Preparing for a low-energy future

Futures 44 (2012) 883–892

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Preparing for a low-energy future Patrick Moriarty a,*, Damon Honnery b a b

Department of Design, Monash University-Caulfield Campus, 900 Dandenong Rd, Caulfield East, Victoria 3145, Australia Department of Mechanical and Aerospace Engineering, Monash University-Clayton Campus, P.O. Box 31, Victoria 3800, Australia

1. Introduction In a 2009 paper, Beddoe et al. [1] employed the useful terms ‘empty world’ and ‘full world’ to describe the past and present of our planet. As Table 1 shows, by comparison with 2010, even the world in 1950 could be considered an empty world, with global primary energy, and particularly oil and electricity use, only a small fraction of today’s values. By 2010, with global GDP ten times that of 1950, the world could be considered full, in that according to the ‘ecological footprint’ measure, we are using the biocapacity of at least 1.2 Earths. Or, as Randers [2] has argued, we are in unsustainable overshoot, which can only be temporary. The world of 1950 was, as today, one with an inequitable distribution of the world’s wealth. But at least the less developed countries of 1950—largely the same list as today—could hope to join the ranks of wealthy countries, since resources of minerals and energy, and global pollution absorption capacity were not important constraints. Today, all has changed. The industrialising countries as a group cannot achieve even the present per capita GDP or energy levels of the wealthy countries, although this insight has yet to be understood by nearly all political leaders. With further expected growth in the world population [3], high levels of material consumption for all are even less likely. In a full world, roughly equal access to resources is the only ethical, and politically feasible, approach. In thinking about the future, some areas subject to human influence, such as urban infrastructure and buildings, are easier to predict than others. The physical layout of towns changes very slowly, given the long useful life of buildings. Roads, canals and railway infrastructure also have a low rate of technical obsolescence, and are continuously maintained, with lives measured in centuries. Of course, nothing is certain in the future—an earthquake and/or a tsunami can destroy a town in minutes, and the site may be abandoned. At the other extreme, clothing fashions change annually, driven by the need for sales. In principle, planning can make the future more predictable; for example, governments can publish schedules for future allowable vehicle emission levels, or carbon tax levels. Of course, if it is widely believed that these levels will not be enforced (perhaps because opposition parties oppose them), then such planning will not reduce uncertainty. What about annual energy use? Power stations and other energy infrastructure have useful lifetimes measured in decades. Even longer lived is the infrastructure supporting the delivery of fossil fuel energy: pipelines, high voltage distribution networks, ships and road systems. Recently, a group of researchers [8] examined what cumulative CO2 emissions would result if all existing power stations and vehicles in use today continued operation until the end of their economic lives. Their central estimate was 496 Gt of CO2, 15 times the 2010 emissions from all fossil fuel use of 33.16 Gt CO2. (Assuming the 2010 value of 76.5 megatonne (Mt) of CO2 for each EJ (EJ = exajoule = 1018 J) of the fossil fuel mix, this is the equivalent of around 6500 EJ of energy [4].) However, given the risks of both fossil fuel depletion and global climate change, we cannot allow past decisions on fossil fuel use to determine future energy patterns. Hence advocates for rapid change in our energy system argue that past energy use patterns need not be a guide for the future, that we can rapidly change our energy system, given another ‘Manhattan Project’. (Incidentally, from an energy viewpoint, the Manhattan Project is not a good example: the only three bombs available to August 1945—the Trinity test bomb, and the Hiroshima and Nagasaki bombs—had a total yield of about 60,000 tonnes of TNT, the energy equivalent of only

* Corresponding author. Tel.: +61 03 9903 2584; fax: +61 03 9903 2076. E-mail addresses: [email protected] (P. Moriarty), [email protected] (D. Honnery). 0016-3287/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.futures.2012.08.002

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884 Table 1 Global parameters, 1950 and 2010.

Population (billion) GDP (US$ trillion, 1990 PPPa) Atmospheric CO2 (ppm) Primary energy use (EJ) Oil use (EJ) Electricity use (EJ)

1950

2010

2.53 5.3 310 89 20 3.1

6.90 53.1 390 503 169 76.8

Sources: [3–7]. a

Purchase parity pricing.

0.00005% of present global energy use. And even 70 years after first achieving criticality in 1942, nuclear power today accounts for less than 6% of world primary energy.) The result is the growing and partly contradictory calls for a rapid shift from fossil to alternative fuels, deep energy efficiency improvements, carbon capture and storage, and for geoengineering the planet by raising the albedo (the proportion of incoming solar radiation reflected back to space). One general method of planning for the future is backcasting. Backcasting involves choosing a preferred future, then devising new policies to attain this preferred state. The complexities of backcasting, which can be regarded as a normative scenario approach, are extensively discussed in [9]. Clearly, this preferred energy future must be chosen from possible energy futures. Science often advances by specifying what is not possible—the first and second laws of thermodynamics are examples. Unfortunately, this approach is of little help in limiting energy futures—the energy futures we will discuss in Section 2 may be overly optimistic, but do not violate energy laws. Hence the energy futures considered possible are strongly contested; a central aim of this paper is to try to narrow down the range of possibilities. We argue that it is unlikely that anywhere near even present levels of global energy use will be available in 2050. However, as Sardar [10] has observed, ‘the notions of control and certainty are becoming obsolete.’ We can only say that assuming less energy will be available in 2050 is a better bet than business-as-usual assumptions of continued energy growth. In Section 2 we demonstrate the close link in recent decades between global primary energy consumption and global GDP. We then present a number of official energy forecasts, showing that their common assumption of continued global GDP growth leads to estimates of roughly 1000 EJ global energy use in 2050. In Section 3 we first show why overall fossil fuel use is likely to peak soon and then fall. We then argue that the only alternatives to fossil fuels, nuclear and renewable energy (RE), cannot form the basis for continued economic growth. Instead, in Section 4, we propose that the world will need to make do with far lower levels of energy than we use today, and give an outline of the way western societies must change to accommodate lower levels of per capita energy use and income. As Sorman and Giampietro [11] argue, moving to lower energy levels will be very difficult for present high energy societies. Further, by promoting economic growth as a key aim, policy makers are still moving us in the wrong direction, thus compounding the difficulty. 2. Recent global energy forecasts Fig. 1 demonstrates the tight relationship between global gross domestic product (GDP) and annual global primary energy consumption. Global GDP is given in 1990 $US and is in purchase parity pricing (PPP) values. Primary energy uses the

Fig. 1. Global GDP vs annual global commercial energy consumption, 1980–2010. Sources: [4–7].

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BP global primary energy data series, and so is for commercial energy only (i.e. excluding firewood in industrialising economies). The graph only shows values since 1980, just after the second oil crisis. Before then, a steeper curve fitted the data better [5]. The graph indicates that the energy intensity of the global economy (the slope of the line), has been constant over the years 1980–2010. Consistent with this close relationship, the fall in global GDP in 2009 was matched by a fall in global energy use. When similar graphs are drawn for national OECD economies, an improvement in energy intensity in recent decades is often found (the curves flatten over time). Much of this improvement occurs because of structural change in their economies, in particular away from energy intensive manufacturing, and towards services. Plotting the global relationship removes this bias, particularly that resulting from the higher energy intensity of China, with its high level of manufacturing exports to OECD countries [5]. Given this close connection between energy consumption and GDP, global energy use is usually anticipated to grow in future. A number of international organisations, as well as the US Energy Information Administration (EIA), have projected global energy use for various years in the future, Table 1. For comparison, in 2009, according to the International Energy Agency (IEA) [12], of the 509 EJ primary energy used globally, fossil fuels accounted for 80.9%, nuclear 5.8% and renewable energy (including fuel wood) the remaining 13.3%. Developing energy forecasts is, in principle, simple: assume an average rate of economic growth, along with a rate of energy intensity improvement, and a future energy use results. But as So¨derholm et al. [13] have recently warned: ‘Quantitative analysis lends coherence to scenario exercises by elaborating the possible consequences of future events and policies, but they may often result in futures that too narrowly resemble current patterns of behaviour.’ These forecasts are really demand projections; it seems to be implicitly assumed that supply will continue to be able to match demand, at a cost not too different from today’s. If all the expected 9.31 billion human population in 2050 [3] used primary energy at the present per capita rate, energy consumption by 2050 would be almost 700 EJ. Given the relatively static nature of OECD per capita energy consumption, differences in the values listed arise mainly from assumptions about growth in energy demand from non-OECD economies. Further, if the per capita rate was at the present US value (which is less than that of some OPEC countries), global energy consumption would be nearly 2900 EJ, about three times higher than the levels shown in Table 1. Accordingly, the estimates in Table 1 appear quite modest. Our argument, developed in the following two sections, does not rest on assumptions about future demand; but rather on the feasibility of annually producing around 1000 EJ of primary energy by 2050, given the environmental constraints. 3. Constraints on future energy production Future energy production can only be derived from three sources: fossil fuels, nuclear energy (including, possibly, fusion energy), and the various types of RE. If fossil fuels lose their dominance of the past 1–2 centuries, the alternatives must either fully replace fossil fuels, or energy use will have to contract. In the following discussion we first examine the future constraints on fossil fuels production, then explain why neither nuclear power nor renewables can replace the one-off fossil fuels bonanza. 3.1. Fossil fuels Before 1800, the world’s energy was derived almost entirely from biomass, with small contributions from water and wind energy. Animals also contributed, mostly to agriculture and transport, but they too were powered by biomass. By about 1850, coal supplied half global energy needs, and today fossil fuels combined supply over 80% of the total [4,5]. It is not difficult to find reasons for this changeover. Fossil fuels are superior to RE sources in the following important ways:  Annual production, and so use, can be continuously expanded, provided there is sufficient investment in the relevant energy infrastructure and reserves are adequate.  Extractive and use costs can be low if externalities (such as those resulting from CO2 emissions) can be mostly ignored.  They are continuously available (not intermittent like wind or solar energy) and can be readily stored.  The energy density (the energy in joules per kg of fuel) is high, and so fossil fuels can be economically transported long distances. Today, over 65% of oil used worldwide crosses international borders, imported mainly from the OPEC countries [4], with natural gas and coal imported to a lesser extent. Until now, global fossil fuel output has been demand-driven, except for some production constraints on oil exports imposed for political reasons in the 1970s. The second half of the ‘Age of Oil’ will occur when production permanently constrains oil use; later, natural gas and even coal will be similarly constrained. Although it is widely agreed that conventional oil production has already peaked [19], the world has large resources of non-conventional oil and gas. These have much higher energy, environmental and monetary costs for extraction compared with conventional resources [5,20]. The notion of Peak Oil is contested, although most researchers believe it will occur sometime in the coming decades. (Nevertheless, on a per capita basis, peak oil occurred in 1978, and has fallen 20% since then [4]. Even the upbeat EIA report [12] does not expect any rise from the 2008 value.) There is even less consensus for the notion of a peak for fossil fuel

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Fig. 2. Historical production and two projected fossil fuel production profiles (EJ), 1950–2050. Squares from [21,22], circles from [12].

production in general. Fig. 2 combines the production forecasts for oil and gas from the Association for the Study of Peak Oil and Gas (ASPO) [21] with that for coal from Patzek and Croft [22]. (The coal depletion study by the Energy Watch Group [23] gives similar coal production declines.) In addition to the composite curve from these peak theorists, Fig. 2 also shows expected production according to the US EIA reference forecast [12]. The contrast in future fossil fuel production is evident, with the EIA (and other official forecasts) assuming that consumption will not be constrained for decades to come. Clearly, both views cannot be correct, but a useful concept for attempting to resolve the issue is the energy ratio. The energy ratio is the gross energy output from an energy source as a multiple of the input energy to explore and develop the field or mine, extract the energy, and transport it to the point of use. Murphy and Hall [24] have documented the global decline in the energy ratio for oil. If this energy ratio falls below one, then more energy is needed to extract the energy than the gross output, and the net energy output falls below zero. (Actually, the energy ratio may need to be much higher, perhaps as much as five, for the energy source to be economically viable, if only because of the uncertainty in the energy ratio calculations.) At present, all fossil fuels have energy ratios of 10 or more, so gross energy output (the values shown in Figs. 1 and 2) is not much different from net output. It is net energy which powers the non-energy sectors of the global economy— without net energy, economic activity would soon cease. The energy ratio for oil will fall as extraction from non-conventional sources rises, reducing oil available to the non-energy sector [5]. The point at which extraction becomes unviable for each fossil fuel is yet unknown, principally because of energy inputs uncertainty. Some researchers think that the question of fossil fuel reserves is irrelevant, because of the threat of global climate change. We would not fully deplete fossil fuels, they argue, because they are too polluting to use. The more popular (and official) view is that one or more tech fixes to the CO2 emissions problem will enable continued fossil fuel use. These fixes include carbon capture and storage (CCS), and a variant, air capture of CO2, and geoengineering. Another possibility is biological removal of CO2 by reforestation. However, given that net deforestation is still occurring, and that the stillexpanding human family could require an increase in agricultural land [25], merely retaining the present levels of carbon in our soils and forests will be difficult, and probably all that can be expected [5]. CCS involves capturing the CO2 from large fossil fuel plants such as power stations and oil refineries, compressing the CO2, then transporting it by pipeline to a sequestration site, where the plan is to bury the CO2 deep underground in suitable locations such as saline aquifers, or in disused oil or gas fields. Although CCS has been discussed for two decades, only a few million tonnes are now sequested annually, compared with total CO2-equivalent emissions from all sources in 2010 of around 58 billion tonnes [4,5]. The energy penalty for CO2 capture alone is high—estimates vary from a low 10% for plants optimised for carbon capture to 40% or more for non-optimised plants (which includes virtually all existing power stations). Even if all large fossil fuel plants implemented CCS, only about 20–25% of CO2-equivalent emissions could be captured [5], either because most fossil fuel emissions are not suitable for capture (e.g. those from vehicles) or because of non-CO2 emissions and CO2 emissions from land use change, especially net deforestation. Given this limitation, air capture (removal of CO2 directly from the air) has been advocated [26]. Since CO2 is a well-mixed gas, air capture could be done anywhere, with zero transport costs if done at the sites for underground sequestration. Unlike CCS, air capture would not be limited to current emissions; in principle past emissions could also be removed. However, the energy costs for carbon capture are enormous, because although CO2 might form around 10% of the exhaust gases from a power station, it comprises only 0.04% of the general atmosphere [27]. A recent cost estimate for air capture was $1000 per tonne of CO2 [28]. Even if CCS or air capture were implemented on a large scale—which, judging by progress to date, seems unlikely [27,29]—they would greatly

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reduce the net energy available to the economy from a given level of gross energy use, and so would hasten the depletion of fossil fuels. Recently, the Royal society in the UK and others have advocated geoengineering as a way of counteracting global climate change. The most discussed method would mimic the natural cooling effect of large volcanic eruptions such as Mt Pinatubo in 1991 by injecting millions of tonnes of sulphate aerosols into the lower stratosphere. The aerosols increase the planetary albedo, thus cooling the planet. The artificial loading of aerosols can be readily adjusted according to the level of global warming needed to be counteracted [5]. Because the aerosols would be gradually removed from the atmosphere by gravity and rain-out, they would need to be continuously replaced. If in the future their replacement was stopped, global temperatures would quickly rise to the level corresponding to the ‘climate forcing’ from the levels of greenhouse gases at that time. The effect of rapid temperature rises on ecosystems could be disastrous. Geoengineering would also tend to lower global precipitation levels, a serious problem in a water-short world. Further, since CO2 emissions would continue unabated, acidification of the oceans would continue, again with potentially adverse effects on ocean ecosystems [5]. Given these serious problems, together with the difficulties of achieving international political consensus when there will be winners and losers, it would seem geoengineering faces insuperable difficulties. Finally, given the long atmospheric residence time of CO2 (much of that emitted will remain there for thousands of years), it is cumulative, rather than annual, emissions that matter [5]. Declining levels of annual emissions from fossil fuel combustion will also add to cumulative emissions, which some scientists regard as already too high [30,31]. Even the ASPOPatzek curve in Fig. 2 implies the combustion of about 14,000 EJ between 2010 and 2050. At 76.5 megatonne of CO2 for each EJ of the 2010 fossil fuel mix [4], 1071 Gt of CO2 would be cumulatively emitted, enough to raise atmospheric concentrations by about 73 ppm [27]. We have, then, several reasons why fossil fuel use can be expected to decline, rather than rise, in the coming decades. The peak theorists believe that a combination of geological, economic and energy return constraints will limit annual production. Cumulative CO2 emissions are a further constraint. Even if, which seems unlikely, geoengineering was implemented and achieved climate stabilisation, progressive ocean acidification could still make this approach too risky. Since CCS would only lead to partial reduction in emissions, progressive ocean acidification would occur with this approach as well. In brief, past use of fossil fuels is important in a negative sense. Because the planet has both a finite endowment of fossil fuels and a finite pollution absorption capacity, cumulative past fossil fuel production will act to limit future use. 3.2. Alternative energy sources: nuclear energy Even before the earthquake and subsequent tsunami that struck Japan on 11 March 2011 and severely damaged several of the reactors at the Fukushima nuclear plant, global nuclear power production was stagnant. Its share of global electricity production reached 17.5% as early as 1993, but in 2010 was only around 13% [4]. Between 2002 and 2011, 28 new reactors were brought on line, but 42 were decommissioned [32]. Given that the reactor fleet is ageing, with many reactors nearing the end of their design life, the long lead times and rising costs for reactor construction, and the decline in public acceptance of nuclear power, especially in Germany and Japan, future nuclear power plant construction will be hard-pressed even to retain present market share for nuclear electricity. Nevertheless, forecasts for nuclear power show wide divergence. The 2011 EIA forecasts do not expect nuclear’s electricity share to rise much, with the baseline forecast predicting only 14% by 2035 [12]. The International Atomic Energy Agency (IAEA) [15] is also cautious, and even before Fukushima, projected nuclear power to modestly increase its share in their high nuclear growth case, and to lose share in their low growth case. On the other hand, the projections from the World Nuclear Association (WNA) [33], the trade organisation for nuclear power, are far more optimistic, with electric output in 2035 18–70 EJ, compared with less than 10 EJ today, and with 2100 output as much as 30 times higher than today. However, the global forecasts for the other organisations listed in Table 2 are similar to the far lower forecasts of the IEA and the IAEA. Given the probable low output of nuclear power in coming decades, uranium reserves are not likely to be a limiting factor. On the other hand, if nuclear electric power had to supply 500 EJ annually (roughly the world’s present primary energy use), proven and inferred reserves would only last for 5.6 years [5]. Only if breeder reactors, which in principle can use a far higher share of the natural uranium for energy, were to replace present-day thermal reactors, would this limit be overcome. But breeder reactors are inherently more difficult to operate safely than conventional reactors, and would necessarily require

Table 2 Global primary energy projections, 2020–2100, in EJ. Organisation and year

2020

2030

2050

2100

EC (2006) [14] EIA (2011) [12] IAEA (2009) [15] IEA (2011) [16] IIASA (2007) [17] WEC (2008) [18]

570–610 659–659 585–650 NA 555–630 615–675

650–705 730–816 670–815 605–705 NA 700–845

820–935 NA NA NA 800–1175 845–1150

NA NA NA NA 985–1740 NA

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spent fuel reprocessing. Experience shows that reprocessing is difficult and expensive, and also greatly enhances the potential for plutonium diversion [5,34]. 3.3. Alternative energy sources: renewable energy Great hopes are placed on RE to save us from the problems of both fossil fuel depletion and their emissions. The recently released IPCC Special Report on RE is particularly optimistic about its potential [35]. Sadorsky [36], in his scenario most favourable for RE, considers that RE could provide as much as 80% of energy, but only by 2100, and only if a serious commitment is made by all parties. As we have seen, before 1800 the world largely ran on RE (including the food consumed by humans and fodder by farm animals). In future, the world will need to return to RE as the source for most of our energy. But what levels of energy consumption can the various RE sources support, and in what time frame? Many researchers think the transition to RE will be as painless as earlier energy transitions from wood to coal, and later to natural gas and oil. Fig. 3a shows how RE and nuclear shares in global electrical output has varied since 1970. Although RE output has more than quadrupled [4], its share fell steadily until the early 2000s, but has risen slightly since. Both nuclear electricity’s share

Fig. 3. Global RE and nuclear electric output (a) nuclear, total RE and all non-fossil output share of total electricity (%) 1970–2030; (b) wind, biomass, geothermal and solar electricity output (TWh) 1970–2010. Sources: [4,5,40,41].

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and even output are now falling. The BP forecasts [41] expect both RE and nuclear to modestly increase their market share by 2030 (Fig. 3a). Although hydropower still dominates RE electricity [4], its global potential is only around 25–30 EJ. Future additions cannot be large, even if its manifest environmental problems can be ignored [38]. Geothermal electric potential is also limited to a few EJ, although there is much larger potential for low-temperature geothermal heat [5]. (In addition to RE and nuclear electricity, traditional fuel wood use is probably about 45 EJ, liquid fuels from biomass about 5 EJ, and direct use of geothermal heat 0.42 EJ [4,16,37].) The fastest growing RE sources are wind and solar power (Fig. 3b). These sources also have by far the highest potential of all RE [39]. But they are intermittent energy sources, making it difficult to accommodate large quantities in present electricity grids, which must match supply to instantaneous power demands. Since the other, non-intermittent, sources have limited potential, any high-energy future (such as those shown in Table 1) could need 500 EJ or more from these two sources by the year 2050. Is this possible? The Global Wind Energy Council (GWEC) is an advocacy body for wind energy, so their estimates can be expected to be optimistic. Their 2010 report [42] presented two scenarios for wind power in 2030, ‘Moderate’ and ‘Advanced’. The Moderate one sees global installed capacity rising to 8.9 times the 2010 value of 200 GW (GW = gigawatt = one billion watt), the Advanced scenario, to 11.7 times. In contrast, the EIA base case has only a 2.2-fold rise by 2030 [12]. Even the GWEC Advanced scenario would give only about 18 EJ wind electric output in 2030, a minor contribution to the 605–845 primary EJ projected for all energy in Table 1. Until 2008, global installed wind GW grew exponentially, but since then has been only growing linearly [4]. Reasons include resident opposition, increased subsidy costs in turbulent economic times and rising difficulty in finding acceptable high-wind locations for wind farms [43]. Importantly, the decline in annual growth in leading countries such as Spain and Germany occurred when wind penetration of the electricity grid was only a fraction of the 20% figure often given as the maximum possible before significant dumping of wind energy is needed (because at times output from this intermittent source would then exceed instantaneous electricity demand.) Beyond this penetration level in any given grid, conversion of wind energy (possibly to hydrogen) and storage will be increasingly needed [44], except in those few fortunate countries/ grids with high levels of hydroelectric power penetration. Solar energy is much more expensive than other RE electricity, but at its present very low levels of output (Fig. 3b), the high costs and subsidies can be tolerated. Present growth of solar output is extremely high (in 2010, manufacture of PV cells was 72.6% greater than in 2009 [4]), but like wind, growth will likely slow to linear when output rises to around one per cent of the electricity market [45]. Governments can attempt to strengthen growth through increased subsidies, but such an approach is ultimately self-defeating if costs remain high relative to other power generation technologies. Unlike wind energy, the global potential for solar energy seems vast, in theory enough to meet any level of global energy demand. The real problem is that the energy ratios for RE sources, and particularly solar energy, are very low compared with those for fossil fuels today [46]. Accordingly, the net energy available to the rest of the economy as a fraction of gross energy will be much lower than for fossil fuels. As the only two RE sources with major potential, wind and solar energy will have to supply nearly all our future energy needs, not just electricity. Thus, conversion of the intermittent electricity produced to an energy carrier such as hydrogen, together with energy storage (and, possibly, later reconversion to electricity), will be increasingly needed. Such conversion and storage could reduce the energy ratio by as much as a factor of two, drastically reducing the net energy obtained from each unit of gross output [44]. The energy return for all energy sources will also be affected by rising energy costs of input materials, as stressed by Bardi [47], who estimated that the mining industry may already be using 10% of global primary energy. This share can only rise as ore grades fall. For wind and solar energy particularly, such increased input energy costs could affect viability as an energy source in many locations. Another factor that could further reduce the energy ratio in the desert areas favoured for their available land and clear skies is the energy costs of supplying fresh water for washing PV cells or solar mirrors, and perhaps also for electrolysis of water for conversion of intermittent electricity to hydrogen [5,48]. There is a further reason why very rapid sustained increases in wind and solar energy are unlikely. Given their generally low energy ratios compared with fossil fuels [46], their input energy costs can be a substantial fraction of gross output energy, a fraction which will be greatly increased if conversion of intermittent electricity and energy storage is needed. RE sources also differ from fossil fuels in that the total input energy costs for RE must largely be expended before any output is produced at all. For example, most of the input energy costs for wind power are for manufacture of the turbines and tower and their erection, and for any electricity transmission infrastructure. Operation, maintenance and eventual dismantling of turbines have only minor energy costs. In an earlier paper [49], we showed using dynamic energy analysis that if capacity growth in an RE source such as wind is too high, net energy may be negative if rates of capacity-building are high. While later years will show an energy surplus, wind energy will represent an energy drain during this period. A ‘crash programme for RE’ therefore gives rise to a boom-and-bust cycle in energy output unless it is balanced by energy produced from non-RE sources—which will be themselves progressively in decline. If left too late, capacity building in RE could well be limited by the energy available once other demands are met. In brief, the past use of RE is important for future use, but in a very different way from fossil fuels. Unlike fossil fuels, RE resources are not used up; solar energy, for example, is continuously renewed during daylight hours. We do not have to worry about RE depletion, except for geothermal energy [5]. But as we have argued for the high-potential wind and solar resources, their lower energy ratio and high upfront energy costs act to constrain the rate of growth, regardless of their eventual installed capacity. The energy ratio will further decline with the shift to progressively lower-quality RE resources

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(e.g. lower average wind speeds) [39], the increasing need for storage/conversion, and declining ore grades for input materials. 4. Adjusting to a low-energy world Given that we will most likely face a low-energy future, how can we, particularly those of us who live in presently highenergy use countries, adapt our communities and lives to a per capita energy use a fraction of today’s? One answer is by increasing the efficiency of energy use. Some energy researchers, notably Lovins [50], think we can drastically cut our energy consumption, with no adverse effects on economic growth, by careful design of energy-using devices and systems. Similarly, Cullen et al. [51] have argued that ‘practically achievable design changes’ could greatly reduce global energy use. But can energy efficiency improvements be translated into overall deep absolute reductions in global energy use, which we argue is needed? A study of the recent past suggests this is unlikely; for example aircraft seat-km per litre of fuel has risen 70% since the 1950s [5], but total aviation fuel used has also risen many-fold [4], the result of greatly increased volumes of air travel. One reason why actual energy efficiency improvements do not always translate into energy reductions is because of energy rebound. An improvement in energy efficiency of a device lowers its cost of operation, with the result that it is used more often, or more of the devices are purchased. (Alternatively, if demand is inelastic, the money saved may be spent on other energy-consuming goods and services.) A study of global energy use shows that our energy efficiency gains have not reduced the growth in global energy use [4]. Since many energy efficiency measures are available at low or even negative cost (i.e. money can be saved) [50,51], why have they not been more widely adopted? One possible reason: efficiency can conflict with other desirable aims. For instance, stand-by power needs of domestic and office equipment lower efficiency, but add to user convenience [5]. Energy efficiency can also conflict with other efficiency measures. For transport, energy efficiency can conflict with time efficiency (i.e. travel speed); faster modes such as air travel are less energy efficient, as formalised in the Karman– Gabrielli diagram [52]. Similarly, industrial agriculture is much less energy efficient than traditional agriculture [53], but now dominates agricultural production because it is both more land-use efficient (agricultural output per hectare is far larger), and enables higher productivity per agricultural worker. For large efficiency gains, we will evidently need to forego some advances in convenience of use and even desirable properties, and also stress energy efficiency above other efficiency measures. What about technology breakthroughs, in either energy efficiency, or in new or improved sources of non-fossil energy? Increasingly, scientific advances in energy-related R&D are becoming more difficult to translate into viable technologies. Compared with 1950, implementation today is subject to a rising number of constraints. For example, the very high RE output needed leads to rising environmental opposition and siting constraints, partly because of rising population. Wind and solar energy conversion devices also use some exotic materials which are in scarce supply [5]. Fresh water availability could constrain not only wind and solar energy, as shown above, but also geothermal and bioenergy. All these limitations mean that energy ratios, and so monetary costs, will be adversely affected. Many technological breakthroughs are forecast over the next decade or so, as in the detailed Japanese Delphi studies [54]. However, retrospective surveys have shown Delphi forecasts to be over-optimistic, possibly because of the constraints on implementation just listed [5]. Similar considerations apply to other forecasts on the implementation of new energy-related technology. Instead, if the world is to avoid ecological collapse and political instability, it will be necessary to move to a human needs approach [27]. The UN Millennium Development Goals [55] include the following aims:       

eradicate extreme poverty and hunger; achieve universal primary education; promote gender equality and empower women; reduce child mortality; improve maternal health; combat HIV/AIDS, malaria and other diseases; ensure environmental sustainability.

This list not only illustrates the nature of basic human needs, but also indicates some official support for this approach. The driving idea is to meet the demands for food, shelter, sociality, access to educational, health and cultural services for all humans, in the context of the need for progressively lower levels of global greenhouse gas emissions and fossil fuel use [5]. Presently, the emphasis on technical fixes has more to do with meeting corporate needs for protecting revenue rather than meeting human needs. Sorman and Giampietro [10] caution against trying to plan for energy de-growth, arguing that the implications of lower energy per capita are unpredictable. Accordingly, only what seem to us essential features of a low energy future are sketched here. A low-energy future will require major reductions in the use of energy-using equipment—vehicles of all types, household appliances, air-conditioning equipment, lighting and so on. With less use of these devices, less power station output will be needed. For buildings, we will need far more use of passive solar energy for temperature control and for lighting. Households will probably need to provide at least some of their energy from rooftop solar systems, and food (fruit and vegetables) from their gardens. In drier areas, households will also need to catch and store rainwater in tanks.

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In previous research we have described what changes can be expected for urban settlements and for transport systems [5,56–58]. In summary, the focus for transport must shift from mobility provision in the form of more vehicular travel to provision of needed access. Not only will public transport have to take over a far greater share of the vehicular travel task, but total vehicular passenger-km will also have to be cut. This requires activities, including production, to be much more localised than is the case today. (Rising freight costs from oil price increases should also favour local production.) Localisation in turn raises the potential for non-motorised modes to be a major form of travel, particularly in urban areas. Overall, we can learn much from how our earlier generations coped with the much lower per capita energy levels of their day, despite inefficient use of coal for power plants, domestic heating and locomotive power. In 1900, per capita energy use was 133 GJ/capita in the US, compared with 304 GJ/capita in 2010, down from 330 GJ in 2005 [4,59]. One aspect of the recent past that we could emulate is the more sparing use of power equipment for jobs that would not need great physical effort if instead performed manually. More physical effort at work and for personal transport could substitute for much of the physical exercise so popular in OECD countries, and even help counteract their rising obesity levels. We can also learn from the experience of other countries. The Human Development Index (HDI) is a welfare indicator devised by the UN Development Program as a better measure of satisfaction of human needs than per capita GDP. Steinberger and Roberts [60] studied the variation of HDI with per capita primary energy use for the world’s nations. They found that ‘high human development can be achieved at moderate energy and carbon levels; increasing energy and carbon past this level does not necessarily contribute to higher living standards.’ Countries with an HDI of 0.9 or more (1.0 is the maximum possible) showed a nearly 5-fold variation in per capita primary energy use. If global energy use must be cut to accommodate the challenges of fossil fuel depletion, global climate change and ocean acidification, energy use per capita in different countries will be much more equal than the 100-fold variation found today [5]. Low energy use countries will demand rough emissions parity with presently high energy use countries. It is difficult to see how a many-fold reduction in energy use in high consumption countries could be accommodated in market-driven economies. 5. Conclusions In the historically short period of a half-century, we have moved from a relatively empty world to a full world, resulting in us pushing against limits and nearing various tipping points in the Earth system. As evidenced by official projections of both global GDP and primary energy use, policy makers are slow to realise this point. For example, the present 390 ppm atmospheric CO2 concentration may need to be reduced to 350 ppm to avoid eventual irreversible loss of the Greenland ice sheet. When used at the scale necessary to meet Table 2 levels, no energy source can be perfectly green. Although fossil fuels pose the greatest threat to our environment, all sources produce at least some climate change effects and these effects scale with use. Future production of fossil fuels will be increasingly constrained by both the need to drastically limit GHG emissions and by resource depletion. Failure to constrain fossil fuel use could also reduce RE potential through continued climate change. All energy sources are subject to diminishing energy returns to energy inputs, because of declining resource quality as cumulative production rises for fossil and nuclear energy, or annual output for RE. Although each source responds differently, the result is the decline in net energy as a share of gross energy—and only net energy can power the non-energy sectors of the economy. This is especially problematic for RE, since most energy inputs occur during equipment production and installation. Where we once relied on our understanding of past demand to predict future energy needs, we must now base our forecasts on what can be supplied to our increasingly full world. To balance consumption with future supply, energy use will have to decline several-fold, with greater cuts in presently high energy use countries. Our focus will need to shift from economic growth to meeting human needs. 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