ELSEVIER METHODS Ecological Economics (Géoparc Jbel Bani)
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ELSEVIER METHODS Ecological Economics (Géoparc Jbel Bani)

Estimating sustainability gaps: methods and preliminary applications for the UK and the Netherlands

By DR. Sandrine Simon, DR. Paul Eakins

School of Politics, International Relations and the Environment (SPIRE) Keele University of Kee/e Staffs ST5 5BG, UK

Abstract

Received 7 February 2000; received in revised form 13 November 2000; accepted 15 November 2000 this paper sets out and applies a new methodology for determining whether or not economic activity is environmentally sustainable. By comparing current environmental impacts with standards of environmental sustain- ability, it calculates the ‘sustainability gaps’ (SGAPs) with respect to different environmental impacts. For the UK, SGAPs are computed for C0 SO and other air pollutants. Across none of these environmental themes can current UK use of the environment be said to be sustainable. The SGAP indicators can be combined with current trends to show how long it would take, on continuation of the trends, for the sustainability standard to be attained. The paper calls this indicator the ‘Years to Sustainability’ (YS) measure. For the policy objective of sustainable development to be made fully operational, it is necessary for the concept of environmental sustainability to be clearly defined by quantitative indicators. On the basis of the indicators set out in this paper it can be judged whether economic activity is moving towards or away from environmental sustainability, and at what speed.

Keywords: Environmental sustainability; Sustainability indicators; Sustainability gap

© 2001 Elsevier Science B.V all rights reserved.

1. Introduction

Sustainable development has become one of the core organizing concepts of environmental policy. One element of this concept is environmental sustainability, which has been defined as the maintenance of important environmental functions into the indefinite future (Eakins, 1997,

p. 39). Eakins and Simon (1998, PP. 54ff.) have shown how, from this definition of environmental sustainability, sustainability principles can be formulated and how, from them in turn, sustainability standards for each individual environmental impact may be derived. This paper seeks to develop this work further to demonstrate how the standards may be used to generate indicators that show in an easily comprehensible way the extent to which current development patterns are environmentally sustainable.

For the concept of environmental sustainability to be used in this way, it is necessary for the sustainability standards to be derived as far as possible on the basis of objective considerations deriving from environmental science concerning the maintenance of important environmental functions, rather than being influenced by considerations of cost or political feasibility. There are a number of criteria which determine the levels at which the environmental sustainability standards should be set. In general, the level should be such those anthropogenic impacts on the environment:

• Do not threaten critical ecosystems (e.g. pollution is kept below the ecosystems' critical loads, biodiversity is conserved)

• Do not threaten biogeochemical systems (e.g. anthropogenic emissions do not destabilize the climate or destroy the ozone layer)

• Do not have a detrimental effect on human health

• Do not harvest renewable resources faster than their rate of regeneration

• Do not deplete non-renewable resources faster than the rate of development of substitutes for them.

In cases where there is uncertainty over which ecosystems or environmental functions qualify as important, or critical, or over the appropriate standard to be applied, an approach based on the idea of 'safe minimum standards' or the Precautionary Principle would seek to avoid situations where there was a risk of irreversible changes or of immoderate costs in the future. The difference between the current level of environmental impact from a particular source, and the sustainable level of impact according to the sustainability standard, may be termed the 'sustainability gap' (SGAP). Two earlier papers (Ekins and Simon, 1998, 1999) have discussed the sustainability gap idea in general terms. This paper presents some results for the UK and the Netherlands of applying the SGAP idea to various environmental themes.

Government environmental policy is often associated with targets, which may or may not be aligned with the sustainability standards. If the sustainability standard is considered too expensive or demanding politically, then perhaps a less demanding target will be set, or the timescale for achieving the sustainability standard will be lengthened, with interim targets being adopted. This seems a preferable way for political considerations to enter into sustainability policy, rather than such considerations leading to the adjustment of the sustainability standards themselves, because the trade-off between achieving environmental sustainability and other political objectives is then apparent. Where government targets are aligned with sustainability standards, these targets become 'sustainability targets'. Where they are not, they are just 'policy targets', and the gaps between current environmental impacts and the policy targets are 'policy gaps'.

This paper derives sustainability standards, policy and sustainability targets, and SGAPs for carbon emissions, and various other air emissions which degrade air quality. The criteria for sustainability set out above are discussed further in Ekins, 2000 (pp. 92ff.), but here there is only space to note a few general points. First, it can be seen that for a given environmental impact (e.g. a polluting emission) there may be different sustainability standards with respect to different criteria (e.g. a critical load for an ecosystem may be above the level that causes a detrimental effect on human health, or vice versa). For environmental sustainability overall, it is the most stringent standard that is binding. Second, environmental effects may be local, regional or global. For example, the effects of sulphur deposition, or of air quality, are local (although the emissions that give rise to them may be very non-local, and may even have come from another country), while those of carbon emissions are global. In the local case, local environmental unsustainability in some places may be compatible with local sustainability elsewhere (although care should always be taken to take into account ecosystem connections and interactions). In the global case, a particular impact can only be pronounced sustainable in the context of the aggregate impacts from everywhere else. It has not been possible to explore all these complexities in this paper. Their implications will, however, be noted where appropriate.

Section 2 looks at a number of sources of local (or regional) air pollution in the UK. Particular instances of environmental unsustainability are identified in relation to the relevant sustainability standards. However, there has been no attempt either to combine the sustainability standards across the different criteria (e.g. to compare the standards for impacts on ecosystems with those for human health), or to estimate whether synergies between impacts (e.g. different air pollutants), or the possible effects of accumulation of pollutants, imply different sustainability standards to those given. The results in Section 2 should therefore be interpreted with caution. They certainly suggest widespread unsustainability across the issues studied, but they should be regarded as indicative of unsustainability, rather than definitive as to its absolute extent. In particular the actual SGAPs quoted should be treated as minima, and might turn out to be greater if all relevant considerations were to be taken into account.

The overall sustainability standards for environmental themes, which are not available for the UK, have been estimated in official sources for the Netherlands for a number of themes.

Section 3 uses this data for the Netherlands to carry out the next stage in the development of sustainability indicators, for which the UK data are inadequate. The indicators that are derived show the extent to which development patterns in the Netherlands from 1980-1991 were environmentally unsustainable, the improvements in environmental sustainability which were achieved over this period, and the length of time, on trends then pertaining, before environmental sustainability would be reached.

Section 2 looks only at local environmental impacts. Section 3 does not make explicit the kind of approach that is necessary to derive a sustainability standard for a global impact. This is the subject matter of Section 4, with regard to carbon emissions and global climate change. Sections 2, 3 and 4 are concerned only with physical environmental impacts and SGAPs ex- pressed in physical terms. Section 5 outlines a methodology for converting these SGAPs into monetary units. The paper does not apply this methodology in practice, which remains a task for future research, but the indicators which could emerge from such an application could give useful insights into the economic implications of seeking to achieve environmental sustainability, as well as into the sustainability performance of the economy as a whole. Section 6 concludes.

2. Various sustainability gaps for local air pollution in the UK

2.1. SO2 emissions

Emissions of SO2 cause important problems of air pollution and acidification. In the UK the effects on human health are now confined to some urban areas. Those on ecosystems may be local or regional. The emission reduction targets that have been agreed internationally have been driven mainly by ecosystem (critical load) considerations, but they do not yet represent sustainability standards, in that they do not yet envisage the reduction of anthropogenic sulphur depositions to levels below all ecosystems' critical loads.

The sustainability standards for SO2 are related to both ecosystem and health effects. Table 1 includes standards for both ecosystem and health effects, but no attempt is made to compare them in order to derive an overall sustainability standard. In general, it is clearly possible for emissions overall to fall within standards of ecosystem sustainability, but for instances of unsustainable impacts on human health to remain, or for there to be no unsustainable impacts on human health, but for emissions (and subsequent deposition) to be too great for some ecosystems. Full environmental sustainability would require both sets of standards to be met. The main sources of SO2 pollution are the power generation and industrial sectors, which in 1995 contributed 67% and 22%, respectively (DETR, 1997, Table 2.3, p. 32).

The use of coal as a fuel in the generation of electricity is a major cause of SO2 pollution. Power generation using natural gas is sulphur-free. Various studies have focused on the cost of SO2 abatement techniques. The costs of the main abatement techniques (not including fuel- switching) are presented in Table 1. Table 1 also presents summary results concerning the sustainability and policy gaps for SO2 emissions. The current situation is compared with the level of the targets, and the gaps are deduced from the comparison.

The 1993 and 1998 targets of the EC Large Combustion Plant (LCP) Directive (Council Directive 88/609/EEC, as amended by Council Directive 94/66/EC) have already been reached.

It seems very likely that the 2003 t£ also be reached on current policy. The year 2000 target of the UN E Commission for Europe (UNECE) Sec phur Protocol (SSP) has already been The July 2000 forecast of Cambridge metrics (CE, 2000, Table 5.3, p. 37) that 2010 UK SO2 emissions will be 732 thousands tonnes (kt), so easily achieving the 2010 target of 983 kt..

In December 1999 the UK Government signed a new UNECE agreement limiting UK SO2 emissions in 2010 to 625 kt. The CE projections for 2010 indicate a gap of 107 kt with respect to this target. It is likely that even this new UNECE target will not give full protection to UK ecosystems (or those outside the UK) from UK sulphur pollution, so that further SO2 reductions from this level are likely to be necessary if no ecosystem critical loads in the UK are to be exceeded (the sustainability standard).

•In 1996 Her Majesty's Inspectorate of Pollution (HMIP, now absorbed into the Environment Agency, EA) set a limit for the power sector of 365 kt, to be achieved by 2005 (DOE, 1996a, pp. 8-9). In 1998, having estimated that, to bring depositions of SO2 within critical loads, power stations would have to reduce their emissions to 200-300 kt (EA, 1998, p. 11); the Environment Agency proposed bringing the achievement date forward to 2001, but this has yet to be agreed. Cambridge Econometrics' (CE) forecast in July 2000 estimated that the power sector's emissions in 2005 with current policy would be 363 kt (CE, 2000, Table 5.3, p. 37), implying a sustainability gap, but no policy gap. By 2010, when SO2 emissions are forecast (by CE) to have fallen further, there will be a sustainability gap for the sector of 89 kt.

•With regard to human health and the sustainability standards prescribed by the UK Government's       Expert            Panel  on Air Quality Standards (EPAQS), the UK Government admitted in its National Air Quality Strategy that 'peak concentrations are such that the level recommended by EPAQS is widely exceeded' (HMG, 1997, p. 167). The same is true for the guidelines issued by the World Health Organisation (WHO). The exceedances are observed locally. In the case of the EPAQS limit value, only the two most remote continuous SO2 monitoring sites in the UK recorded no exceedances in 1993-94, while the highest exceedances were recorded in Bexley, Belfast, Leeds, and Newcastle (HMG, 1997 Table II 10.2, p. 166). The WHO limit values were most exceeded in 1993-94 in Belfast, Barnsley and Grimethorpe (DOE, 1996b Table II 10.3, p. 172).

2.2. Air quality (apart from SO2)

The UK National Air Quality Strategy has emphasised that air quality is an issue that is clearly related to sustainable development. The strategy attempts to set standards and help provide adequate measures in line with the objective in Agenda 21 to 'minimise hazards and maintain the environment to a degree that human health and safety is not impaired or endangered, and yet encourages development to proceed' (HMG, 1997, p. 4).

The substantially lower levels of some forms of airborne pollution that are experienced today in the UK, compared to earlier decades, still generate, directly or indirectly, premature mortality and chronic illness. Although much attention has focused on human health, the issue of ecosystem health, particularly related to the issue of acidification, is also of importance for sustainability. As a consequence, as noted earlier with regard to SO2, policies related to air pollution in the UK have focused both on urban air pollution, most recently in relation to transport, and on trans-boundary pollution such as pollution by acidifying substances.

Table 2 provides a summary concerning the sustainability gaps expressed in physical units for the main air pollutants in the UK, apart from SO2, which has already been considered. In the first column, Table 2 gives the current situation concerning air pollutants. It shows that certain towns or areas experience serious problems of air pollution. In the second column, sustainability standards are presented; they deal with levels, as defined by competent scientific bodies that are acceptable for human health. The targets given in column 3 are the policy targets set by the government. In some cases, they correspond to the sustainability standards; in other cases, the sustainability standards, as scientifically defined, are more stringent than the targets. The sustainability gap and the policy gap (the physical difference between the current situation and the sustainability standard and policy target, respectively) are calculated in the last two columns. Clearly the significance to be attached to these gaps depends on the accuracy with which the air quality has been measured and the representativeness of the site where the monitoring station is located.

It should be noted that ecosystem sustainability standards are not considered in Table 2. Nor are possible synergistic effects between different pollutants, or the possible implications of the accumulation of pollutants. These issues would, of course, need to be taken into account in any overall sustainability assessment. DETR, 1998, discusses the complexities involved in the development of an overall indicator of air pollution concentrations. This looks forward to the kinds of aggregate indicators used in Section 3 to derive more complete SGAPs for environmental themes in the Netherlands, but which are not yet available for the UK.

Table 2 shows that:

•For carbon monoxide: the EPAQS target of 10 pm as a rolling 8 h average corresponds to a sustainability standard. The biggest exceedances of this limit are observed in Belfast (HMG, 1997, Table II 5.2, p. 102).

•In 1995, the UK had made some progress in reducing the emissions of VOCs, although further reductions had to be made if the 1999 limit of the UNECE VOCs protocol was to be achieved. In the specific case of benzene, the highest SGAP in 1995 was in Southampton (DETR, 1997, Table 2.24b, p. 52). For butadiene 1,3, on the other hand, no SGAP is observed.

•For O3, the highest 8 h mean concentration (in Lullington Heath) exceeds the target/standard of EPAQS (HMG, 1997, Table II.8.3, p. 141) and the WHO standard; a 66% reduction is still needed to meet the EPAQS standard.

•The WHO standard for lead has been adopted as a target. Some SGAPs can still be observed. The highest located at a measurement site (Imperial Metal Industries/site 2) exceeds the standard by 520 ng/m3 (DETR, 1997, Table 2.18, P- 45).

•The highest exceedances of the sustainability standard (and policy target) for PM10 is observed in Belfast (DETR, 1997, Table 2.10, p. 37).

•The sustainability standards for NO2 emissions are more stringent than the targets adopted. The highest hourly concentrations (98th percentile) are observed in London (HMG, 1997, Table II.7.2, p. 115), where both the sustainability standard and the policy targets are most often exceeded.

3. from 'sustainability gaps' to 'years to sustainability'

Adriaanse (1993, p. 75) defines sustainability as either a 'no-effect level' or a 'no-major-effect level' of environmental impact. His approach involves aggregating measures of environmental stress into 'theme equivalent units' for different environmental themes, and normalising these units of stress in terms of a sustainability standard. Space precludes detailed examination of the approach here, but it has been used by the Netherlands Government in its National Environmental Policy Plans. Some of Adriaanse's results are laid out in Table 3.

Columns 1 and 2 of Table 3 show Adriaanse's calculations of various stresses across seven environmental themes in the Netherlands for 2 years, 1980 and 1991, measured in various 'theme equivalent' units. Column 3 (ibid., p. 76) gives the sustainability standards he derived according to the definition above (ibid. pp. 78ff). Column 4 gives the 'normalised' stress according to the methodology advocated by Adriaanse, whereby the individual environmental stresses are weighted by the sustainability standards, converting the different environmental theme units (Ceq etc.) into a common unit, here called Environmental Pressure equivalent units (EPeq) (ibid., Table 603, p. 146). Column 6 gives the implied normalised sustainability standards. Because of the way the weighting has been carried out this standard is 100 for each theme, so that the total normalised sustainability standard across the six themes is 600 EPeq.

The final column computes the number of years before sustainability will be reached if the trend established in 1980-91 is extrapolated linearly, both for the individual environmental themes and for all the themes together. It can be seen that the overall sustainability standard of 600 EPeq will be reached after 51 years, although individually climate change, eutrophication, dispersion and waste disposal will still not have reached their sustainability level by then.

Finally it can be seen from Table 3 that the various measures cannot all be derived for all the environmental themes. For ozone depletion, the sustainability standard of 0 means that no figure for normalised stress can be derived, although there is no problem computing the Years to Sustainability For disturbance the increasing trend from 1980-91 means that no figure for YS can be given. However, in this case there is no problem with normalising the stress, and the increasing trend is factored into the total normalised figures, increasing the length of time before sustainability overall will be reached (removing disturbance from the total actually reduces the time before sustainability is reached to 43 years).

In Table 4 the first three columns are repeated from the previous table, but the next two columns go beyond Adriaanse to calculate the 'sustainability gap' (SGAP) for each theme for each year, where SGAP is the distance in theme equivalent units between current conditions and the sustainability standard. Thus in the SGAP columns the standard is subtracted from the stress for each year. The next two columns normalise this SGAP as shown, in the manner suggested by Adriaanse but not actually calculated by him. It can be seen that the NSGAP for climate change, for example, was reduced by 17% from 1980-91, while that for disturbance increased by 30%. The total NSGAP was reduced by 18% over this period. The final column again gives the years required to reach the sustainability standard (to reduce SGAP and NSGAP to zero) given the trend established from 1980-91. The YS is the same irrespective of whether individual or normalised units are used.

Both the normalised SGAP (NSGAP) and Years-to-Sustainability (YS) indicators give useful information on the achievement of sustainable development. Normalising the SGAP figures into the arbitrary units of 'environmental pressure equivalents' of NSGAP permits them to be aggregated across themes into a total NSGAP. As shown in the tables, this then enables an index number to be derived showing overall progress towards sustainability and a Years-to- Sustainability indicator to show how long it will be on current trends before overall sustainability (though not sustainability for each theme, which can only be derived from the individual theme indicators) will be achieved.

4. UK sustainability in terms of carbon emissions

4.1. Introduction

Section 2 calculated UK SGAPs for individual air pollutants with a local and regional impact. This section now explores the concept of the SGAP for the UK in respect of a global pollutant, CO2. CO2 is only one of a number of greenhouse gases (GGs), although it is the most important, and is estimated by the IPCC to be responsible for 70% of projected global warming (Houghton et al., 1996, p. 40). In the UK its share has been calculated to be even higher: 79% in 1990 and projected to rise to 85% by 2010 (DETR, 2000, Table 1, p. 46), so there is some justification in only focusing on CO2 in the first instance, as is done here.

However', it may be noted that in the sustainability analysis that follows what is identified as a sustainable trajectory for UK carbon emissions will only be so if other greenhouse gas emissions are being abated to a similar extent.

One of the core requirements of environmental sustainability is the prevention of the disruption of climate stability by human activities. Such stability depends on the atmospheric concentration of greenhouse gases (GGs), which is affected by the total global emissions of these gases.

Maintaining climate stability is the fundamental objective of the Framework Convention on Climate Change (FCCC), which set aspirational CO2 emissions targets for 2000 and, with the Kyoto Protocol, targets for a basket of six GGs (expressed as CO2-equivalents) by 2010.

These targets are less stringent than potential sustain-ability standards for CO2 emissions, as will be seen below.

4.2. UK policy targets

Much effort has been put into finding measures to reduce CO2 emissions, as well as calculating the cost of reducing the gap between the projected emissions for 2010 and the target. The second column of Table 5 summarises a range of CO2 reduction measures which have been put forward, as well as estimates of costs associated with the implementation of these abatement methods and technologies. It will be noted that a number of the abatement options have negative marginal costs, implying that reducing carbon emissions through these options will increase economic efficiency as well as reduce environmental impacts.

The first column of Table 5 presents information about the atmospheric concentration of CO2, and UK CO2 emissions in relation to various possible sustainability standards and policy targets.

•The fundamental standard for environmental sustainability with respect to the climate is the highest level of atmospheric concentration of CO2 that is compatible with climate stability. There is considerable uncertainty about this level. It would be safest to set a sustainability standard of the pre-industrial CO2 concentration (280 pm). If the calculation of the gap is based on the sustainability standard of returning to this level of concentration, which would require immediate and very great reductions in global CO2 emissions, the sustainability gap would be 78 pm CO2 (Houghton et al, 1996, Table1, p.15, gives the1994 atmospheric concentration of CO as 358 pm.)

•The first target of FCCC was to return CO2 emissions to their 1990 level by the year 2000. Recent projections (DETR, 2000, Table 1, p. 46) suggest that this will be achieved, so that the gap against this target is 0 mtC. The Kyoto Protocol to FCCC, agreed in December 1997, has committed the UK to a reduction of 12^% in the six scheduled GGs, one of which is CO2, by 2010. Again, recent projections (DETR, 2000, Table 1, p. 46) suggest that this will be achieved.

•The UK government has set an aspirational policy target of reducing CO2 emissions by 20% from 1990 levels by the year 2010. Comparing this target with the level of emissions projected by the UK government for that year (DETR, 2000, Table 1, p. 46), the policy gap is 22 mtC.

4.3. Global CO2 considerations

The UK government's 20% CO2 reduction target is motivated by a perception that CO2 emissions will be constrained beyond the 2008-2012 commitment period of the Kyoto Protocol, and a desire to be prepared for these constraints. It is not, and does not pretend to be, a sustainability target.

Because GGs have a global environmental effect, to be sustainable UK GG emissions must be compatible with a sustainable overall global emissions level. Calculating UK sustainability in terms of carbon emissions therefore entails the calculation of the sustainable global level of emissions and the UK share of this level.

The objective of the Framework Convention on Climate Change (FCCC) is "the stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system ... within a time frame sufficient to allow ecosystems to adapt naturally to climate change, to ensure that food production is not threatened and to enable economic development to proceed in a sustainable manner" (quoted in Vellinga and Grubb, 1993). Precisely what this means in terms of the atmospheric concentration of GGs like CO2 and methane is not spelt out. The most that could be done to meet the objective would be to return GG concentrations to their pre-industrial levels before significant climate change gets under way. Given that global CO2 emissions would immediately have to fall very sharply to achieve this, and in fact are still rising, this is not generally to be a feasible objective.

However, it may be possible to satisfy the sustainability requirement of climate stability by limiting both the absolute global temperature increase (affected by the cumulative total of emissions) and the rate of warming (affected by the rate of emissions). The global temperature appears not to have varied by more than 1°C in a century during the last 10 000 years, and the average global temperature during the last ice age was only 5°C below pre-industrial levels. The IPCC has concluded that a rate of warming of 0.1 °C per decade (about half current rates) is probably the maximum over time that many ecosystems could tolerate (IPCC, 1990, quoted in Hadley Centre, 1995, p. 11). In the absence of any more definitive information, these may be taken as the sustainability standards for the rate and level of global temperature increase to 2100.

Table 6 shows that a 'sustainable' level of cumulative carbon emissions (one that is consistent a 1°C warming limit) is 295 GtC (assuming anti-deforestation measures). This should be put in a context of already identified, economically recoverable fossil fuel reserves of over 1000 GtC and carbon emissions to 2100 that, without CO2 reduction policies, are likely to be 1500 GtC (Hare, 1997, Table 5, p. 10, and Table 8, p. 13, derived from Houghton et al., 1996). Clearly, climate stability will require much fossil fuel that has already been discovered not to be burned over the next 100 years, as a result of very substantial efforts at carbon reduction.

The conversion of a 100-year carbon budget to interim targets requires modeling using another whole range of assumptions about economic and population growth, and technical change.

The most detailed modeling exercise of this sort, which envisages a carbon budget in the above range is the Fossil Free Energy Scenario (FFES), elaborated in 1993 by the Stockholm Environment Institute, on commission from Greenpeace International, and reported in a study entitled Towards a Fossil-Free Energy Future (SEI, 1993). The commission stipulated that scenarios had to be consistent with a range of 'acceptable' emission trajectories on CO2, based on Green peace’s earlier work, all of which entailed zero use of fossil fuels by 2100. Some climate modeling suggested that these trajectories would restrict global mean temperature rise to a maximum of 1.7°C above 1990 levels, with temperatures falling from 2070, and that the decadal rate of temperature increase would fall below 0.1 °C by 2040 (SEI, 1993, Figure 3.5, p. 17). The trajectories are therefore somewhat less demanding than the physical targets in the previous paragraph, but they will be used here to illustrate how an SGAP for UK carbon emissions may be calculated.

4.4. A sustainable CO, emissions trajectory for the UK

Table 7 shows CO2 emissions from fossil fuel use for the world and the UK for various years. The columns for 1980, 1990 and 1995 for rows 1 and 2 are actual emissions. For the 2010, 2030 and 2100 columns, the first row gives the projected global emissions from the FFES study for those years. As noted above, this trajectory is identified for the purposes of this paper as one that is compatible with long-term environmental sustainability. The Trend entries in Row 2 for these columns gives UK emissions that will result if the trend that prevailed from 1980 to 1995 continues in a linear manner, entailing a reduction of 1.13 mtC per year. On this trend UK carbon emissions in 2100 will be 29 mtC, when the FFES (sustainable) level is 0. Extrapolating Trend beyond 2100, UK carbon emissions will fall to 0 in the year 2126.

The STrend entries indicate the linear trend that would be necessary for 1995 emissions to fall to 0 by 2100. The FTrend entries indicate what UK emissions would have to be to follow the trend of the FFES trajectory. This is the average trend that all countries would need to follow if the global FFES projections were to be realised. It may be noted that the UK government's current aspirational target of reducing carbon emissions t< of their 1990 level by 2010 implies a 2010 tar 127 mtC, which is in line with both the S and FTrend figures for that year.

However, be seen that between the years 2010 and 2(least UK emissions on the current Trend v substantially in excess of these 'sustainability’ trends. Finally for Row 2 the DETR entry 2010 column, taken from the UK governr Draft Climate Change Programme, indicate 2010 emissions are actually forecast to be sul tially (25 mtC) above even the Trend figu: that year. It seems unlikely that the trend i past will be followed, let alone greater c reductions be achieved, without substantivt policy measures.

Of the figures given, only FTrend relates ble UK emissions to the globally sustainabl jectory of FFES This relation is such the countries, developed and developing, will he reduce their emissions at least according to this trend for the FFES trajectory to be realised. This is highly unrealistic, not least because the FCCC is based explicitly on the understanding that industrial countries will start to reduce their emissions while those from developing countries continue to grow. For the FFES trajectory to be attained with developing country emissions growing, perhaps until 2010, industrial countries will have to reduce their emissions significantly faster than the FFES trend. To decide how much industrial countries should reduce their emissions, and how much those from developing countries can grow, some explicit mechanism is required to share our global carbon emissions.

Any formula for sharing the world's carbon emissions is likely to be disputed. Some will argue that it should be on the basis of current shares. Some will argue that it should be related to a country's share of world GDP. Others still will want it related to a country's share of world population. The 'environmental space' methodology (see Opschoor and Weterings, 1994, 1995, for a theoretical discussion), which has been widely used to calculate global environmental entitlements and limits (e.g. Buitenkamp et al., 1993; FOE, 1995; McLaren et al., 1998; Sachs et al., 1998), normally calls for world carbon emission rights to be allocated on an equal per capita basis.

In 1990 the UK population was about 57 million out of a world population of 5.3 billion (WRI, 1994, Table 16.1 p. 268). The UK therefore had about 1.1% of the world's population. However, Table 7 shows that in that year its carbon emissions were 2.7% of the world total. This means that UK carbon emissions per head were nearly 2.5 times the world average.

The world population is currently expected to reach 8.9 billion in 2050 and 9.5 billion by 2100 (UNPD, 1999), but that in the UK is expected to stay relatively constant. Row 3 of Table 7 shows how the UK's share of world population will decline, from 1.1 to 0.63% from 1990 to 2100, if its population stabilises at 60 million by 2050, but that of the world grows according to the UN's medium projection.

Considerations of fairness seem to argue for relating CO2 emission rights in some way to population. So do considerations of political pragmatism. As developing country emissions increase, it will be imperative, if global emissions are to be constrained, that they accept emission limits under the FCCC. It seems most unlikely that they will be prepared to do so under any formula that is not related to population. Their preferred formula would probably be globally equal per capita emissions.

On the other hand, it hardly seems desirable for a carbon reduction formula to be such as to enable countries to increase their share of carbon emissions by increasing their share of the global population. Given that population growth is an important driver of carbon emissions, it would be better for the formula to give incentives for the restraint of population growth.

For these reasons it seems desirable to relate a country's share of world carbon emission rights to its share of world population in a certain year. This establishes the principle of equal per capita emissions rights for that year. Thereafter, countries whose populations grow more slowly than the world average will have higher per capita emission rights than countries with higher than average population growth. With regard to which year should be chosen, there seems some logic in choosing 1990, which is the FCCC reference year for carbon emissions. For the UK, which formula is chosen makes a substantial difference to its sustainable emissions trajectory, as a share of the FFES global trajectory. Row 4 gives its sustainable emissions with the UK share calculated as 1.1% of the global total, where 1.1% was its population share in 1990. Row 5 gives its sustainable emissions with the UK share calculated according to its current share of world population, which falls to 0.77% in 2030 and 0.63% in 2100. Its 2030 'sustainable emissions' fall to 20 mtC under this formula, from 28 mtC under the constant 1990 share formula.

Rows 1-5 enable various 'sustainability gaps' (SGAPs) to be calculated for the UK, where SGAP is the distance from the current situation, or from the projected outcome in the future according to the current trend (Trend in Table 7), and the relevant sustainability standard.

SGAP1 takes the sustainability standard to be FTrend. SGAP2 takes the sustainability standard to be the result of giving the UK 1.1% of sustainable global emissions (Row 4). SGAP3 takes the sustainability standard to be the -result of allocating the UK a share corresponding to its share of the global population (Row 5).

In can be seen that the current Trend gives an SGAP in 2030 of 46 mtC related to FTrend (SGAP1), 80 mtC related to a 1.1% share of global emissions (SGAP2) and 88 mtC relating the UK share to its share of world population in the year in question (SGAP3). The SGAPs in 2010 are smaller but still substantial, apart from SGAP1, but this small gap depends on the 1980-95 trends being maintained. If the DETR forecast of 2010 emissions on current policies is correct, even SGAP1 in 2010 rises to 28 mtC. Projecting the Trend beyond 2100 shows that it will be 126 years from 2000 before the UK reaches a zero emissions level, which for sustainability will need to remain 0 until carbon sequestration from the atmosphere has reduced atmospheric CO2 concentrations to the 350 pm that corresponds to the upper warming limit of 1°C. Unless and until the science of global warming and its possible impacts starts to rule out some of the potentially disastrous outcomes that can currently be envisaged, this would appear to be the kind of long-term sustainability goal that is in accordance with a precautionary approach to environmental policy.

5. The sustainability gap in monetary terms

While the calculation of the SGAP, NSGAP and YS indicators relate the environmental indicators to the concept of sustainability and, therefore, to sustainable development, none of the physical SGAP indicators give any idea of the economic implications of a sustainability gap or of attempts to reduce it. They give no insights into the scale of the economic resources that might be required, relative to the overall scale of the economy, to bring environmental performance close to the defined sustainability standards.

In order to generate these economic insights, detailed microeconomic analysis is required of the technologies that could be employed to reduce environmental pressures. In general, a number of different technologies are available to reduce environmental impacts, with different levels of potential impact reduction at different costs. Table 1 and Table 5 gave some information on costs for some of these technologies for SO2 and CO2. Putting the microeconomic analyses of these technologies together, it is possible to build up a marginal cost curve of pressure reduction (e.g. emission abatement) on the assumption that the cheapest technologies are implemented first. These marginal cost curves are, of course, affected by technical change, and they can be used to gain insights into the economic implications of such change by projecting their future development.

Marginal cost curves have been derived for carbon dioxide, sulphur dioxide and nitrogen oxides (Simon, 1998). In principle these cost curves could be applied to the physical SGAPs estimated for the different pollutants in order to derive a monetary equivalent for the physical SGAP indicators. These monetary SGAPs (MSGAPs) could then be aggregated across different physical indicators within the same environmental theme (e.g. the different greenhouse gases that contribute to global warming) to arrive at an MSGAP for the environmental theme as a whole. The environmental theme SGAPs could then be aggregated together to derive an overall monetary SGAP (OMSGAP) for economic activity as a whole. As is made clear in Ekins, 2000 (p. 146), it is important to note that the OMSGAP figure thus derived is not commensurable with GNP or the other national accounting product aggregates, and could not therefore be subtracted from, say, NNP, in order to produce a 'sustainable income', or 'Green GNP' figure. It is also important to note that OMSGAP does not represent the amount of money that would have to be spent to achieve sustainability. The OMSGAP is very much a static, partial equilibrium calculation, representing at a moment in time the aggregation of expenditures that would need to be made to reduce the various dimensions of the physical sustainability gap to zero.

The OMSGAP would be an expressive indicator of the potential of an economy, at a certain moment in time, to achieve environmental sustainability. It would reflect both the physical distances from environmental sustainability and the economic possibilities of reducing those distances. Over time, OMSGAP would decrease if either the physical sustainability gaps decreased, or new technologies, processes or materials were developed which enabled those gaps to be reduced at lower cost in the future. OMSGAP/GNP, either in aggregate or for each environmental theme, would also be an interesting indicator with which to make inter-country comparisons of environmental efficiency, in much the same way as energy intensity (Energy Use/GNP) is currently used.

6. Conclusion

This paper has developed a number of new indicators of environmental sustainability which may be used to assess whether the development process of an economy is sustainable or not. The indicators are based on a perception that objective standards of environmental sustainability, within acceptable margins of error and uncertainty, may be derived from considerations of ecology, environmental science and environmental impacts on human health. Comparisons of current states of and pressures on the environment with these standards may then be made, leading to the derivation of physical 'sustainability gaps' (SGAPs) for the environmental issues in question. Development can only be said to be sustainable when the SGAPs, across all environmental themes of concern, are either zero, or may be projected to fall to zero within an acceptable time scale.

For the UK, the paper has shown that SGAPs currently exist with regard to CO2, SO2 and other air pollutants. None of these emissions in the UK can be said to be sustainable.

However, for the local air pollutants the situation is improving, albeit in some cases only slowly. While currently UK CO2 emissions are also decreasing, projections indicate that they may not continue to do so. Even so, on the current trend there is a substantial CO2 SGAP until the third decade of the next century. Given the climate change that is already apparent, it may well be that climatic conditions will have changed drastically and irreversibly before the SGAP has been closed.

By combining the physical SGAP indicators with figures for the marginal cost of abatement, avoidance or environmental restoration, a monetary indicator of the overall distance to sustainability (OMSGAP) may be derived. Expressed as a ratio with respect to GDP, OMSGAP would give a meaningful indication of the relative environmental sustainability of economic performance.

The indicators SGAP, YS and OMSGAP are potentially useful new indicators of environmental sustainability. Until these, or similar, indicators, come to be employed, it will not be possible to judge whether development is in fact becoming environmentally sustainable and how far current performance is from environmental sustainability. While much of the data to compute the indicators already exists, a considerable statistical effort is still needed in the UK and elsewhere to address the complexities identified in the paper, so that they may be derived across all relevant environmental themes.

Acknowledgements

The research reported in this paper received financial support from WWF-UK and the Environmental Research Trust, to whom gratitude is warmly expressed.

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