the means to mine and process the metal, plus the inauguration of recycling technology for lithium. One can immediately take issue with the practicalities of both arms of this scheme, however. Roughly one fifth of all fuel in the UK is used for aircraft, or around 13 million tonnes. At a yield of 952 L/ha and a density of 0.88 g/cm3, to produce this much biodiesel would take 15.5 million hectares of arable land, of which the UK has only 6.5 million hectares. Thus if we were to stop growing food crops entirely and just rapeseed, we could still only fuel 42% of our aviation fleet. It is obvious that just a few percent at best of our current number of planes can be kept in the air by means of biofuels. Clearly, the days of cheap air-travel are numbered and this may be one reason why the coalition government has scrapped plans to build the controversial and vexed third runway at Heathrow Airport. Given the 30 million cars on the roads here currently fuelled by oil, the case for a wide-scale implementation of electric-cars might appear compelling. However, the lead-in time to make a dent in that number of vehicles and the 60 million tonnes of crude oil used for fuel would be decades at best, even if the necessary supplies of REEs, lithium and overall manufacturing capacity for them could be achieved. The most practical use for electricity is to power mass transportation, e.g. tramways and railway networks rather than individual vehicles.
Endangered Elements: Threat to Green Energy.
Underpinning the above political agenda, a list2 of "endangered elements" has been published in a new report, including the rare earth elements (REEs), in particular neodymium, production of which, it is reckoned3, will have to increase five-times to build enough magnets for the number of wind-turbines deemed necessary for a fully renewable future. Nonetheless, my rough calculations indicate that this would still take 50 - 100 years to implement, depending on exactly what proportion of the renewable electricity budget would be met from wind-power, and if the manufacturing capacity and other resources of materials and energy needed for this Herculean task will prevail.
Neodymium is a rare earth metal used extensively to produce permanent magnets found in everything from computer hard disks and cell phones to wind turbines and cars. Neodymium magnets are the strongest permanent magnets known, and a neodymium magnet of a few grams can lift a thousand times its own weight. The magnets that drive a Toyota Prius hybrid’s electric motor use around 1 kilogram of neodymium, while 10 - 15 kg of lanthanum is used in its battery3. Interestingly, neodymium magnets were invented in the 1980s to overcome the global cobalt supply shock that occurred as the result of internal warfare in Zaire (now Congo). Around one tonne of REE-based permanent magnets is needed to provide each MW of wind-turbine power.
Of the other REEs, demands for dysprosium and terbium, which are harder elements to extract than their lighter relatives, are such that supply will be outpaced within a decade. The latter have been described as "miracle" ingredients for green energy production since small quantities of dysprosium can result in magnets with only one tenth the weight of conventional permanent magnets of similar strength, while terbium can be used to furnish lights that use as little as 20% of the power consumed by normal illumination. By alloying neodymium with dysprosium and terbium, magnets are created that more readily maintain their magnetism at the high temperatures of hybrid car engines3.
However, far more dysprosium relative to neodymium is required than occurs naturally in the REE ores, meaning that another source of dysprosium must be found if hybrid cars are to be manufactured at a seriously advancing rate. As noted, almost all REEs come from China whom it appears will run out of dysprosium and terbium within 15 years, or sooner if demand continues to soar, notwithstanding that Chinese hegemony for its own future energy projects may mean that the current amount of REEs being released onto the world markets will be severely curbed. Almost certainly, new sources of REEs will be sought, given their vital importance to providing future renewable energy, and Japanese geologists have reported that there may be 100 billion tonnes of REEs in the mud of the floor of the Pacific Ocean.4 Since the minerals were found at depths of 3,500 to 6,000 metres (11,500-20,000 ft) below the ocean surface, the undertaking required to recover them will not be trivial, however, and the practicalities of the enterprise remain to be seen.
Peak Oil - Peak Minerals.
According to the Hubbert theory5, all resources are finite and will ultimately be extracted only to the limit where it is feasible to do so, whereupon either financial costs or those of energy dictate that to proceed further only yields diminishing returns. The Hubbert theory was originally applied to oil, in which the production curve "peaks" at the point of maximum output (when half the original resource has been used), beyond which it falls remorselessly. Similar fits can also be made to gas and coal production data and a recent analysis was reported using the approach to a study6 of 57 different minerals by Ugo Burdi and Marco Pagini. These authors have fitted both logistic and Gaussian functions to mineral production data from the United States Geological Survey (USGS), and it is interesting that for mercury, lead, cadmium and selenium, there is good accord found between the "ultimate recoverable resources" URR determined from the curve-fitting to the data and those reported as remaining in the USGS tables (plus the amount of each already extracted). For tellurium, phosphorus, thallium, zirconium and rhenium, the agreement is quite close but tends to smaller values than are indicated from the figures for cumulative production plus the USGS reserves. For gallium, the figure obtained from the fitting analysis is significantly lower than the USGS estimate (by about a factor of seven).
Evidence of peaking is found for a number of minerals, e.g. mercury around 1962; lead in 1986; zirconium in 1990; selenium in 1994; gallium in 2000. The results for gallium are significant, both in that the peak occurred seven years ago and in the size of its total reserve, which when compared with the amount used worldwide by the electronics industry, implies that we may run short of gallium any time soon. Tellurium and selenium are two other minerals that underpin the semiconductor industry and it appears that their fall in production may also impact negatively on future technologies that are entirely reliant upon them, since there are no obvious substitute materials with precisely equivalent properties.
For vanadium, although a production peak is indicated in 2005, the data in the "mineral commodities handbook" show a later and sudden surge in production, which is not fully explained but thought may potentially relate to uncertainties in reporting from countries like China. So, there may be a real and ongoing upsurge in production from particularly the Chinese economy which is quoted as being "out of sync" with the rest of the world, such is its massive expansion, or it might be a red herring.
Hafnium, another metal whose days are numbered, is an essential component of computer-chips and is also employed as a thermal-neutron absorber in nuclear control-rods, is thought may literally run-out within 10 years. Peak oil we all know about, but peak gas, peak uranium and peak coal will follow. There is in fact a peak in the production of all materials that were laid down in the distant past, and we are using them up at an expanding rate.
Interestingly, copper, zinc, tin, nickel and platinum show an almost exponential increase in production; however, the stocks of some metals may be insufficient to supply the technological demands of the modern developed world into the far (or even near) future. There is also the issue of how quickly a rare and difficultly extractable metal such as platinum might be produced in comparison with an overall demand for it. Copper production can be fitted with an exponential function up to 2006, while a logistic function provides about the same quality of fit, yet indicates a peak in about 2040. The latter agrees reasonably well with the USGS estimated copper reserves of 0.5 - 1.0 Gigatons, while the fit gives 2 Gigatons. Notably, the world price of copper has skyrocketed during the past few years, which is again attributed to demand in China, as was the cost and shortage of wood earlier in the year.
The above analyses rest upon the case that the determined "peaks" represent actual global production maxima. Indeed, more reserves of all minerals may yet be found if we look assiduously enough for them; but herein lies the issue of underpinning costs, both in terms of finance and energy. It is the latter that may determine the real peaking and decline of minerals, which extend beyond the simple facts, say, of mining and refining a metal from its crude ore. There is also the cost-contribution from the energy needed to garner energy-materials such as oil, gas, coal and uranium, and thence to turn them into power and machinery; and since fossil fuels are being relentlessly depleted, it takes an inexorable amount energy to produce them, resulting in a cumulative and rising energy demand overall.
The whole "extractive system" is interconnected through required underpinning supplies of fossil fuels, and it is perhaps this that explains why the production of so many minerals seems to be peaking during the period between the latter part of the 20th century and the start of the 21st, in a virtual mirror-image of the era when troubles in the production of fossil fuels were experienced across the globe. Hence, it may be the lack of fossil fuels which determines the real amount of all other minerals that can be brought onto the world markets6. Even if we manage to solve our energy problems, we may not have enough "stuff" to make things from. Some salient points about potential metals shortages are apparent from the list of elements in Table 17, which gives the world total reserve of each, the expected time of exhaustion based on current rates of production, and their principal uses. The figures therein are based on known reserves, noting that more might be found if they were explored for with sufficient assiduousness. However, emerging new technologies and a growing world population mean that some key-metals are likely to be exhausted more quickly, as indicated in Table 27. The reserve lifetime of a resource (also known as the R/P ratio) is defined as the known economically recoverable amount (R) divided by the current rate of use (P) of it, hence the values in Table 1 and Table 2. Economics predicts that as the lifetime of a reserve shortens so its price increases. Consequently, demand for that reserve decreases and other sources, once thought too expensive, enter the market. This tends to make the original reserve last longer, in addition to the volume of the new reserves. For example, there is enough bauxite reckoned to provide aluminium for 70 years, but the latter is an abundant element and there are many alternative known sources of it, thought to add-up to over 1000 years worth. In practice many other factors are involved, particularly geopolitical situations, but the basic geological fact remains: reserves are limited and hence their present patterns of consumption and growth are not sustainable over the longer term. While some elements are very plentiful compared to the total amount of them required, the rate at which they can be recovered sets a limit on how quickly a given reserve can be exploited. The R/P ratio analysis is of course a gross approximation, as the Hubbert-type fits to production show, since a given amount of a resource/year cannot be produced up to the bitter end. Production must eventually decline, mainly as the Energy Returned on Energy Invested (EROEI) falls.
The Role of Recycling.
In the face of resource depletion, recycling looks increasingly attractive. In this stage of development of the throw-away society, now might be the time to begin "mining" its refuse. It has been shown that there are part-per-million (p.p.m.) quantities of platinum in road-side dust8, which is similar to the 3 p.p.m. concentration in South African platinum ore. It is suggested that extracting platinum from this dust, which originates in catalytic converters, might prove lucrative and would expand the limited amount of platinum available, which even now does not meet demand for it. Discarded cell-phones too, might be a worthwhile source. For metals such as hafnium and Indium, recycling is the only way to extend the lifetime of critical sectors of the electronics industry. This is true also of gallium, tellurium and selenium, since all of them are past their production peak, which forewarns of imminent potential production shortages and escalating prices. While recycling of base-metals from scrap is a mature part of an industry worth $160 billion per year, current efforts to recover and recycle rare-metals are far less well advanced. However, in view of its present high-price, rhenium is now recovered from scrap bimetallic catalysts used in the oil refining industry. I expect to see an expansion of this top-end of the metals-market since rising demand for rare-metals will confer highly lucrative profits. It might be argued that we will never "run-out" of metals because their atoms remain intact, but the more dispersion that occurs in converting concentrated ores into final products, the more difficult and hence energy intensive it becomes to reclaim those metals in quantity. In a sense the problem is the same as deciding which quality of ore to mine in the first place: we now need to either find richer sources to recycle from or arrange how we use these materials in the first place to facilitate recycling. Ultimately, recycling needs to be deliberately designed into an integrated paradigm of extraction, use and reuse, rather than treating it as an unplanned consequence.
Table 1. Metals under threat: the world total reserve of each, and the expected time of exhaustion based on current rates of production and their principal uses.7
Aluminium, 32,350 million tonnes, 1027 years (transport, electrical, consumer-durables)
Arsenic, 1 million tonnes, 20 years (semiconductors, solar-cells)
Antimony, 3.86 million tonnes, 30 years (some pharmaceuticals and catalysts)
Cadmium, 1.6 million tonnes, 70 years (Ni-Cd batteries)
Chromium, 779 million tonnes, 143 years (chrome plating)
Copper, 937 million tonnes, 61 years (wires, coins, plumbing)
Gallium 1000 - 1500 tonnes, 5 - 8 years (semiconductors, solar cells, MRI contrast agents).
Germanium, 500,000 tonnes (US reserve base), 5 years (semiconductors, solar-cells)
Gold, 89,700 tonnes, 45 years (jewellery, "gold-teeth")
Hafnium, 1124 tonnes, 20 years (computer-chips, nuclear control-rods)
Indium, 6000 tonnes, 13 years (solar-cells and LCD's)
Lead, 144 million tonnes, 42 years (pipes and lead-acid batteries)
Nickel, 143 million tonnes, 90 years (batteries, turbine-blades)
Phosphorus, 49,750 million tonnes, 345 years ( fertilizer, animal feed)
Platinum/Rhodium, 79,840 tonnes, 360 years for Pt (jewellery, industrial-catalysts, fuel-cells, catalytic-converters)
Selenium, 170,000 tonnes, 120 years (semiconductors, solar-cells)
Silver, 569,000 tonnes, 29 years (jewellery, industrial-catalysts)
Tantalum, 153,000 tonnes, 116 years, (cell-phones, camera-lenses)
Thallium, 650,000 tonnes, 65 years (High Temperature Superconductors, Organic Reagents)
Tin, 11.2 million tonnes, 40 years, (cans, solder)
Uranium, 3.3 million tonnes, 59 years (nuclear power-stations and weapons)
Zinc, 460 million tonnes, 46 years (galvanizing).
Table 2. It is predicted that the growth in world population, along with the emergence of new technologies will result in some key-metals being used up quite rapidly7, e.g.
Antimony, 15 - 20 years.
Gallium, 5 years.
Hafnium, 10 years.
Indium, 5 - 10 years.
Platinum, 15 years.
Silver, 15 - 20 years.
Tantalum, 20 - 30 years.
Uranium, 30 - 40 years.
Zinc, 20 - 30 years.
Stolen Catalytic Convertors and Platinum Prices.
The price of platinum has just hit $1,722 an ounce9, in consequence of fears that the major producers of the metal in South Africa will be unable to keep pace with rising demand for it and that it is seen as an “investment” commodity, along with gold. Around 40% of "new" platinum, extracted at a rate of close to 150 tonnes annually, is used for jewellery which is about the same as is used to make catalytic converters (“cats”). It is reckoned that scrapping one million such cats would yield 40,000 ounces of platinum (which works out at 40,000 x 31.10 g/Troy ounce = 1.244 tonnes or 1.244 g per cat, as an average). It is thought that the worldwide "scrap-platinum" market might eventually provide 1 million Troy ounces per year, or 31.1 tonnes; meanwhile, those unwilling to wait have resorted to stealing cats, which we can reckon to be worth $69 each. Equivalent to £43, this is not quite a pedigree beast, but since the devices are quite easily stolen from parked cars (if you know where and how) this is now an increasing phenomenon.
There is a considerable limitation in the rate at which platinum can be recovered in relation to the amount of it we would need to make fuel cells for vehicles powered by hydrogen on any significant scale. I have assumed there are 600 million "cars" on the highways of the world, but this does in fact err on the side of caution. At the end of 2004, the figure was closer to 500 million cars and 200 million trucks etc., (up from around 40 million vehicles altogether in 1945), and 500 million of that total are fitted with cats. It is less demanding in terms of platinum to make a cat than a fuel cell, since the latter use up to 100 g of platinum per unit, e.g. that employed by Daihatsu.
The US based consulting firm TIAX have concluded that world platinum will not run-out, and certainly if the amount of Pt required in fuel cells falls (as is claimed, to perhaps one third of the amount currently used, and there are far more optimistic claims too of about one sixth), there would be enough of it in existing mine-holdings to make those 680 million fuel cells, but it is a rare metal which is only laboriously wrestled from its ore, usually over a period of about 6 months. As noted earlier, 80% of world Pt comes from two mines in SA and most of the rest from another mine in the Urals. Enhancing new Pt output will be very difficult if not impossible in any significant amount.
It is highly unlikely that we will give-up all our jewellery and we need the existing cats to keep nitrogen oxide pollutants (NOx) and other traffic exhaust-emissions within acceptable limits. It is difficult to predict the date of breakthroughs in research and even more so to predict timelines for their commercial development. Notwithstanding, I am looking at a period of about 10 years, by when according to almost all estimates we will be past the point of peak oil production, and oil-supplies worldwide will be down, probably to 90 % of current levels, which is really going to hurt our lifestyle.
In this interim of the "Oil Dearth Era", we cannot expect fuel-cells to help us much, and even if we surrendered half the world's new platinum (100 tonnes) plus another 30 tonnes (which would involve taking 24 million vehicles off the road once their cats had been scrapped) from recycled platinum, we could introduce an optimistic 130 x 106 g/say 60 g/vehicle = 2.17 million fuel cells per year. If we could do this starting now, in a 10 year period, we could have 21.7 million new "fuel cell" cars, but we would have taken 240 million off the road for their cats. This would leave us with 680 - 240 = 440 oil-powered vehicles left (having scrapped their cats for the Pt they contain, and ignoring those that had been stolen) plus 21.7 million hydrogen-powered cars, making 68%, or two thirds of the current number.
Rising fuel prices and shortages of fuel will force that number down significantly, and in 25 years we would be left with 54 million hydrogen vehicles, but if the cats are scrapped for their Pt, that will require the loss of 600 million oil-powered vehicles, or most of the current number, leaving us with just 9% of the current level of car transport power by oil, then powered by hydrogen. These sums are merely illustrative and are open to criticism, but I am simply trying to stress the point that the hydrogen economy, if it could be implemented will provide for less than 10% of current levels of transportation, while the shortages of oil expected over that same 25 years and the inexorably rising monetary and energy costs of its extraction and processing will force the great majority of current vehicles off the roads.
In the immediate future (a period of 10 years, starting now) we can forget about a hydrogen-based transport infrastructure. While making diesel from biomass and from algae by so-called second generation processes offers some hope (and does not compromise food production, unlike first generation biofuels, which ultimately must do), probably only 15% of current transport levels can be so maintained. The notion that we can simply change-over almost overnight to hydrogen or to anything else on a scale that will allow us to preserve our current measure of energy profligacy is simply wrong. Accordingly, society will begin to relocalise into smaller self-sustaining communities - if people can't move around so easily they will stay where they are, and will need to find a means for living at the local level. Deconstructing populous cities will be the most testing effort, and may prove impossible, but the world needs a clear plan of cooperative transformation and not further war and bloodshed over relentlessly depleting resources.
Agricultural Phosphorus Shortage Made Worse by Biofuels?
I read an article a few years ago on the subject of “Peak Phosphorus”10 which was called to mind again by a more recently published article.11 Phosphorus is an essential element in all living things, from plants to you and me, along with nitrogen and potassium - known collectively as, P, N, K, in the form of micronutrients that drive growth. Global demand for phosphate rock is predicted to rise at 2.3% per year, but this is likely to increase in order to produce biomass for biofuel production. If the transition is made to cellulosic ethanol as a fuel, because whole plants are consumed in the process, not merely the seeds etc., yet more phosphorus will be required and less of the plant (the "chaff") will be available to be returned as plant rubble after the harvest, which is a traditional and natural provider of K and P. However, the resource of phosphate rock is in decline, posing a threat to global food production. Similarly to the well-known Hubbert Peak analysis which predicts that individual oil wells or indeed the global production of oil reaches a maximum, beyond which it declines relentlessly, a similar function can be fitted to world phosphate production.10 The method can be adapted in terms of the Hubbert Linearization, which was used recently to predict that only around half the proven world coal reserve (903 Gt) will actually be extractable at some 435 Gt.12 This involves plotting the annual production (P) divided by total production to date (Q), i.e. the ratio, (P/Q), against total (cumulative) production to date (Q), yielding an intercept on the x-axis which corresponds to the ultimate recoverable reserve.
The result indicates that the peak for phosphate production happened in the US in 1988 and for the world in 1989. The really telling aspect of the article is the inclusion of a plot of world oil production versus world population, for which the two quantities can be seen to follow one another closely. The conclusion is that we literally eat oil, since it underpins almost all agriculture, certainly in the developed nations, but also N and P, as required by the Green Revolution, which has preserved us from a Malthusian die-off scenario - so far, at least. Population has only grown as it has because of cheap phosphate deposits and cheap energy to produce the mineral and to get it onto farms around the world. The timing of the production peak for phosphorus has been challenged by another analysis which instead predicts that it will occur in 2034.11 In analogy with the peaks for oil production in the 1970s, it is concluded that the observed peak at the end of the 1980s was not a true maximum production peak, and was instead a consequence of political factors such as the collapse of the Former Soviet Union and a decreased demand for fertilizer from Western Europe.11 In any event, it is clear that the reserve of phosphate rock will at some point fail demand for it and without an alternative source of phosphorus fertilizer humanity will begin to starve, let alone produce biofuels.
In contrast to fossil fuels, say, phosphorus can be recycled, but if phosphorus is wasted, there is no substitute for it. The evidence is that the world is using up its relatively limited supplies of phosphates in concentrated form. In Asia, agriculture has been enabled through returning animal and human manure to the soil, for example in the form of sewage sludge, and it is suggested that by the use of composting toilets, urine diversion, more efficient ways of using fertilizer and more efficient technology, the potential problem of phosphorus depletion might be circumvented. It all seems to add up to the same thing, that we will need to use less and more efficiently, whether that be fossil resources, or food products, including our own human waste. We are all bound on this planet and depend mutually on the various provisions of her. There are now so many of us that we will be unable to maintain current profligacy. In the form of localised communities as the global village will devolve into by the inevitable reduction in transportation, such strategies would seem sensible to food production at the local level. "Small is beautiful" as Schumacher wrote those many years ago, emphasising a system of "economics as if people mattered"13.
Running Low on Gas.
Helium is a remarkable material, with some unique properties, especially in liquid form in which it is used as a coolant, for example to run superconducting magnets inter alia in MRI (magnetic resonance imaging; the safer alternative to x-ray body scanners) applications. It is also used as a blanket-gas to shield sensitive materials from atmospheric oxygen, and enable certain chemical reactions to be performed, and in specialist welding operations in which the weld is stronger when the metal surface has not been exposed to reactive atmospheric gases. Helium finds further application in gas-cooled nuclear reactors, as a heat-transfer agent.
Most of the world's helium is found in the United States, and it is recovered by separating it from natural gas with which it is coincident. Helium arises from the decay of radioactive elements like thorium and uranium, whose atomic nuclei decay to form alpha-particles - helium nuclei - which form elemental helium by capturing a couple of electrons from their surrounding media. The majority of helium - since it is a material of low mass - simply rises into the atmosphere and escapes the Earth's gravitational pull to dissipate into outer-space, but some of it becomes trapped in the rocky formations of gas-wells, from which it may be recovered in concentrations of up to 7%.14
As is the case for all fossil-materials, natural gas was laid-down in long times past and we will eventually use it up, especially against current rising demand for it. It is the same story for oil, ultimately coal, and indeed uranium, so most of our current energy production methods are living on borrowed time. Helium is also a fossil material, but it can be recycled, as I recall from working at the Paul Scherrer Institute (PSI) in Switzerland, which uses huge amounts of liquid helium to cool the vast array of magnets used to steer beams of charged particles, particularly muons, toward particular experimental arrangements. At PSI, the helium is recovered and liquefied on site so it can be recycled, since it is a comparatively expensive substance, and another recollection about it is that it diffuses through the steel walls of cylinders in which it is stored under high pressure. If you get a new helium cylinder and don't use it for say, 6 months, when you attach the pressure valve, about half of it has gone!
While the world would certainly not grind to a complete halt if all its particle physics institutes had to close-down in the absence of helium, modern medicine would be disadvantaged and need to return to using x-rays as a means to "photograph" the inside of human bodies as in the CT-scanner alternatives to MRI. If we run short of natural gas, however, the world won't run on with this fact largely unnoticed, and peak gas looks to hit at around 202515... a mere 10 years time, and more and more of it is used each year, along with all other sources to slake a dust-dry thirst for energy.
(2) Davis, E. (2011) "Critical Thinking.” http://www.rsc.org/chemistryworld/Issues/2011/January/CriticalThinking.asp
(3) Inman, M. (2011) "Going "All The Way" With Renewable Energy?" http://news.nationalgeographic.com/news/energy/2011/01/110117-100-percent-renewable-energy/
(5) Hubbert, M.K. (1956) “Nuclear Energy and the Fossil Fuels.” Presented before the Spring meeting of the Southern District, American Petroleum Institute, Plaza Hotel, San Antonio, Texas, March 7-9.
(6) Bardi, U. and Pagani, M. "Peak Minerals”. http://www.theoildrum.com/node/3086.
(7) Rhodes, C.J. (2010) Sci. Prog. 93, 37.
(8) Cohen, D. (2007) "Earth Audit", New Scientist, 26th May, p. 35.
(13) Schumacher, E.F. (1973) Small Is Beautiful: A Study of Economics as if People Mattered. Vintage, London.
(15) De Sousa, L. (2006) "Natural Gas: how big is the problem?" http://www.theoildrum.com/story/2006/11/27/61031/618