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Defining absolute environmental limits for the builtenvironmentRobert Lowe aa Bartlett School of Graduate Studies , University College London , Gower Street, London,WC1E 6BT, UK E-mail:Published online: 03 Feb 2007.

To cite this article: Robert Lowe (2006) Defining absolute environmental limits for the built environment, Building Research& Information, 34:4, 405-415, DOI: 10.1080/09613210600772482

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De¢ning absolute environmental limitsfor the built environment

Robert Lowe

Bartlett School of Graduate Studies,University College London,Gower Street, LondonWC1E 6BT,UKE-mail: robert.lowe@ucl.ac.uk

The question addressed is whether it is possible to define working limits on environmental impacts from the built

environment in terms of global carrying capacity. The main focus is on energy-related impacts, since these are global and

relatively well-understood. Four possible approaches to defining limits are explored: static equilibrium, asymptotic,

integral of excess and planned future. The conclusions that emerge from this exploration are that global environmental

constraints are very tight, but also that they are dynamically and strongly influenced by the trajectory of social and

technological development over the coming century. Their use as the basis for practical, quantitative metrics of

sustainability, therefore, involves a large measure of subjectivity. A fifth approach – the developmental approach – is

identified, which instead of focusing on long-term external constraints to human activity, focuses instead on the internal,

short- to medium-term dynamics of the built environment itself. It appears likely that the developmental approach,

guided by qualitative conclusions from the analysis of global carrying capacity, is likely to be most fruitful.

Keywords: building performance, building sustainability assessment, environmental assessment, environmental

performance targets

La question traitee ici est de savoir s’il est possible de definir des limites operationnelles a l’impact du milieu bati sur

l’environnement, en terme de capacite porteuse globale. L’auteur insiste sur les consequences liees a l’energie qui sont

de nature globale et qui sont relativement bien comprises. Il explore quatre methodes possibles visant a definir ces

limites: equilibre statique, asymptotique, integrale de l’exces et avenir planifie. Il ressort de cette etude que les

contraintes environnementales globales sont tres pesantes mais aussi qu’elles sont fortement influencees par la

dynamique du developpement social et technologique pour le siecle a venir. Leur utilisation en tant que base d’une

metrique pratique et quantitative de la durabilite implique donc une large mesure de subjectivite. Une cinquieme

methode, dite developpementale, est definie qui, au lieu de se concentrer sur les contraintes exterieures a long terme

qui pesent sur l’activite humaine, s’interesse a la dynamique interne du milieu bati proprement dit, a court et moyen

terme. Il est probable que la methode developpementale, guidee par les conclusions qualitatives de l’analyse de la

capacite porteuse globale, sera vraisemblablement la plus fructueuse.

Mots cles: performances des batiments, evaluation de la durabilite des batiments, evaluation environnementale, objectifs

des performances environnementales

IntroductionThe nature, purpose, impact and limitations ofenvironmental performance targets for buildings arereconsidered. The main reasons for engaging in thisdiscussion, for revisiting questions that have beenaddressed on numerous previous occasions, is a perva-sive concern that environmental performance targets in

use at the beginning of the 21st century are more arbi-trary, more subjective, than one would like.

Efforts to construct environmental targets as part of thedevelopment of a range of environmental performanceassessment tools began more than 15 years ago (Build-ing Research Establishment (BRE), 1990; Cole, 2004)

BUILDING RESEARCH & INFORMATION (2006) 34(4), 405–415

Building Research & Information ISSN 0961-3218 print ⁄ISSN 1466-4321 online # 2006 Taylor & Francishttp: ⁄ ⁄www.tandf.co.uk ⁄journals

DOI: 10.1080/09613210600772482

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and have continued with the development of theEnvironmental Preference Method (Aninck andBoonstra, 1996), the Green Building Tool (Cole andLarsson, 1999), Leadership in Energy and Environ-mental Design (LEED) (US Green Building Council(USGBC), 2003) and others. These broad environ-mental performance assessment tools establishedmetrics in a range of environmental impactcategories; the categories used by BREEAM ’98 wereas follows:

. operational energy and carbon dioxide (CO2)

. transport

. pollution of air and water other than by CO2

. materials

. water

. ecology and land use

. health and well-being

The Ecopoint scale that underpinned BREEAM ’98was established through a series of focus groups withindustry representatives and academics in the UKundertaken in 1997 and 1998. The scale was set sothat 100 000 ecopoints would be roughly the annualenvironmental impact of a UK citizen. The functionof the focus groups was to establish a broad consensuson the weighting of different environmental impact cat-egories (Howard, 1998). The Ecopoint systemis, however, not normative in the sense that no clearview is taken of what it is necessary to achieve inglobal terms, as opposed to what is currently typical.The establishment of these performance metricstherefore leaves open the question of whether it ispossible to define absolute standards of performance.

The distinction between relative and absolute environ-mental standards is one that was explored by a numberof authors during and following GBC ’98. Cooper(1999, p. 323) observes that unless methods for asses-sing the built environment are capable of measuringperformance against carrying capacity criteria, ‘theirability to contribute to the sustainability debate islikely to remain limited’. Kohler (1999, p. 310) statesthat environmental performance assessment methodsbased on relative performance hid ‘the real mass andenergy flows which determine the effective environ-mental impact’ and obscured the ‘differences inimpact between individuals and different countries’.

Cole (1999, p. 231) writes:

Broadly speaking, three distinct roles for build-ing environmental assessment methods were

evident during the development and testing ofGBTool:

† Providing a common andverifiable set of cri-teria and targets so that building ownersstriving for higher environmental standardswill have a means of demonstrating thateffort, i.e., a mechanism to influencemarket receptivity and demand for higherenvironmental performance standards.

† Providing the basis for making informeddesign decisions, i.e., a design tool that canprovide direction and guidance at all stagesduring the design development by highlight-ing priority issues and suggesting the poss-ible trade-offs between options.

† Providing anobjective assessment of a build-ing’s impact on the environment, i.e., a toolto evaluate energy and mass flows betweenbuilt and natural systems and provide acommon yardstick for measuring progresstowards sustainability.

He continues:

This requires making a distinction between thethree roles identified above and making the dis-tinction explicit in the structuring of the asses-sment method. The paper emphasizes this pointby differentiating between what can be termed‘green’ assessment methods that offer relativeassessments and ‘sustainable’ assessments thatoffer absolute ones.

In an unpublished note written at GBC ’98 and referredto by Cole, the present author argued that:

Physical indicators of sustainability can be nor-malised . . . using the word ‘normalised’ to mean:

divide one physical quantity by another withthe same units so that when the resulting ratiois one, something significant is happening.

Wherever possible, each physical indicator of sus-tainability should be normalised (in the abovesense) by some measure of the total sustainablelevel of activity described by that indicator. Thehuman principle of equity intrudes here, in thatdenominators should represent equitable sharesof the total sustainable level.

This will give rise to a series of physically based,carrying capacity normalised indicators that willbe defined on the following scale:

0 no impact

,1 you have some headroom

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1 you are using your sustainable fair share

.1 you are using more than your sustainablefair share

�10 try harder . . .

There will be physical indicators where we find itdifficult to define the equitable share of a sustain-able level of activity. Furthermore sustainablelevels are not necessarily constant. Uncertaintyabout normalisation does not make it pointless totry. Honesty, humility and transparency are theonly sustainable attitudes in the face of uncertainty.

I have elsewhere suggested using total sinkcapacity to normalise carbon emissions.Although we have not identified and quantifiedall carbon sinks, we can measure total sinkcapacity directly and quite accurately. We alsoknow how many people there are in the world.It is therefore comparatively easy to construct anormalised indicator of carbon emissions.

The objective of the present paper is to revisit thisrobust, but potentially simplistic, position to seewhether it is indeed supportable. Is it possible toplace environmental targets on an absolute, objectivebasis? Is it possible, to use Cole’s terminology, todevelop viable and uncontentious, absolute indices ofsustainable building practice?

Possible approaches to de¢ningabsolute limitsIn this core section of the paper, a series of possibletheoretical approaches to defining absolute limits isdiscussed. Where numerical illustrations are needed,these are framed in the context of global limits toland and sea areas, and energy and climate change-related limits. Some of these approaches are clearlysimplistic, but their inclusion in this discussionthrows useful light on important issues.

Static equilibrium approachThe static equilibrium approach to limits divides thecurrent size of any given resource by the currenthuman population of the earth. This will be illustratedbelow by calculations of land area and CO2 emissions.Of these, the simplest are the calculations of land areaper person, which underpin Wackernagel and Rees’(1996) work on ecological footprints:

Fair earth share per person

¼ surface area of the earth=population

¼ 5� 1014 m2=6:6� 109

� 8 ha per person

Fair land share per person

� 5� 1014 m2=6:6� 109 � 0:25

� 2 ha per person

Fair ecologically productive land share per person

� 0:66 fair land share

� 1:3 ha per person

In presenting such calculations, the author is mindful ofthe criticisms that have been levelled at the ecologicalfootprint approach as a policy tool (e.g. Pearce, 2005).These criticisms are, however, not relevant to thepurpose of this section of the paper. The approach todefining limits illustrated in the above calculationssuffers from three obvious problems: neither the humanpopulation (Figure 1) nor, in many cases, the total sizeof the resource are constant, and while total areas ofland and sea are easy to measure, definitions of ecologi-cally productive areas are problematic, for severalreasons. In the case of land (Cox et al., 2000, 2004):

. they require a judgement about how unproductiveland must be before it can be excluded from thetotal

. they ignore the large variations in productivity ofproductive land, and the potential for dramaticallyincreasing the productivity of unproductive landthrough technologies such as irrigation1

. they take no account of the possibility that directand indirect impacts of human activities may signifi-cantly reduce productive land areas over the rest ofthis century. For example, it has been suggested thatclimate change could lead to the die-back of tropicalforests from the middle of the century

The fundamental nature of the above problems meanthat detailed calculations of land areas are complex

Figure 1 Projections of world population. Source: PopulationDivision of the Department of Economic and Social Affairs of theUnited Nations Secretariat (2005)

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and of limited value in the search for absolute targetsfor the built environment – it is this conclusion andthe reasons for that will be carried forward in thispaper. However, the author is unwilling to leavethe subject of ecological footprinting without onefurther observation. The primary value of Wackerna-gel and Rees’s (1996) work lies not in the detailedquantitative work that it supports, but in the quali-tative insights that it provides. For example, the eco-logical footprint concept makes clear that undercurrent conditions, land and sea are the ultimatedeterminants of wealth. This assertion would bedeeply unsurprising to observers from before theindustrial revolution, one of whose effects has beento obscure this connection for those of us who havecome after. Yet it is clear that the primary functionof modern technology is not to replace, but toextend, intensify and mediate the grip of wealthysocieties on land and sea.

The discussion of global limits presented in this paperassumes that global resources are divided equallybetween different societies. It is worth acknowledgingthe utopian nature of the proposition implicit in thisapproach. If acted upon, it would mean the end oflarge-scale, systematic differences between rich andpoor, the end of the distinction between North andSouth, the end of imperialism. Whether it is acceptedas a guide to action, or even as a useful analyticaldevice, it is in this last respect at least that it entirelyin step with the likely outcome of the next halfcentury.

For the second example of how difficult it can be todefine the size of the resource, one can consider theproblem of defining limits to CO2 emissions. Startingfrom the Brundtland definition of sustainable develop-ment as development that satisfies the needs of peopletoday without harming the ability of future generationsto satisfy their needs, one may argue that, at the veryleast, the current generation should not change thecomposition of the atmosphere.2 To hold the atmos-pheric concentration of CO2 at its current levelwould require net emissions of CO2 to be reduced tothe level of the net global sink capacity for the gas.This is currently in the region of 3 gigatonnes (Gt) ofcarbon per annum (Houghton et al., 2001). The result-ing per-capita limit on CO2 emissions would beapproximately 0.46 tonnes (t)/annum. For compari-son, emissions per capita in the UK in 2005 are pro-jected to have been 2.53 t/annum, 5.5 times largerthan the estimate based on the static equilibriumapproach. Globally, anthropogenic emissions exceedsink capacity approximately 2.5-fold.

The main problem with using net sink capacity as thebasis for setting limits to CO2 emissions is that it isunlikely to remain constant. Two-thirds of the globalnet sink capacity is accounted for by oceanic uptake.

The ability of the sea to absorb CO2 is subject,among others, to the following uncertainties:

. the solubility of gases in water falls as the tempera-ture rises

. the possibility that both changes in the thermo-haline circulation and increases in the temperaturegradient in the surface layers of the ocean maychange the rate of transport of CO2-rich surfacewater into the depths3

. reduced sea ice cover will increase the area of waterin contact with the atmosphere

The first and second of the above mechanisms reducethe ability of warmer oceans to absorb CO2,while the third may increase it.

The probability of policies that would curtail furthergrowth of atmospheric CO2 concentration beingimplemented in the near future is vanishingly small, butif such policies were implemented, and discounting theabove effects, the flux of CO2 into the oceans would fallas the concentration of the gas dissolved in the surfacelayers of the ocean moved into equilibrium with theatmosphere. Assuming that the capacity of land-basedsinks did not vary significantly from the present value of1 Gt/annum, the flux into the oceanic sink would falltowards zero, roughly exponentially, with a time constantin the region of 80 years. The total sink strength wouldtherefore fall by approximately half over the century fol-lowing a decision to stabilize the atmosphere (Figure 2).

Asymptotic approachThe above leads naturally onto a discussion of asymp-totic limits. The asymptotic approach to limits

Figure 2 Possible trajectory for the global net carbon dioxidesink capacity following a decision to stabilize the atmosphere in2010. It is assumed that land-based sinks are unaltered over theperiod shown and that changes to the strength of the oceanicsink arise solely from the stabilization of the oceanic mixed layer,with a time constant of 80 years

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recognizes that the world is currently far from a stableor sustainable state. It assumes, perhaps optimistically,that the human population will stabilize and thatresource use will progressively and asymptoticallyapproach sustainable levels. For any given resource,the asymptotic limit may then be defined as the asymp-totic size of the resource divided by the asymptotichuman population of the earth. In the case of landareas, taking the asymptotic population to be 10billion:

Asymptotic earth share per person

¼ surface area of the earth= population

¼ 5� 1014 m2=1010

� 5 ha per person

Asymptotic land share per person

� 5� 1014 m2=1010 � 0:25

� 1:3 ha per person

As noted above, there is no reason to suppose that theproportion of ecologically productive land will remainconstant and it is possible to conceive of a number ofscenarios in which the proportion would fall. But onthe assumption that the proportion retains the valuecurrently ascribed to it by Wackernagel and Rees, theasymptotic ecologically productive land share perperson would be in the region of 0.9 ha per person.

The asymptotic approach to limits is close to the philo-sophy of the contract-and-converge movement. Thebig advantage of limits based on the asymptoticapproach is that they are not rendered out of date bypredictable changes in the relationship betweenhuman society and its ecological niche. But in thecase of CO2 emissions, the approach leads to asimple result. For several hundred thousand yearsbefore the advent of large-scale human activity, theconcentration of CO2 in the atmosphere remained inthe range 180–300 ppm (Houghton et al., 2001;Barnola et al., 2003). This suggests that in theabsence of more complex arguments, the long-runasymptotic sink magnitude should probably be takenas zero.4

It is unfortunately hard to avoid more complex argu-ments. It is unlikely that in the face both of the impend-ing extent of human development over the course ofthis century and of climate change itself, that land-based sinks will remain constant or that absorptionby the oceans will be unaffected. As noted above,there are grounds for expecting significant land-basedsinks of CO2 to reverse in the second half of thecentury and, in addition, for higher temperatures totrigger significant emissions of methane. It is clearthat once the biosphere became a major net source ofgreenhouse gases, the importance of anthropogenic

emissions of fossil carbon from energy conversionwould diminish sharply, possibly to the point of irrele-vance. Were such a point to be reached, the luxury ofbeing able to consider measures to slow and/or haltclimate change would have been foregone, and themost pressing problem would be how to adapt to thenew conditions with the minimum loss of human lifeand culture (Lovelock, 2006).

That such an outcome might be possible is indicatedmost persuasively by the fact that it appears to have hap-pened once before, at the end of the Permian. Thereremains more uncertainty about the sequence of eventsthat resulted in the largest mass extinction event of thelast 600 million years than there is about the smallerand more recent extinction event of 65 million yearsago. Furthermore, the nature of the planet at the endof the Permian was significantly different from today– not least in the disposition of the continents. Neverthe-less, recent literature (Benton and Twitchett, 2003;Kiehl and Shields, 2005) establishes the plausibility ofa model based on an initial release of CO2 from large-scale and prolonged volcanic eruptions in what is nowSiberia, which caused an initial rise in global tempera-ture, which in turn triggered a massive release ofmethane from methane hydrates on sea floors andtundra, which led, in the words of Benton and Twitchett(2003, p. 358), ‘to an ever-worsening positive-feedbackloop, the “runaway greenhouse”’.

It has proved difficult to discuss the possibility andpolicy implications of such a runaway greenhouseeffect without appearing to undermine the case forprompt and vigorous action to reduce CO2 emissions.The case for such action would indeed be weak if thescientific community were certain that such actioncould affect neither the extent nor the rate ofwarming, nor reduce the probability that initialanthropogenic warming would be large enough totrigger large-scale biospheric releases of greenhousegases. If, on the contrary, there appeared to a signifi-cant probability that prompt action might achieve allof the above goals, then the case for such actionwould be strong. This statement can be framed econ-omically: the amount that can rationally be spent toavoid a severely negative outcome depends on theextent to which expenditure is expected to reduce theprobability of that outcome multiplied by the coststhat would be ascribed to the severely negativeoutcome, should it come about. The non-zero prob-ability that a runaway greenhouse effect may alreadybe unavoidable is therefore balanced by the very highcosts that would be likely to be ascribed to it.5

The balance of opinion among climate scientistsappears to be that several more tipping points almostcertainly exist, but they are unlikely to be reachedbefore the end of the 21st century and are still, in prin-ciple, avoidable (Hansen, 2005; Schellnhuber et al.,

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2006). The prevailing uncertainty about where, interms of the magnitude of initial warming that wouldbe needed to reach them, such tipping points may liestrengthens rather than weakens the case for promptand vigorous action to avoid reaching them.

The reason for this extended discussion is that it raisesthe interesting possibility that the range of possiblefuture climates accessible as a result of human activitymight in the long-term be discontinuous. The accom-panying uncertainties make it impossible to defineasymptotic limits except in probabilistic terms, and,over a significant range of possible futures, likely thatif they were so defined, they would be negative.

Integral of excess approachWith different degrees of sophistication, both the staticequilibrium and the asymptotic approach focus on therequirements of sustainability. The integral of excessapproach focuses instead on the magnitude and dur-ation of departure from sustainability.6 The argument,in the case of climate change, is that because of thepressing nature of the problem, what is important isnot the asymptote, but how fast it can be approachedand how much damage is done on the way. Theapproach is illustrated below (Figure 3).

One of the earliest examples of the integral of excessapproach was provided by the International Project forSustainable Energy Paths (IPSEP) project (Krause et al.,1989).7 Despite its age and the fact that is has sincebeen overtaken by the work of the IntergovernmentalPanel on Climate Change (IPCC), this early study veryclearly illustrates the principles, implications and limit-ations of the integral of excess of approach.

On the basis of a detailed review of the sensitivity ofland-based eco-systems to climate change, the IPSEPteam proposed an upper 400 ppm limit on atmosphericCO2. Further work suggested that this would be equiv-alent to a global fossil carbon budget of 300 Gt. Thenext step was to propose a set of rules for partitioningthis budget between industrialized and developingcountries – in the end, the approach taken was toassume a half-and-half division. The resulting scen-arios are shown in Figure 4.

It is worth noting that very similar arguments to thosepresented by Krause et al. were used as the basis for theGerman Parliamentary Enquete Kommission’s rec-ommendations (Deutsche Bundestag, 1991).8 Thisdocument and the analysis that it represented providedthe strategic background for the highly influential Pas-sivhaus standard (Feist, 1998).9

As a basis for well-defined absolute limits to CO2 emis-sions, the approach presents a number of problems. Theleast convincing part of the IPSEP study was the attemptto argue for 400 ppm as an upper limit to atmosphericCO2 concentration, which was then used to define aglobal fossil carbon emission budget for 1990–2100.This is not because such a limit is unsupportable, butbecause arguments in this area are unavoidably specu-lative, complex and dependent upon judgement.

Two further problems associated with this approachrelate to limits to integration and the assumption thatimpacts on the environment are identical regardlessof when, within these limits, emissions take place.The first problem is largely side-stepped by the IPSEPcarbon emission scenarios, since under these, emissionrates are close to zero by the end of the 21st century.But scenarios in which CO2 emissions remain high at

Figure 3 Anthropogenic carbon emissions underIntergovernmental Panel on Climate Change (IPCC) scenarioIS92c (Nakicenovic and Swart, 2000). The area under the curve,shaded grey, represents the integral of the excess of actualemissions over the assumed long-term sink capacity ^ in thecase of this scenario, a total of some 770 Gt of carbon between1990 and 2100

Figure 4 Carbon emission scenarios based on a300-Gt globalfossil carbon budget, split equally between industrializedcountries (IC) and developing countries (DC). Source: afterKrause et al. (1989)

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the end of the century beg the question of the signifi-cance of emissions in the 22nd century. This secondproblem arises because the earlier a given quantity ofCO2 is emitted, the larger its impact on climate byany given subsequent date – because of time lags inthe climatic system, primarily associated with thermalcapacity of the ocean, emissions that take place overthe next quarter of a century will have more impacton the climate of 2100 than emissions that take placein the last quarter of the century.10

Problems of contextA problem common to all of the approaches consideredabove is how to convert global limits to human activityinto constraints on the built environment. Related tothis is the problem of accounting for the interactionsbetween the built environment and of the wider tech-nological context. To be a useful guide to action forthe built environment community, it must be possibleto translate whole system limits into limits on thebuilt environment and ultimately into design targetsfor individual buildings and building sub-systems.

In the case of energy, the first of these two problems iscommonly addressed by assuming that reductions inCO2 emissions required at the level of the economyas a whole also apply to the built environment. Thecase for this approach is based on the fact that, formost advanced economies, the built environmentaccounts for about 50% of total emissions. Rigorousglobal emissions limits become unachievable unlesssuch limits are applied in full to the built environment.

The problem of the wider technological context relatesin the main to the energy conversion and distributioninfrastructure that supports the built environment.The history of energy supply over the last centuryshows sustained and powerful downward trends inthe CO2 emissions associated with the supply of bothelectricity and fuels for direct combustion to buildings.The trend for electricity is shown in Figure 5. The pointabout this figure is that the trend can be extended forseveral more decades based on incremental improve-ments to existing technology. Crucially, the futurepaths indicated in Figure 5 can be achieved by arange of different mixes of technology, all more orless credible. For example, zero-carbon electricity canbe delivered by renewables, nuclear and so-calledclean coal using carbon sequestration in almost anyproportions. Given the wide range of technological sol-utions available, coupled with recent political pro-nouncements on the importance of reducing CO2

emissions, it appears likely that the long-term trendwill continue downward and that a factor of tenreduction between 1950 and 2050 is possible.

At present, in most countries, the carbon intensity ofdelivered electricity is two or three times as high as thecarbon intensity of natural gas. This means that

electricity consumed in buildings accounts for around50% of total CO2 emissions from the built environment.Yet much of the research and most of the practicalmeasures undertaken over the last quarter of a centuryto reduce the environmental impact of buildings havebeen directed at reducing emissions from space andwater heating.

The potential impact of combining continued vigorousend-use measures in the housing sector with continueddecarbonization of the electricity supply system wasexplored by Johnston (2003) and Johnston et al. (2005).The result was to make a 60% reduction in sectoralCO2 emissions by 2050 comparatively straightforwardto achieve. Indeed, Johnston and Johnston et al. foundmuch larger reductions to be possible if low-carbon elec-tricity were combined with heating systems based on heatpumps. The point can be illustrated by reference to thePassivhaus standard. As noted above, this reduces emis-sions from space heating by 80–90%, but actually onlyhalves emissions for all end uses under current conditions(Schnieders and Hermelink, 2006). Remaining emissionsfrom dwellings built to this standard are dominated byelectricity used by lights and domestic appliances, whichis unaffected by the thermal envelope measures thatform the core of the standard. But a halving of thecarbon intensity of delivered electricity in conjunctionwith the Passivhaus standard would achieve an overallreduction in direct emissions of the order of 75%.11

The main conclusion from this brief discussion ofthe interactions between the built environment and elec-tricity supply systems is that, in addition to other sourcesof uncertainty, technical performance goals for the builtenvironment are powerfully dependent on parallel

Figure 5 Carbon intensity of UK-delivered electricity from1950to 2004; and possible future paths assuming the electricitygenerated by combined cycle gas turbines (ef¢ciency rateincreasing from 47% in 2004 to 60% in 2050), plus a proportion ofzero-carbon electricity varying from zero to 50% in 2050. Dataare fromDigest of UK EnergyStatistics (DTI, 2005)

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developments in other categories of infrastructure.Studies that assume a static electricity supply systemare likely to overestimate significantly the difficultiesinherent in reducing the CO2 emissions from the builtenvironment (e.g. Boardman et al., 2005).

Planned future approachThe importance of technology and policy contextrevealed in the foregoing text brings us to theplanned future approach to limits. The plannedfuture approach deals head-on with context by specify-ing the context in as much detail as is necessary to con-strain all major sub-systems. Examples of such futuresinclude the scenarios presented in the Energi 21 planfor the Danish economy (Danish Ministry of Energyand Environment, 1997) and, going further back intime, the work of Leach et al. (1979) and Olivieret al. (1983). The recent work of Lovins et al. (2005)contains significant elements of a planned future.

Planned futures are most commonly generated either tosupport or to challenge national policy. In the formercase, the overt goal of the work is to provide a detailedplanning framework for decision-making throughoutthe economy. It is however often possible to discern asecondary goal, of advocacy for the futures thusdescribed. In the second case, that of studies aimed atchallenging existing policies, the balance of these twogoals is reversed and such studies are probably betterthought of as alternative rather than planned futures.Despite the different goals of the two approaches,there do not appear to be systematic differences inthe levels of detail present in each. Detail is requiredboth to support strategic decision-making and to chal-lenge a prevailing climate of opinion.

From the perspective of attempts to define absoluteindices of environmental performance planned futuresovercome some of the formal problems identified forthe static equilibrium, asymptotic and integral ofexcess approaches. But indices based on such futuresare contingent on structures of analysis and assumptionsthat delineate but ultimately cannot banish the funda-mental uncertainties associated with guessing thefuture – which is unknown because it has not happenedyet. More practically, planned futures are rarely builtwith the objective of calculating absolute environmentallimits to human activity and are rarely global in scope –not least because of the cost of constructing them.Finally, planned futures are often overtly partisan – con-structed to build rather than to represent consensus.12

They are therefore unlikely to be able to deliver uncon-tentious measures of environmental impact.

Developmental approachOne of the most important messages from the above isthat it is effectively impossible to define precise absol-ute environmental targets for the built environment.The most that one can conclude is that in key

categories, current environmental impacts exceed bymany times what can be sustained. One can gofurther and note that it does not matter greatlywhether targets are based on the assumption that athreefold reduction in environmental impact is suffi-cient to achieve sustainability, or that nothing lessthan a factor of ten is enough. Either case rendersmost of the existing infrastructure environmentallyobsolete, particularly if continued economic growthat anything approaching historical rates is assumed.Against this background, the absolute level at whichtargets are set is less important than the direction inwhich they point. The empirical measure of the effec-tiveness of such targets is their impact on innovation,not the degree to which they embody an absolute exter-nal measure of sustainability.13

The author’s own experience of the process of developingthe energy performance requirements for the 2006 revi-sion of Part L of the Building Regulations for Englandand Wales14 suggests that key factors in success of devel-opmental targets are likely to be the following:

. their relationship to regulation and other systemsfor incentivizing the reduction of environmentalimpact – targets that form the basis for regulatorystandards can be powerful agents for change

. their relationship to currently available technology –energy targets must challenge but not overwhelm thecapacity of industry to deliver improved performance

. they must be pragmatic, reflecting current andemerging technological and market opportunities

. they must be dynamic, with clear programmes forrevision – targets which are not regularly revisedeventually act as a brake on rather than a spur totechnological innovation

. the process for setting and revising targets musttake account of the lead times of the systems andindustries that they affect; the further ahead stake-holders can see, the more demanding the targetsthat can be set

. the importance of devising targets that segmentrather than homogenize markets – targets arelikely to have the largest impact on innovation ifthey increase rather than reduce the range anddiversity of business opportunities within the sector

. the importance of framing regulation in a way thatmaximizes tendency for integration throughout theindustry, including the design process and thesupply chain

. the importance of consultation as part of a process forbuilding consensus, coupled with a clear recognition

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within the business environment of the importanceand validity of corporate social responsibility

. the importance of an overarching long-termenvironmental agenda established by government– in the case of the 2006 revision to Part L of theBuilding Regulations for England and Wales, thishas been provided by the 60% reduction commit-ment in the Energy White Paper (Department ofTrade and Industry (DTI), 2003).15

Developmental targets need to be both backward andforward looking. Rather than being defined in terms ofglobal limits to carrying capacity, they are probablybest defined in terms of current performance. Thus theenergy standards that underpin the 2006 revision toPart L of the Building Regulations for England andWales, are based on the estimated performance of build-ings built to the preceding 2002 standard, coupled with aconsensual view of the rate of change that could be toler-ated by the construction industry in this market. Whilethe 60% reduction commitment contained in the 2003Energy White Paper that preceded the review played apowerful role in shaping the climate of opinion withinwhich it took place, the resulting standards are notdirectly or mechanically related to the 60% reductioncommitment. Finally, there was a clear recognition inthe consultation document that initiated the publicreview process (Office of the Deputy Prime Minister(ODPM), 2004), that a single review of energy perform-ance standards would be insufficient to move the con-struction industry of a major industrial country to asustainable state, and that this review would thereforebe merely the first of a series. The function of this particu-lar standard was not to move the industry across thechasm to a sustainable future but to define the next step-ping stone and enable the industry to move to it.

The relationship between the above process of regulatorydevelopment and environmental performance assessmentschemes in use in the UK is instructive. The unexpectedlyrapid development of Part L since the publication of theEnergy White Paper caused it to overtake environmentalperformance assessment schemes such as BREEAM, par-ticularly in the housing sector (Rao et al., 2000; Office ofthe Deputy Prime Minister (ODPM), 2005). These inturn are now undergoing rapid development to ensurethat they can continue to fulfil their most important orig-inal function – to provide guidance for and means ofrecognizing developments that go beyond minimumregulatory requirements. This is a clear empirical demon-stration of the developmental and pragmatic nature ofthese environmental performance schemes.

During the writing of this paper it became clear that theemphasis on pragmatic incrementalism in the descriptionof developmental targets was potentially misleading.Such an approach does not automatically mean that pro-blems such as climate change are not being taken

seriously nor does it mean moving slowly. Moreover adevelopmental approach does not remove the need fora long-term strategic vision nor preclude medium orlong-term strategic decision making. But it does dependon the view that not all factors can be dealt with bysuch decision making, that many decisions are besttaken over shorter cycles and that the complexity of thefactors and processes that determine energy use in thebuilt environment renders it poorly suited to approachesto change that are unable to harness and promote learn-ing within relatively short review cycles.16 If such reviewcycles can be effectively implemented it is at least possiblethat a developmental approach may result in faster ratesof change than other approaches.

ConclusionsThe central purpose of this paper has been to examinethe proposition that it is possible to define environ-mental performance standards for the built environ-ment in terms of global carrying capacity.

The main focus has been on energy-related impacts,reflecting the perception that this is the category ofimpact where such an approach is most likely to meetwith success. Four possible approaches to defining limitsare explored: static equilibrium, asymptotic, integral ofexcess and planned future. The central qualitative con-clusion that emerges from this exploration is confirmationthat current environmental impacts are unsustainable bylarge factors. But while analyses of global constraintsprovide essential qualitative insights into the problemsof sustainability, the quantification of global constraintsis unavoidably uncertain. Major sources of uncertaintyinclude the potential impact of climate change on thebehaviour of the main carbon sinks, with the possibilitythat land-based sinks will reverse during the second halfof the 21st century. Such changes are also likely toimpact on timber production in the currently mostproductive tropical regions. A second area of currentconcern is the possibility that current models of ice sheetdynamics seriously underestimate the risk of ice sheet col-lapse within one to three centuries (Hansen, 2005).

The process of translating global targets into specifictargets for buildings and building systems introducesfurther uncertainties. Some of the most interesting sur-round possible developments in energy supply systems,most obviously in electricity generation. Significantreductions in the carbon intensity of electricity impacton overall CO2 emissions both directly, by reducingemissions associated with current end-use systems, andindirectly, by increasing the viability of new end-usetechnologies such as heat pumps. The potential for suchinteractions is considerable and increases the further intothe future one looks. Global constraints cannot thereforeprovide a unique, objective and unambiguous metric forbuilt environment performance standards.

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The alternative to basing targets on long-term externalconstraints to human activity is to base them on theinternal short- to medium-term dynamics of the builtenvironment itself. This approach is referred to as thedevelopmental approach. Such targets, set pragmati-cally, regularly revised, forming the basis for regulationand para-regulatory systems, and promoting and sus-taining innovation, appear likely to be most fruitful.On this view, the future emerges from a series of short-and medium-term decisions and the importance of thelong-term global analysis is largely symbolic.17 This is,however, to underestimate the importance of a clearqualitative view of long-term environment constraints.Experience both from the UK and Germany suggeststhat such a view is essential to the establishment of anenvironment and climate of opinion that makes it pos-sible to make and implement appropriate short- tomedium-term decisions. Within this model of environ-mental targets and decision-making, the analysis of pos-sible long-term goals for the built environment providesa qualitative framework within which to build the con-sensus needed to drive developmental goals and vigorousand concrete short- to medium-term action.

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Endnotes1The historical record of sustainability of irrigation systems ismixed, and many systems have failed completely. Failuresinclude much of the ‘fertile crescent’, reduced to desert since clas-sical times, and, more recently, the Aral Sea basin (InternationalFund for the Aral Sea (IFAS), 2000). Partial failures include theMurray–Darling basin in Australia (Natural Resource Manage-ment, 2004).

2However, note that holding the composition of the atmosphereconstant does not prevent further climate change. There is nosimple argument arising from the Brundtland definition of sus-tainability that will lead to a prescription for reduced emissionsof greenhouse gases.

3A complete collapse of thermohaline circulation as a result ofwarming of surface layers appears to have played a part in theend of the Permian extinction event (Benton and Twitchett,2003; Kiehl and Shields, 2005), though against a very differentarrangement of land masses.

4Significant departures have probably taken place, e.g. during thePermian extinction event. Lovelock (1989, p. 125) has suggestedthat there may be a long-term tendency for atmospheric CO2 con-centration to fall, balancing the increased intensity of the sun.

5The author is, however, unaware of any economic analyses ofrunaway climate change.

6The difference between these two approaches can be thought ofin terms of the cry of St Augustine: ‘Oh Lord, make me virtuous,but not yet!’ The asymptotic approach focuses on the nature ofthe future state of grace, while the integral-of-excess approachfocuses on the extent of the sin in the meantime.

7For some of the implications of the IPSEP scenarios, see Lowe(2000).

8Wilfrid Bach, a co-author of the final IPSEP report, also servedon the Enquete Kommission.

9The Passivhaus standard is built around an absolute primaryenergy target of 120 kWh/m2/annum and a target of 15 kWh/m2/annum for space heating (Informationsgemeinschaft Passiv-haus Deutschland, n.d.). These limits are intended toreduce energy demand for space and water heating by approxi-mately 90% compared with the German housing stock and byapproximately 80% compared with new housing, in line withthe goals set out in the early 1990s in the report of the EnqueteKommission of the German Parliament (Deutsche Bundestag,1991, pp. 156–192).

10One reviewer of the present paper questioned the emphasisgiven to the 18-year-old IPSEP study of climate mitigation com-pared with more recent work under the aegis of the IPCC. Themain reason for this is that the IPCC scenarios are basedheavily on the internal dynamics of the global socio-economicsystem. In contrast, the IPSEP study started from a clearlydefined climate change goal (based on a conservative view ofthe maximum rate of temperature rise to which land-based veg-etation could be safely exposed), and then constructed and ana-lysed a carbon scenario that would achieve this goal. The IPSEPstudy, therefore, more clearly illustrates both the implicationsand limitations of attempting to calculate top-down environmen-tally based limits to human activity than do the IPCC scenarios.Hansen’s (2003) recent 1 K, 475 ppm scenario was based on anapproach similar to that of the IPSEP study, but with awarming limit based on a conservative view of the stability ofpolar ice sheets rather than vegetation. Hansen notes that hefaced criticism for publishing a paper that could be seen as inde-pendent and critical of the position taken by IPCC and states thatit is important that IPCC conclusions not be considered as beingclose to gospel truth.

11At the same time, it would halve emissions associated with elec-tricity used to build dwellings to this standard.

12A final point is that the implementation of planned futures,particularly where these represent a significant move awayfrom business-as-usual, requires high and sustained levels ofconsensus – as illustrated by fate of the Danish Energy 21 planfollowing the change of government in Denmark in 2001.

13This is not to say that the outcome for climate would be thesame regardless of whether in 2050 emissions had actually beenreduced by a factor of three or a by factor of 10. The formerwould be consistent with a world in which global emissionswere more or less at today’s levels, and in which climate changecontinued unabated. The latter would be consistent with asharp reduction in global emissions of CO2 and the possibilitythat potentially dangerous climate change might be avoided.The point that is being maintained here is that in the short-to-medium term, any non-marginal reduction target is potentiallyrevolutionary. The key to the ultimate outcome are the concep-tual shifts and processes of innovation that are precipitated bythe adoption of the long term target, not its precise magnitude.

14The Building Regulations of England and Wales are dividedinto 14 parts, running (with some omissions) from A to P. Allare available online (http://www.odpm.gov.uk/). Separate regu-lations are published for Northern Ireland and for Scotland.

15Note, however, that the relationship is not mechanistic – theEnergy White Paper legitimises Part L, but does not prescribe it.

16However, effective learning does require feedback. A lack ofempirical knowledge about energy use both in new and existingbuildings is a significant constraint on the development ofpolicy and if, left unaddressed, will become even more proble-matic in the future.

17Some readers may find this comes close to being a restatementof the view of history as ‘one damn thing after another’. What dis-tinguishes it from this statement is the long-term qualitativeframework that underpins and guides it.

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