Nuclear Versus Coal plus CCS: a Comparison of Two Competitive Base-Load Climate Control Options

In this paper, we analyze the relative importance and mutual behavior of two competing base-load electricity generation options that each are capable of contributing significantly to the abatement of global CO₂ emissions: nuclear energy and coal-based power production complemented with CO₂ capture and storage (CCS). We also investigate how, in scenarios developed with an integrated assessment model that simulates the economics of a climate-constrained world, the prospects for nuclear energy would change if exogenous limitations on the spread of nuclear technology were relaxed. Using the climate change economics model World Induced Technical Change Hybrid, we find that until 2050 the growth rates of nuclear electricity generation capacity would become comparable to historical rates observed during the 1980s. Given that nuclear energy continues to face serious challenges and contention, we inspect how extensive the improvements of coal-based power equipped with CCS technology would need to be if our economic optimization model is to significantly scale down the construction of new nuclear power plants.     

CO2 Capture and Storage with Leakage in an Energy-Climate Model

Geological CO₂ capture and storage (CCS) is among the main near-term contenders for addressing the problem of global climate change. Even in a baseline scenario, with no comprehensive international climate policy, a moderate level of CCS technology is expected to be deployed, given the economic benefits associated with enhanced oil and gas recovery. With stringent climate change control, CCS technologies will probably be installed on an industrial scale. Geologically stored CO₂, however, may leak back to the atmosphere, which could render CCS ineffective as climate change reduction option. This article presents a long-term energy scenario study for Europe, in which we assess the significance for climate policy making of leakage of CO₂ artificially stored in underground geological formations. A detailed sensitivity analysis is performed for the CO₂ leakage rate with the bottom-up energy systems model MARKAL, enriched for this purpose with a large set of CO₂ capture technologies (in the power sector, industry, and for the production of hydrogen) and storage options (among which enhanced oil and gas recovery, enhanced coal bed methane recovery, depleted fossil fuel fields, and aquifers). Through a series of model runs, we confirm that a leakage rate of 0.1%/year seems acceptable for CCS to constitute a meaningful climate change mitigation option, whereas one of 1%/year is not. CCS is essentially no option to achieve CO₂ emission reductions when the leakage rate is as high as 1%/year, so more reductions need to be achieved through the use of renewables or nuclear power, or in sectors like industry and transport. We calculate that under strict climate control policy, the cumulative captured and geologically stored CO₂ by 2100 in the electricity sector, when the leakage rate is 0.1%/year, amounts to about 45,000 MtCO₂. Only a little over 10,000 MtCO₂ cumulative power-generation-related emissions are captured and stored underground by the end of the century when the leakage rate is 1%/year. Overall marginal CO₂ abatement costs increase from a few €/tCO₂ today to well over 150 €/tCO₂ in 2100, under an atmospheric CO₂ concentration constraint of 550 ppmv. Carbon costs in 2100 turn out to be about 40 €/tCO₂ higher when the annual leakage rate is 1%/year in comparison to when there is no CO₂ leakage. Irrespective of whether CCS deployment is affected by gradual CO₂ seepage, the annual welfare loss in Europe induced by the implementation of policies preventing “dangerous anthropogenic interference with the climate system” (under our assumption, implying a climate stabilisation target of 550 ppmv CO₂ concentration) remains below 0.5% of GDP during the entire century.

Modeling a Carbon Capture,Transport, and Storage Infrastructure for Europe

In this paper, we develop a model to analyze the economics of carbon capture, transport, and storage (CCTS) in the wake of expected rising CO₂ prices. We present a scalable mixed integer, multiperiod, welfare-optimizing network model for Europe, called CCTS-Mod. The model incorporates endogenous decisions on carbon capture, pipeline and storage investments, as well as capture, flow and injection quantities based on given costs, CO₂ prices, storage capacities, and point source emissions. Given full information about future costs of CCTS-technology, and CO₂ prices, the model determines a cost minimizing strategy on whether to purchase CO₂ certificates, or to abate the CO₂ through investments into a CCTS-chain on a site by site basis. We apply the model to analyze different scenarios for the deployment of CCTS in Europe, e.g., under high and low CO₂ prices, respectively. We find that beyond CO₂ prices of €50 per t, CCTS can contribute to the decarbonization of Europe’s industry sectors, as long as one assumes sufficient storage capacities (onshore and/or offshore). We find that CCTS is only viable for the power sector if the CO₂ certificate price exceeds €75 per t.     

Atmospheric Change and Biodiversity

Strategies to conserve biodiversity need to include the monitoring, modelling, adaptation and regulation of the composition of the atmosphere. Atmospheric issues include climate variability and extremes; climate change; stratospheric ozone depletion; acid deposition; photochemical pollution; suspended particulate matter; and hazardous air pollutants. Coarse filter and fine filter approaches have been used to understand the complexity of the interactions between the atmosphere and biodiversity. In the first approach, climate-based models, using GIS technology, helped create future biodiversity scenarios under a 2 × CO₂ atmosphere. In the second approach, the SI/MAB forest biodiversity monitoring protocols helped calibrate the climate-forest biodiversity baseline and, as global diagnostics, helped identify where the biodiversity was in equilibrium with the present climate. Forest climate monitoring, an enhancing protocol, was used in a co-location approach to define the thermal buffering capacity of forest ecosystems and their ability to reduce and ameliorate global climate variability, extremes and change.     

Development of environmental impact monitoring protocol for offshore carbon capture and storage (CCS): A biological perspective

Offshore geologic storage of carbon dioxide (CO₂), known as offshore carbon capture and sequestration (CCS), has been under active investigation as a safe, effective mitigation option for reducing CO₂ levels from anthropogenic fossil fuel burning and climate change. Along with increasing trends in implementation plans and related logistics on offshore CCS, thorough risk assessment (i.e. environmental impact monitoring) needs to be conducted to evaluate potential risks, such as CO₂ gas leakage at injection sites. Gas leaks from offshore CCS may affect the physiology of marine organisms and disrupt certain ecosystem functions, thereby posing an environmental risk. Here, we synthesize current knowledge on environmental impact monitoring of offshore CCS with an emphasis on biological aspects and provide suggestions for better practice. Based on our critical review of preexisting literatures, this paper: 1) discusses key variables sensitive to or indicative of gas leakage by summarizing physico-chemical and ecological variables measured from previous monitoring cruises on offshore CCS; 2) lists ecosystem and organism responses to a similar environmental condition to CO₂ leakage and associated impacts, such as ocean acidification and hypercapnia, to predict how they serve as responsive indicators of short- and long-term gas exposure, and 3) discusses the designs of the artificial gas release experiments in fields and the best model simulation to produce realistic leakage scenarios in marine ecosystems. Based on our analysis, we suggest that proper incorporation of biological aspects will provide successful and robust long-term monitoring strategies with earlier detection of gas leakage, thus reducing the risks associated with offshore CCS.     

Robust Energy Portfolios Under Climate Policy and Socioeconomic Uncertainty

Concerning the stabilization of greenhouse gases, the UNFCCC prescribes measures to anticipate, prevent, or minimize the causes of climate change and mitigate their adverse effects. Such measures should be cost-effective and scientific uncertainty should not be used as a reason for postponing them. However, in the light of uncertainty about climate sensitivity and other underlying parameters, it is difficult to assess the importance of different technologies in achieving robust long-term climate risk mitigation. One example currently debated in this context is biomass energy, which can be used to produce both carbon-neutral energy carriers, e.g., electricity, and at the same time offer a permanent CO₂ sink by capturing carbon from the biomass at the conversion facility and permanently storing it. We use the GGI Scenario Database IIASA [3] as a point of departure for deriving optimal technology portfolios across different socioeconomic scenarios for a range of stabilization targets, focusing, in particular, on new, low-emission scenarios. More precisely, the dynamics underlying technology adoption and operational decisions are analyzed in a real options model, the output of which then informs the portfolio optimization. In this way, we determine the importance of different energy technologies in meeting specific stabilization targets under different circumstances (i.e., under different socioeconomic scenarios), providing valuable insight to policymakers about the incentive mechanisms needed to achieve robust long-term climate risk mitigation.     

Constructing “not Implausible” Climate and Economic Scenarios for Egypt

A space of “not-implausible” scenarios for Egypt's future under climate change is defined along two dimensions. One depicts representative climate change and climate variability scenarios that span the realm of possibility. Some would not be very threatening. Others portend dramatic reductions in average flows into Lake Nassar and associated increases in the likelihood of year to year shortfalls below critical coping thresholds; these would be extremely troublesome, especially if they were cast in the context of increased political instability across the entire Nile Basin. Still others depict futures along which relatively routine and relatively inexpensive adaptation might be anticipated. The ability to adapt to change and to cope with more severe extremes would, however, be linked inexorably to the second set of social–political–economic scenarios. The second dimension, defined as “anthropogenic” social/economic/political scenarios describe the holistic environment within which the determinants of adaptive capacity for water management, agriculture, and coastal zone management must be assessed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Wildfire-induced reduction in the carbon storage of Mediterranean ecosystems: An application to brush and forest fires impacts assessment

Wildfire is one of the most dangerous and harmful phenomena in the world. Hence, fire impacts assessment could become very important in forest areas according to its environmental and landscape values. This paper suggests an approach to identify fire effects on biomass, in consonance with the potential carbon storage of each area used, and its biomass consumption based on fire behavior.Dense mature forests were the most vulnerable landscapes based on its aboveground biomass, mainly tree stem biomass. A significant correlation was found between fire intensity and biomass consumption. Biomass consumption ranged from 16.59% to 98.75% from the two studied wildfires. It is necessary to provide a scenario analysis according to the uncertain CO₂ market. As an example, carbon storage impacts in one fire were between 100,340.66 € (using the minimum price of CO₂) and 741,057.44 € (using the maximum price of CO₂). Differences between scenarios ranged from 35.30% to 46.51% of the total carbon storage impacts. This approach might be a solution to identify and prioritize areas for restoration activities and optimize the allocation of the resources.     

Numerical Modeling of a Potential Geological CO 2 Sequestration Site at Minden (Germany)

We study opportunities for CO₂ sequestration in geological formations of the state North Rhine Westphalia in Germany. Simulations are performed for evaluating a potential site within the Bunter sandstone formation near the town of Minden in a depth?of around 3,000 m using the numerical simulator TOUGHREACT. Our focus is on three CO₂ storage mechanisms: (1) hydrodynamic trapping, (2) dissolution trapping, and (3) mineral trapping. The results show that due to buoyancy the injected CO₂ phase initially migrates towards the top of the reservoir and is hydrodynamically trapped beneath the confining layer of the cap rock. Then, the CO₂ spreads laterally and dissolves partially in the formation water. The dissolution of CO₂ results in an increase of the density of the brine causing a downward migration until it settles after 10,000 years at the bottom of the reservoir. The simulations indicate that after 10,000 years, 15% (17 Mt) from a total of 114 Mt injected CO₂ are trapped hydrodynamically, 20% (23 Mt) are trapped by dissolution, and 65% (74 Mt) are fixed in newly formed carbonates such as dawsonite, ankerite, and siderite. Within our study pressure increases near the injection well by a factor of 1.1 which is lower than the upper limit usually accepted in gas storage operations. The mineral reactions cause a net decrease of porosity and in turn a decrease of permeability down to 9% of the initial value in parts of the reservoir.     

Meta-Modeling to Assess the Possible Future of Paris Agreement

In the meta-modeling approach, one builds a numerically tractable dynamic optimization or game model in which the parameters are identified through statistical emulation of a detailed large scale numerical simulation model. In this paper, we show how this approach can be used to assess the economic impacts of possible climate policies compatible with the Paris Agreement. One indicates why it is appropriate to assume that an international carbon market, with emission rights given to different groups of countries will exist. One discusses the approach to evaluate correctly abatement costs and welfare losses incurred by different groups of countries when implementing climate policies. Finally, using a recently proposed meta-model of game with a coupled constraint on a cumulative CO₂ emissions budget, we assess several new scenarios for possible fair burden sharing in climate policies compatible with the Paris Agreement.     

Mesoscale ecohydrological modelling to analyse regional effects of climate change

Hydrological processes and crop growth were simulated for the state of Brandenburg (Germany) using the hydrological/vegetation/water quality model SWIM, which can be applied for mesoscale river basins or regions. Hydrological validation was carried out for three mesoscale river basins in the area. The crop growth module was validated regionally for winter wheat, winter barley and maize. After that the analysis of climate change impacts on hydrology and crop growth was performed, using a transient 1.5 K scenario of climate change for Brandenburg and restricting the crop spectrum to the three above mentioned crops. According to the scenario, precipitation is expected to increase. The impact study was done comparing simulation results for two scenario periods 2022–2030 and 2042–2050 with those for a reference period 1981–1992. The atmospheric CO₂ concentrations for the reference period and two scenario periods were set to 346, 406 and 436 ppm, respectively. Two different methods – an empirical one and a semi-mechanistic one – were used for adjustment of net photosynthesis to altered CO₂. With warming, the model simulates an increase of evapotranspiration (+9.5%, +15.4%) and runoff (+7.0%, +17.2%). The crop yield was only slightly altered under the climate change only scenario (no CO₂ fertilization effect) for barley and maize, and it was reduced for wheat (–6.2%, –10.3%). The impact of higher atmospheric CO₂ compensated for climate-related wheat yield losses, and resulted in an increased yield both for barley and maize compared to the reference scenario. The simulated combined effect of climate change and elevated CO₂ on crop yield was about 7% higher for the C₃ crops when the CO₂ and temperature interaction was ignored. The assumption that stomatal control of transpiration is taking place at the regional scale led to further increase in crop yield, which was larger for maize than for wheat and barley. The regional water balance was practically not affected by the partial stimulation of net photosynthesis due to higher CO₂, while the introduction of stomatal control of regional transpiration reduced evapotranspiration and enlarged notably runoff and ground water recharge.     

Energy forecasting and atmospheric CO2 perspectives: two worlds ignore each other

Macroeconomic models predict that the global primary energy demand will increase by a factor of 2–4 by the year 2050. In contrast, climate analyses made by the IPCC claim that CO₂ emissions in 2050 should not exceed the values of 1990 or even be 20% lower. By 2100 emissions should be reduced to one third of the present value. The common wisdom to deal with these opposing trends is the concept of de-carbonization, i.e., the continuous decrease of the carbon emission per unit energy utilization. De-carbonization rates needed to compensate for the growing demand while keeping the CO₂-emissions constant should at least be 2% per year compared to actual values of 0.3%. The potential of different de-carbonization rate measures is analyzed. It is argued that the goal can only be met if per capita energy utilization in the industrialized countries is significantly reduced from their typical level of 5000–10 000 W. As a realistic target we suggest 2000 Watt per capita, the present global average. This would leave expansion capacity for the developing countries which presently have per capita demand between 300 and 1000 W. Based on the example of Switzerland it is shown that the two key issues to attain this goal are the quality of buildings and the demand for mobility. It is concluded that the conversion of the present energy system into a 2000 W system is neither limited by technology nor by finances but by the acceptance of a new life style in which energy is used more efficiently and more intelligently than today. This revised version was published online in July 2006 with corrections to the Cover Date.     

Evaluating Uncertain CO2 Abatement over the Very Long Term

Climate change research with the economic methodology of cost–benefit analysis is challenging because of valuation and ethical issues associated with the long delays between CO₂ emissions and much of their potential damages, typically of several centuries. The large uncertainties with which climate change impacts are known today and the possibly temporary nature of some envisaged CO₂ abatement options exacerbate this challenge. For example, potential leakage of CO₂ from geological reservoirs, after this greenhouse gas has been stored artificially underground for climate control reasons, requires an analysis in which the uncertain climatic consequences of leakage are valued over many centuries. We here present a discussion of some of the relevant questions in this context and provide calculations with the top–down energy-environment-economy model DEMETER. Given the long-term features of the climate change conundrum as well as of technologies that can contribute to its solution, we considered it necessary extending DEMETER to cover a period from today until the year?3000, a time span so far hardly investigated with integrated assessment models of climate change.     

On a Holistic Modeling Approach for Managing Carbon Emission Ecosystems

Effective use of historical volumes of heterogeneous and multidimensional data is a major challenge, especially projects associated with potential applications of carbon emission ecosystems. Data science in these applications becomes tedious when such varied data are accumulated and or distributed in multiple domains. Design, development, and implementation of sustainable geological storages are crucial for managing carbon dioxide (CO₂) emissions and its modeling process. The purpose of the research is to address major challenges and how best a robust “ontology-based multidimensional data warehousing and mining” approach can resolve issues associated with carbon ecosystems. The conceptualized relationships deduced among multiple domains, integration of domain ontologies, data mining, visualization, and interpretation artefacts are highlights of the study. Several data, plot, and map views are extracted from metadata storage for interpreting new knowledge on carbon emissions. Statistical mining models describe data attributes’ correlations, patterns, and trends that can help in predicting future forecast of CO₂ emissions worldwide.     

Changes in Near-Surface Temperature and Sea Level for the Post-SRES CO2-Stabilization Scenarios

Changes in global near-surface temperature and sea level are calculated from 2000 to 2100 for the Post-SRES (Special Report on Emissions Scenarios) scenarios that stabilize the CO₂ concentration early in the 22nd century. Seven stabilization scenarios are examined together with their corresponding SRES marker scenarios – A1, A1/S450, A1/S550, A1/S650, A2, A2/S550, A2/S750, B1, B1/S450, B2, and B2/S550 – where the number following the S indicates the stabilized CO₂ concentration in parts per million by volume (ppmv). The calculations are performed using an energy-balance-climate/upwelling-diffusion-ocean model for three values of the climate sensitivity, ΔT _2x =1.5, 2.5 and 4.5°C. The resulting reductions in global warming and sea-level rise for the stabilization scenarios relative to their corresponding marker scenario increases with ΔT _2x and are greater the lower the stabilized CO₂ concentration. For the S550 stabilization scenarios, the reductions in global warming and sea-level rise in 2100 range from 0.29°C and 3.31 cm for B2/S550 with ΔT _2x =1.5°C, to 1.23°C and 11.81 cm for A2/S550 with ΔT _2x =4.5°C. The percent reductions for the global warming and sea-level rise for each stabilization scenario are almost independent of ΔT _2x and range respectively from about 16% and 12% for the A1/S650 scenario to about 39% and 30% for the A1/S450 scenario. The geographical distributions of near-surface temperature change are constructed using a method to superpose the patterns simulated by our atmospheric general-circulation/mixed-layer-ocean model, individually for doubled CO₂ concentration and decupled SO₄ burden. Results are illustrated for the B2 and B2/S550 scenarios for ΔT _2x =2.5°C. The near-surface temperature changes of the B2/S550 scenario in 2100 are everywhere smaller than those for the B2 scenario, with values ranging from about 0.3°C in the tropics to 0.5°C over Antarctica and 0.7°C in the Arctic. The global results of this study are available on the web at: http://crga.atmos.uiuc.edu/research/post-sres.html. We would be pleased to collaborate with other researchers in using these results in impact and integrated-assessment studies. This revised version was published online in July 2006 with corrections to the Cover Date.     

Impacts of ocean CO2 disposal on marine life: I. A toxicological assessment integrating constant‐concentration laboratory assay data with variable‐concentration field exposure

Feasibility studies suggest that the concept of capturing CO₂ from fossil fuel power plants and discharging it to the deep ocean could help reduce atmospheric CO₂ concentrations. However, the local reduction in seawater pH near the point of injection is a potential environmental impact. Data from the literature reporting on toxicity of reduced pH to marine organisms potentially affected by such a plume were combined into a model expressing mortality as a function of pH and exposure time. Since organisms exposed to real plumes would experience a time‐varying pH, methods to account for a variable exposure were reviewed and a new method developed based on the concept of isomortality. In part II of this paper, the method is combined with a random‐walk model describing the transport of passive organisms through a low pH plume leading to a Monte‐Carlo‐like risk assessment which is applied to several candidate CO₂ injection scenarios.     

The Kyoto Protocol and non-CO2 Greenhouse Gases and Carbon Sinks

Many trace constituents other than carbon dioxide affect the radiative budget of the atmosphere. The existing international agreement to limit greenhouse gases, the Kyoto Protocol, includes carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF₆) and credit for some carbon sinks. We investigate technological options for reducing emissions of these gases and the economic implications of including other greenhouse gases and sinks in the climate change control policy. We conduct an integreated assessment of costs using the MIT Emissions Prediction and Policy Analysis (EPPA) model combined with estimates of abatement costs for non-CO₂ greenhouse gases and sinks. We find that failure to take advantage of the other gas and sink flexibility would nearly double aggregate Annex B costs. Including all the GHGs and sinks is actually cheaper than if only CO₂ had been included in the Protocol and their inclusion achieves greater overall abatement. There remains considerable uncertainty in these estimates, the magnitude of the savings depends heavily on reference projections of emissions, for example, but these uncertainties do not change the overall conclusion that non-CO₂ GHGs are an important part of a climate control policy.     

The Influence of Temperature on Chemical Fluid-Rock Reactions in Geological CO2 Sequestration

For a Bunter formation in the German Federal State of North Rhine Westphalia, we use numerical models to consider reactions between the supercritical, aqueous, and solid phases. These reactions may occur in a CO₂-water system representing a saline aquifer CO₂ storage scenario. Thus, the models are used for determining the extent of fluid–rock reactions during mineral dissolution or precipitation. In particular, we study the effect of temperature by comparing results for our system set at 100 °C and at 58 °C. Results show that the abundance of dissolved ions changes as a result of elevated temperature. For the entire 10,000-year simulation period, the overall geochemical behavior of the Bunter reservoir rock at the Minden site is explained in terms of different mineral transformations, although some of them are not changed significantly. This mainly comprises the alteration of carbonate minerals such as calcite, and aluminium silicates such as oligoclase, chlorite, illite, albite, kaolinite, and Na-smectite. Another chemical behavior derives from the generation and consumption of new secondary minerals such as dawsonite, pyrite, and Ca-smectite. In contrast to a system temperature of 58 °C, the mineralogical transformations of other minerals such as siderite, ankerite, dolomite, and magnesite are not observed at 100 °C. Also, the numerical simulation results show that at elevated temperature, the dominant role played by hydrodynamic mechanism dwarfs the role of other trapping mechanisms including dissolution and mineralization. Results also demonstrate how geological, petrophysical, and geochemical data can be integrated to estimate quantitatively the magnitude of the fluid–rock reactions. These reactions may entail new geotechnical problems, such as rock self-fracturing which ultimately decreases the CO₂ sequestration projects security.     

Spatial and Temporal Performance of the MiniFACE (Free Air CO2 Enrichment) System on Bog Ecosystems in Northern and Central Europe

The Bog Ecosystem Research Initiative (BERI) projectwas initiated to investigate, at five climaticallydifferent sites across Europe, the effects of elevatedCO₂ and N deposition on the net exchange ofCO₂ and CH₄ between bogs and the atmosphere,and to study the effects of elevated CO₂ and Ndeposition on the plant biodiversity of bogcommunities. A major challenge to investigate theeffects of elevated CO₂ on vegetation andecosystems is to apply elevated CO₂concentrations to growing vegetation without changingthe physical conditions like climate and radiation.Most available CO₂ enrichment methods disturb thenatural conditions to some degree, for instance closedchambers or open top chambers. Free Air CO₂Enrichment (FACE) systems have proven to be suitableto expose plants to elevated CO₂ concentrationswith minimal disturbance of their natural environment.The size and spatial scale of the vegetation studiedwithin the BERI project allowed the use of a modifiedversion of a small FACE system called MiniFACE. Thispaper describes the BERI MiniFACE design as well asits temporal and spatial performance at the five BERIfield locations. The temporal performance of theMiniFACE system largely met the quality criteriadefined by the FACE Protocol. One minute averageCO₂ concentrations measured at the centre of thering stayed within 20% of the pre-set target for morethan 95% of the time. Increased wind speeds werefound to improve the MiniFACE system's temporalperformance. Spatial analyses showed no apparentCO₂ gradients across a ring during a 4 day periodand the mean differences between each sampling pointand the centre of the ring did not exceed 10%.Observations made during a windy day, causing aCO₂ concentration gradient, and observations madeduring a calm day indicated that short term gradientstend to average out over longer periods of time. On aday with unidirectional strong winds, CO₂concentrations at the upwind side of the ring centrewere higher than those made at the centre and at thedownwind side of the ring centre, but the bell-shapeddistribution was found basically the same for thecentre and the four surrounding measurement points,implying that the short term (1 sec) variability ofCO₂ concentrations across the MiniFACE ring isalmost the same at any point in the ring. Based on gasdispersion simulations and measured CO₂concentration profiles, the possible interferencebetween CO₂-enriched and control rings was foundto be negligible beyond a centre-to-centre ringdistance of 6 m.