Emission scenarios and global climate protection

Global Environmenlol

Change, Vol. 6, No. 4, pp. 305334.

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Pergamon

Cotwinht

SO959-3780(96)00030-l

0

1996 Elsvier

1996

Science Ltd

Printed in Great Britain. All rights rcwved 0959-3780/96 $15.00 + 0.00

Emission scenarios and global climate protection Joseph Alcamo and Eric Kreileman This paper evaluates the effec6veness of a wide range of emission scenarios in protecting climate (where ‘Protecting climate Is used here to mean minimizing ‘dangerous anthropogenk Interference wlth the climate system’ which results In impacts to society and the natural environment). Under baseline (no action) conditions there is a slgntftcant increase in and climate emissions, temperature impacts. Ccntroiiing only CO2 emissions (ie freezing emissions in year 2ooo at 1996 levels, and decreasing them afterwards at 1 %/yr) and only in Annex I countrfes, does not signtficantiy reduce the impacts observed under ths baseline scenario. However, Impacts are sLIbotalltiaiiy reduced when emissions are controiied in bolh Annex I and non-Annex I countries, and when both COx and non-CO, emissions are controiied. it was also found that stabiiixing COx in the afmosphere below 466ppm substantially reduces climate impacts. But in order to follow the pathway to stabittxation at 466ppm specified by the IPCC, global emissions can only slightly increase in the coming decades, and thsn must be sharply reduced. On the other hand, stabilizing COx In the atmosphere above 460ppm can have slgnifkant impacts, whkh indicates that stabilixation of greenhouse gases in the atmosphere will not nscessariiy provide a htgh level of climate protection. Resutts from these and other scenarios are synthesized and related to climate protection goals through a new concept ‘s& emlsdon corridors’. These corridors indicate the aiiowabie range of near-term global emissions (equivalent CO3 which compiles with qecifled short- end iongterm ctlmatsgoals. For an iiiustra6ve sst of climate goals, the allowable anthropogenk global emissions in 2010 are computed to range from 7.3 to 14SGtCIyr equivalent CO1 (1996 level = approximately 9.6GtCl yr); when these iimlts are set twice as strkt (ie divided by two), the allowable range becomes 7.6 to 9.3Gt Clyr. To fafi within this lower corridor, global emissions must ba lower in 2010 than in 1996. Copyright 0 1996 Eisevier Bcience Ltd

The Framework Convention on Climate Change (FCCC) provides a backdrop for climate negotiations and the process for adopting climate policies. Although the Convention has many passages that speak of long-term climate protection, there is a critical gap between the long time scales of the earths climate system (by human standards) and the much shorter time scales of climate policy making. The basic problem is that climate change and its impacts will unfold over several decades or centuries, while climate policies deal largely with the immediate political and economic situation of different countries. The general aim of this paper is to bridge this gap by using results from an integrated climate model that couples emission scenarios with long-term climate protection. Here we take climate protection to mean minimizing ‘dangerous anthropogenic interference with the climate system’ which results in impacts to society and the natural environment. This paper has two specific objectives: 1. 2.

To evaluate the effectiveness of different emission pathways in achieving both short- and long-term goals for climate protection. To identify the allowable range of emissions in the short run that would achieve short- and long-term climate protection goals.

To accomplish the first objective, we evaluate the environmental consequences of different emission scenarios. We do this by running an integrated global change model in the ‘forward’ direction, that is, we first make assumptions about driving forces such as population, or about emission pathways, and then run the model to compute climate change and its impacts. This is described in the first part of this paper. To accomplish the second objective, we first specify climate goals and then run the model ‘backwards’ to compute the allowable range of emissions that would achieve these goals. This allowable range is called a ‘safe emission corridor’, and is the topic of the second part of this paper. This paper is unique in the wide range of different policy scenarios it deals with, and in the methods it presents to compare and evaluate these scenarios. It is also fairly unique in that it uses calculations from an integrated global change model, IMAGE 2, which (1) explicitly couples emissions, climate change and its impacts; (2) provides global information with geographic and regional detail; (3) simulates transient climate conditions. This information is especially relevant to climate policy making because it provides insight into where and when different kinds of policy action should be taken to mitigate climate change, and what their expected consequences will be on the global environment. We discuss this further in the following section. 305

Emission scenarios and global climate protection: J Alcamo and E Kreileman

IMAGE 2: instrument for analysis Joseph Alcamo is with the Center for Environmental Systems Research, University of Kassel, Kurt-Wolters Street 3, D-34109, Kassel, Germany. Eric Kreileman is with the National Institute of Public Health and Environment (RIVM), PO Box 1, 3720 BA, Bilthoven, The Netherlands. The IMAGE Project is supported by the National Institute of Public Health and the Environment, the Netherlands (RIVM), the Dutch Ministry of Housing, Physical Planning and Environment (VROM), the Dutch National Programme on Global Air Pollution and Climate Change (NRP), and the Center for Environmental Systems Research, University of Kassel, Germany. The authors are grateful to M Berk, R Leemans, B Lubkert-Alcamo, B Metz, and R Swart for reviewing an earlier manuscript upon which this paper is based (J Alcamo and G J J Kreleman. The global climate system: near-term action for long-term protection. RIVM Research Report 481507001). The authors also acknowledge the comments and advice of participants of three workshops held in Delft, The Netherlands in 1995 and 1998 on ‘Using the IMAGE 2 model to negotiations’. We support climate especially thank W Hare and B Metz for contributing key ideas to what evolved into the Safe Emission Corridor’ concept presented in this paper.

‘J Alcamo (ad) /IWAGE 2.0: integrated Modelling of Global Climate Change, Kluwer Academic Publishers, Dordrecht, 1994. Also published as special issue of Water, Air and Soil Pollution, Volume 76. Nos l-2,1994

The integrated global change model used for analyses in this paper is the IMAGE 2 model.’ IMAGE 2 provides comprehensive, geographically detailed, and quantitative information about trends in greenhouse gas emissions and their impact on climate, the biosphere, and society. The driving forces of the model, and the scenarios in this paper as well, are population, economy, and economic activity. Based on these assumptions about these driving forces, IMAGE 2 computes future changes in the consumption of energy, food, and timber. This consumption leads to greenhouse gas emissions from energy and industry, shifts in land use and land cover, and changes in the fluxes of gases from the terrestrial environment. The emissions and fluxes of gases lead to changes in the atmospheric composition of various gases, as well as changes in the flux of heat and moisture between the terrestrial, oceanic and atmospheric compartments. Eventually these fluxes affect regional climate, and these changes in regional climate then feed back to the terrestrial and oceanic compartments in different ways, for example, by changing the productivity of crops and the required amount of future agricultural land. The model is constructed from 13 individual global submodels organized into three fully linked subsystems: Energy-Industry, Terrestrial Environment, and Atmosphere-Ocean (Figure 1). The Energy-Zndustry models compute the emissions of greenhouse and other gases from five sectors in 13 world regions (Figure 2) based on estimates of industrial production and energy consumption. The Terrestrial Environment models simulate changes in global land use and cover on a grid scale taking into account shifts in the demand and potential productivity of land. These models also compute the subsequent fluxes of gases between the terrestrial environment and atmosphere. The Atmosphere-Ocean models calculate the changes in atmospheric composition of greenhouse and other gases, changes in the heat and moisture balance of the earth, and subsequent shifts in temperature and precipitation patterns on the regional scale. Each submodel has been tested either with data from 1970 to 1990, or long-term averages, depending on suitability and avail-

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Figure IMAGE

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1.

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2 model.

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Emission scenarios and global climate protection: J Alcamo and E Kreileman

World regions in IMAGE 2

2. Aggregation of countries into world regions as used in the IMAGE 2 model.

F@n

‘J Alcamo, G J J Kreileman,

M Krol and G Zuidema, ‘Modelling the global societybiosphere-climate system, Part 1: model description and testing’, Water Air Soil Pollution, Vol 76, Nos 1-2, 1994, pp l-36 3H J M de Vries, J G J Olivier, R A van den

Wijngaart, G J J Kreileman and A M C Toet, ‘A model for calculating regional energy use, industrial production and greenhouse gas emissions for evaluating global climate scenarios’, Water Air Soil Pollution, Vol 76, Nos 1-2, 1994, pp 79-132 4K Klein Goldewijk, J G van Minnen, G J J Kreleman, M Vloedbeld and R Leemans. ‘Simulating the carbon flux between the environment terrestrial and the atmosphere,’ Water Air Soil Pollution, Vol 76, Nos l-2, 1994, pp 199-230; G J J and A F Bouwman, Kreileman ‘Computing land use emissions of greenhouse gases’, Water Air Soil Pollution, Vol 76, Nos l-2, 1994, pp 200230; R Leemans and G J van den Born, ‘Determining the potential distribution of natural vegetation, crops and agricultural productivity’, Water Air Soil PolluHon, 76(1/2): 133-162. Vol 76, Nos 1-2, 1994, pp 133-162; G Zuidema, G J van den Born, J Alcamo and G J J Kreileman, ‘Simulating changes in global land cover as affected by economic and climatic factors’, Water Air Soil Pollution, Vol 76, Nos l-2, 1994, pp 163-196 continued on page 308

Canada

6 Eastern Europe

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USA LatinAmerica

7 CIS 8 Middle East

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12 Oceania 13 Japan

ability of data. An overview of model development and testing is given in Alcamo et al.’ Details of development and testing of the Energy-Industry subsystem are given in de Vries et ai, for the Terrestrial Environment subsystem in Klein Goldwijk et al, Kreileman et al, Leemans and van den Born et al and Zuidema et a1,4 and for the Atmosphere-Ocean subsystem in de Haan et al and Krol and van der Woerd.’ Later we discuss other critical points of the calculations used to generate the scenarios in this paper.

Evaluating emission scenarios Consequences

of no action: baseline scenarios

Method and assumptions. As a starting point for evaluating climate policies it is necessary to have a benchmark scenario that specifies the consequences of no policy action. In this paper we use the intermediate of three baseline scenarios developed with the IMAGE 2 model. This scenario is called Baseline A, and is described in detail in Alcamo et a1.6 Assumptions about the key driving forces in Baseline A come from the intermediate emissions scenario of IPCC, IS92a.7 In this scenario world population grows to 11.3 billion in 2100 and GDP grows at a worldwide average of 2.3%/yr from 1990 to 2100. Specific assumptions are made for each region. In general, however, most industrialized regions level off in population and slow down in economic growth in the coming decades, while most developing countries are assumed to sharply increase in both population and income until the second half of the next century. What are the consequences of not acting? Because of assumed trends in population and income, Baseline A shows steady increases in global energy consumption, industrial activity, and deforestation in the first

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continued from page 307 5B J de Haan, M Jonas, 0 Klepper, J Krabec, M S Krol and K Olendrzynski, for ‘An atmosphere-ocean model integrated assessment of global change, Water Air Soil Pollution, Vol 76, Nos 1-2, 1994, pp 263-318; M S Krol and H J van composition der Woerd, ‘Atmospheric calculations for evaluation of climate scenarios’, Water Air Soil Pollution, Vol 76, Nos 1-2, 1994, pp 259-282 6J Alcamo, G J J Kreileman, J C Bollen, G J van den Born, Fi Gerlagh, MS Krol, A M C Toet and H J M de Vries, ‘Baseline environmental scenarios of global change’, in this issue ‘J Leggett, W J Pepper and R J Swart, ‘Emission Scenarios for the IPCC: an Update’, in IPCC, Climate Change 7992: The Supplementary Report to fhe lPCC Cambridge Scientific Assessment, University Press, Cambridge, 1992 “Significant’ is defined as 5% of the theoretical yield that would occur under ideal climate conditions, as computed by the FAO Crop Suitability model ‘FAO (Food Agricultural and Organization) Report on the agrozones project. Vol 3. ecological Methodology and results for South and Central America. World Soil Resources Report 4813. FAO, Rome, 1978 “Leemans and van den Born, op tit, Ref 4

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half of the twenty-first century and a slowing down in the second half. Following from these trends, global CO2 emissions reach 22.0 Gt C/yr (Figure 3A), and cumulative CO2 emissions 1691 Gt C (Table 1) between 1990 and 2100. A brief pause in the upward trend of CO2 emissions occurs in 2025 when African forests are depleted. As a result, the global deforestation rate temporarily slackens and along with it global CO* emissions. However, the reduced emissions are offset by the smaller amount of CO2 taken up by the biosphere because of the extinction of African forests. Because of this factor, and because the atmosphere does not immediately respond to a change in global emissions, there is no apparent slowing of the buildup of CO2 in the atmosphere (Figure 3B). The concentration of CO* reaches 737ppm in 2100, more than double its 1990 level (Figure 3B). The accumulation of CO2 and other greenhouse gases bring about an average increase in surface temperature of 2.8”C between 1990 and 2100 (Figure 3C), ranging from around 2.5”C in the tropics to 4.O”C in the higher latitudes. Changes in patterns of temperature and precipitation lead to a wide variety of potential impacts on nature and society. As impact indicators in this paper we select crop impacts, threat to natural vegetation, and sea level rise because they are related to specific issues raised in the Framework Convention on Climate Change. Article 2 of the Convention (the Objective) calls for stabilization of greenhouse gas concentrations at a level to allow ‘ecosystems to adapt . , to ensure that food production is not threatened, and to enable economic development to proceed in a sustainable way’. There are, of course, many ways to assess the impact of climate change on crops. In this paper we use the area with a significant decrease in crop yield due to climate change.’ This is a measure of the amount of area where the current agricultural industry must adapt to new climate conditions. If these conditions change too fast, then there will be increased risk that farmers and the agricultural industry cannot adapt to the new conditions. To compute the areas of decreased yield, IMAGE 2 uses an adapted version of the FAO Crop Suitability model’ which gives constraint free rainfed yields based on local climate conditions. The model has been implemented on the global scale, and modified to include CO2 fertilization and other factors. It has also been tested against current crop growing conditions and found to give reasonable results, as described in Leemans and van den Born.” The results for maize show that under Baseline A the area with decreased yield rapidly expands in the first half of the next century (Figure 3E), eventually reaching 32% of current maize growing areas in the world. As partial compensation for reduced yield, changes in climate will increase maize yield in 16% of current maize growing areas; in addition, entirely new areas will become climatologically suitable for maize growing. The affected area of temperate cereals has a similar temporal pattern (not shown). However, a smaller fraction of the total cereals area is ultimately affected by climate change because decreases in yield are partly offset by higher atmospheric levels of CO* concentrations which enhance the fertilization of cereals. This effect is not so important for maize. To assess the threat of climate change to natural vegetation we take an approach different from that used to assess crop impacts. We use the change in climate related potential vegetation as an indicator because it

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“I C Prentice, W Cramer, S P Harrison, R Leemans, R A Monserud and A M Solomon, ‘A global biome model based on plant physiology and dominance, soil properties and climate’, Journal of Biogeography, Vol 19, 1992, pp 117-134 “Leemans and van den Born, op tit, Ref 4

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is a measure that can be computed with existing models, and because it shows where existing vegetation will not be well adapted to new climate conditions. Fast changes in potential vegetation could lead to severe disruption of natural vegetation succession, the main process through which vegetation can respond and adapt to new conditions. If vegetation is disrupted over a large area, then local and regional biodiversity could also suffer. Hence we call this indicator ‘threat to natural vegetation’. For potential vegetation calculations, IMAGE 2 uses a modified version of BIOME” as described in Leemans and van den Born.” BIOME has

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scenarios and global climate protection: J Alcamo and E Kreileman (C)

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been found to compute reasonable spatial patterns of current global vegetation.13 The temporal trend of the area of natural vegetation under threat of climate change is similar to that of crops, with a steep initial increase in the first half of the next century and a slowing down of this trend in the second half of the century (Figure 3F). These results indicate that large areas of natural vegetation may be sensitive to the climate change computed to occur in the first half of the next century. By 2100, climate change will threaten terrestrial vegetation in 41% of the world’s land area (Figure 3F).

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Another key impact of climate change will be rising sea level, which could bring about increased coastal flooding especially in developing countries not able to afford complete coastal protection. Because of the very slow deepwater mixing the ocean, sea level responds more slowly than the atmosphere to the buildup of greenhouse gases. Hence global average sea level rise is 42cm between 1990 and 2100 and is still exponentially increasing in 2100 although temperature increase has begun to slow down (Figure 3D).

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Emission scenarios and global climate protection: J Alcamo and E Kreileman Summing up the consequences 0

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of not acting

Not acting to mitigate climate change (ie baseline conditions) could cause a doubling of current atmospheric levels of CO2 by 2100, and important negative impacts on crops, natural vegetation and sea level. The rate of increase of impacts on world vegetation could be larger in the first half of the next century than before or after. Impacts on vegetation (agricultural and natural) slow down in the second half of next century, but sea level continues to rise exponentially.

Policy Approach

I: controlling

the level of emissions

Method and assumptions. After examining baseline emissions, we now investigate the control of global emissions. We examine five scenarios which represent different broad types of policies. They are called ‘Start 2000’ scenarios (abbreviated ‘St2000’) because they assume that at least some controls begin in some countries in year 2000. In practice, this means that the scenarios depart from the Baseline A scenario in year 2000. The scenarios have the following characteristics:14

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14When no controls on emissions are specified, as in the case of non-Annex I countries before year 2025, emissions from these countries are assumed to follow Baseline A. Also, emissions from natural sources, and emissions of ozone precursors (such as NO,) and halocarbons are assumed to follow Baseline A ?he AOSIS (Alliance of Small Island States) protocol proposals calls for a reduction of the 1990 anthropogenic COP emission level of Annex I countries by at least 20% by the year 2005 ‘6Total equivalent COP emissions are the sum of all greenhouse emissions expressed in Gt Clyr of equivalent CO?. The latest global warming potential factors from IPCC (D L Albritton, R G Derwent, I Isaksen, M Lal and D J Wuebbles, ‘Trace Gas Radiative Forcing Indices’, in J T Houghton, L G Meira Filho. J Bruce, H Lee, B A Callander, E Haites, N Harris and K Maskell (eds), Climate Change 1994, pp 21Ct-231. Cambridge University Press, Cambridge, 1994) are used to convert gases to COP equivalents.

l

St2000-a:

This reflects the thinking of the AOSIS protocol.‘5 For this scenario we assume that CO1 emissions in Annex I regions are frozen after year 2000 at their 1990 levels. (Annex I countries refers to the industrialized countries of the world, which are listed in Annex I of the Framework Convention on Climate Change. See Appendix A of this paper for a list of these countries.) After year 2000, CO2 emissions in Annex I countries decline by 1%/year. 32000-b: The same as 32000-a except that non-Annex I countries take the same actions as Annex I, but with a 25 year delay. 32000-c: The same as St2000-b but both Annex I and non-Annex I countries reduce non-CO* emissions at the same rate as CO2 emissions. St2000-d: The same as St2000-c but no decrease in emissions after they are frozen. St2000-e: The same as St2000-d vut emissions decline by 2%/yr (rather than l%/yr) after they are frozen.

What is the effectiveness of different levels of control? We now report on scenario results for two key indicators - total equivalent CO* emissions16 because they are a comprehensive indicator of greenhouse gas emissions (Figure 4A), and change in global surface temperature as an indirect indicator of climate impacts (Figure 4B). First, we observe that the St2000-a scenario does not effectively slow the increase in emissions and temperature observed in Baseline A (Figure 4A and 4C). This is the scenario in which only CO2 emissions are controlled and only in Annex I countries. By comparison, in the next scenario (St2000-b) (St2000-a) non-Annex I countries also control their CO2 emissions (although with a 25year delay), and as a result global emissions substantially decline and the rate of temperature increase slows. Cumulative emissions in 2100 (equivalent CO*) are 1736 Gt C/yr as compared to 2589 Gt C/yr in Baseline A (Table 1). If non-CO* emissions (ie methane and nitrous oxide) are controlled in all countries at the same rate as CO2 emissions (St2000-c), not only do emissions decrease but temperature stabilizes in the second half of the century. Scenario results also show that another factor of importance is the rate

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Emission scenarios and global climate protection: J Alcamo and E Kreileman

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which emissions are annually reduced after they are frozen in year 2000: (1) If emissions remain constant afterwards (St2000-d), temperature continues to rise steeply increase; (2) if emissions decrease at l%/yr (St2000-c) temperature stabilizes around 2065, and (3) if they are reduced by 2%/yr (St2000-e) temperature not only stabilizes around 2040, but eventually decreases. Hence, to stabilize or reverse climate change it is necessary to do more than stabilize global emissions after 2000. (We note that these conclusions assume that there is no special action taken to enhance the sinks of greenhouse gases.)

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Emissionscenarios and global climateprotection: J AIcamo and E Kreileman

Summing up the effectiveness of controls l

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Control of only CO2 emissions in only Annex I countries will not substantially protect climate. Controlling emissions in both Annex I and non-Annex I countries will significantly slow temperature increases. Controlling non-COz emissions in addition to CO2 emissions is an effective policy to slow temperature increases. If emissions are only stabilized, global temperature will continue to sharply increase. Global emissions must be reduced by l%/yr or more in order to stop the increasing trend of temperature.

Policy Approach II: stabilizing greenhouse gases in the atmosphere Method and assumptions. We have just examined one approach to protecting climate, namely setting goals for controlling global emissions. Another approach is to set goals on the level of greenhouse gases in the atmosphere. This approach is consistent with Article 2 of the Framework Convention of Climate Change which aims to achieve the ‘stabilization of greenhouse gas concentrations in the atmosphere that would prevent dangerous anthropogenic interference with the climate system”. In order to evaluate the implications of this objective, the IPCC sponsored a modeling exercise to compare stabilization calculations from several global models.” The organizers of this exercise selected various longterm targets for stabilizing CO2 and concentration pathways to targets. The models were then run in the ‘inverse mode’ to determine the global CO2 emissions that would be allowed in order to achieve these targets and pathways. Here we elaborate on preliminary results from IMAGE 2 that were included in the IPCC exercise.18 Our procedure is:

(1) “I G Enting, T M L Wigley and M Heimann. Future emissions and concentrations of carbon dioxide. Technical Paper No 31, Australian Division of CSIRO, Atmospheric Research, Mordialloc, Australia, 1994 “J Alcamo, M S Krol, and R Leemans, Stabilizing greenhouse gases: global and regional consequences, in: S Zwerver, A von Rompaey, M Kok, M Berk (eds.) Climate Change Research: Evaluation and Policy Implications, Elsevier Science: Amsterdam, 1994 lgEnting et al, op tit, Ref 17 2@The land use emissions of non-CO2 gases are taken from the Baseline A scenario described in Alcamo et a/, op tit, Ref 6. The energy emissions of non-CO2 gases are computed by assuming that the ratio of non-CO2 emissions at time t over their value in 1990 is equal to this ratio for CO2 emissions. The emissions of CO2 at time t are back calculated by IMAGE 2 based on the specified atmospheric CO2 F,athway In order to calculate the energy emissions of CO2 we must estimate the land use emissions of COP. These land use emissions are taken from the Baseline A scenario described in Alcamo et al, op tit, Ref 6

(2)

(3) (4)

We specify a stabilization target (350, 450, 550, and 650ppm), and a pathway to reach this target. For this purpose we use pathways specified by the IPCC in Enting et af19 (Figure 3B). Note, all of these targets except 450 ppm are to be reached in 2 150. Hence they are not reached by 2100, the time horizon of the IMAGE 2 model presented in Figure 3B. IMAGE 2 is used to compute the buildup of non-CO2 gases in the atmosphere after making additional assumptions about non-CO2 emissions.20 The model is used to compute resulting climate change and its imnacts. The model is then used to back calculate allowable global CO2 emissions from energy combustion for the different pathways.2’

What are the consequences of stabilization? For the scenarios in which CO2 is stabilized at 450ppm or above, global average surface temperature is still increasing in 2100, although the rate of increase is small for 450ppm (Figure 3C). For the 350ppm scenario, temperature stabilizes around 2030 (Figure 3C). The temporal trend of impacts on crop yield and natural vegetation resembles that of temperature change. The maize area affected by climate change is still increasing in year 2100 for the 450ppm and higher targets (Figure 3E), reaching 26% of total current maize growing area under the 650ppm scenario by 2100. Even under the lowest pathway (350ppm) 16% of maize area is affected by climate change (Figure 3E). Hence, some sensitive areas could still be affected by climate change.

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Emission scenarios and global climate protection: J Alcamo and E Kreileman

that in order for CO2 “We note, in stabilize the concentrations to atmosphere at any level, emissions must zero. This is eventually approach because a truly stable, unchanging COP implies that a CO2 concentration exists between the equilibrium atmosphere, ocean and biosphere. At equilibrium, net fluxes of CO2 are zero ‘9 M L Wigley, R Richels and J A Edmonds, ‘Economic and environmental stabilization of in the choices atmospheric COs concentrations’, Nature, Vol379.1998, pp 249-243 24J Alcamo and G J J Kreileman, ‘Interim impacts of environmental delaying emission controls’, prepared for Workshop on Timing the Abatement of Greenhouse Gas Emissions. Paris. 17-18 June, 1998

316

Under the 650ppm pathway, 31% of the natural vegetation area is threatened by 2100 (Figure 3F). By contrast, under the 350ppm scenario, the area stops increasing around 2050, but not before it exceeds 15% of the total terrestrial area of the world (Figure 3F). Because of the lag time in the response of sea level to changes in greenhouse gases, sea level is still increasing for all scenarios in year 2100, reaching 66cm over its 1990 level under the pathway to 650 ppm, and 36 cm under the pathway to 350ppm (Figure 3D). Up to now, we have examined the impacts resulting from specified concentration pathways of CO* in the atmosphere. Now we answer the question, what emissions will achieve these specified concentration pathways? These global emissions have been back calculated using IMAGE 2 for the different stabilization pathways after accounting for the uptake of CO2 by vegetation and the ocean (Figure 3A). For the 450ppm and higher scenarios, emissions can increase up to around 2030, and then they must decrease. For the 450ppm pathway emissions can increase by only about 20%. To achieve the 350ppm pathway, emissions must immediately stabilize and then sharply decline after 2030.22 We note that computed emissions are somewhat above the average computed in the IPCC exercise and by Wigley et aZ23 because the IMAGE 2 model has higher CO2 uptake in its biosphere which compensates for higher emissions. Wigley et al also have noted that different pathways to the same CO2 target will result in about the same temperature and sea level rise after several decades. From this they conclude that different (and cheaper) emission pathways can be used to reach the same long-term stabilization target. Alcamo and Kreileman24 however, pointed out that interim impacts of climate change could be quite different for two different pathways to the same target. As a sensitivity analysis, we also examined the case in which CO2 and all orher greenhouse gases were immediately stabilized in the atmosphere at their 1990 level. This is an unrealistic alternative because it would require a two-thirds reduction of global CO2 emissions between 1990 and 2000 (Figure 3A). Nevertheless, it is interesting as a theoretical case because it indicates the level of climate change that might be ‘unavoidable’. For this case the increase in global temperature is much slower than for any other stabilization scenario, reaching about 0.5”C over its 1990 level (Figure 3C). Nevertheless there are ‘residual’ impacts on maize, natural vegetation, and sea level which are smaller than the other stabilization pathways but still discernible (Figures 3D, 3E, and 3F). This is an indication of the difliculty of avoiding future climate impacts due to the present level of greenhouse gases in the atmosphere, and the likely need to adapt to some of these impacts. Another important conclusion from this analysis is that stabilization targets above 450ppm will have large impacts, and below 450ppm will have lower but still some residual impacts. This is a good reminder that not all stabilization levels may ‘prevent dangerous anthropogenic interference with the climate system’ as called for in Article 2 of the Climate Convention. This also suggests that policy makers should examine whether goals for stabilization comply with goals of climate protection. In the next section we examine more closely the ability of these and other scenarios to protect climate.

Emission scenarios and global climateprotection: J Alcamo and E Kreileman Summing up the consequences of stabilization l

0

a

l

l

For stabilization of COz at 450ppm or higher, temperature and impacts steadily increase up to at least year 2100. For stabilization of CO2 below 450 ppm, temperature and area of vegetation impacts stabilize. However, because of the lag time of the ocean, sea level continues to rise. As was found for the baseline scenarios, the stabilization scenarios indicate a more rapid increase in temperature and some climate impacts in the first half of the next century than before or after. Climate policy making should take this information into account. To stabilize CO2 in the long run at 350 or 450ppm, global emissions cannot substantially increase, and eventually must be signiticantly reduced. Stabilizing greenhouse gases in the atmosphere does not necessarily provide a high level of climate protection. Even immediate stabilization of all greenhouse gases may lead to a 0.5”C increase in global temperature along with some climate impacts. Hence, some climate change may be unavoidable.

Do global emission scenarios comply with climate goals over the next century? Method and assumptions. The purpose of this section is to compare in a

25Emission scenarios in this analysis give the sum of anthropogenic emissions of CO*, CHI, and N20, expressed in Gt Clyr of equivalent COP. *‘Leggett et al, op tit, Ref 7 27J Alcamo, A Bouwman, J Edmonds, A Grtibler, T Morita, An evaluation of the In: IPCC IS92 emission scenarios. Houghton, et al (eds) Climate Change 7994, Cambridge University Press, 1995 *aIt is difficult to estimate the feasible limits of rates of emissions growth or reduction. However, we note that the highest rate of growth of the highest of the six IPCC reference (no control) emission scenarios was about +3%/year (Leggett et al, op tit, Ref 7). As to the possible rate of emission reductions - a 4%lyear reduction rate would reduce global emissions by 56% in 20 years, whereas a 1% rate would reduce them by 18% in 20 years

standardized way the effectiveness of different scenarios in complying with short- and long-term climate goals. We evaluate seven emission scenario? which range from ‘no policy action’ with strongly increasing emissions, to vigorous policy action with rapidly decreasing emissions. All of these scenarios have been covered in previous sections except the ‘IS92e’ scenario from IPCC26 which is used as an upper bound of uncontrolled emissions. The range of these scenarios is about as wide as the range of global CO2 scenarios found in the literature by the IPCC.27 For each of these scenarios the IMAGE 2 model is used to compute changes in climate and its regional and global impacts. We then evaluate if the scenarios comply with climate goals by computing whether or not they exceed limits set on four indicators over the next century. The indicators are: 0 0 l l

Change Rate of Change Rate of

Although these indicators are very simple, they nevertheless provide an overview of global climate impacts. Sample limits for these indicators are given in Table 2. The first three are intended to represent climate change impacts while the fourth indicator gives a rough indication of the technical and economic feasibility of emission reductions. The main idea of this indicator is to avoid assuming unrealistic emission reductions during a short period of time.28

Table 2. Source: WMOIICSUIUNEP Advisory Group on Greenhouse Gases (F R Rijsberman and R J Swart (eds). Targets and indicators of Climafic Change, Stockholm Environment Institute, Stockholm, 1990)

in global average surface temperature relative to 1990. temperature change per decade. in global average sea level relative to 1990. change of global emissions per year.

Example of proposed limits on indicators of cllm8ta chan#o Indicator

Rate of temperature change Temperature change (relative to pre-industrial) Sea level rise (relative to 1990)

VahIO O.i”C/decade

l.o-2.o”c 20-50 cm

317

Emission scenarios and global climate protection: J Alcamo and E Kreileman

For the evaluation, emission scenarios are plotted over time (Figure 5) and a segment is made thick where at least one of the above mentioned limits is exceeded. A thin segment indicates that no limits are exceeded and afuzzy segment indicates a zone of uncertainty.29 To make an explicit connection to climate protection, we will refer to thick segments as being ‘unsafe’ and thin segments as being ‘safe’. Using this approach we answer the following questions: l

0 l

How sensitive are results of the evaluation to the desired climate goals? Which indicators are responsible for violating the desired goals? Are scenarios that are different globally also different regionally?

How sensitive are results to desired climate goals? To answer this question, we specify three sets of limits to the indicators, and then compute where along the emission pathways the indicators are exceeded. The three sets are: (1)

(2)

(3)

‘?his uncertain zone is derived by assigning a &20% range around the specified limits of the indicators. This range takes into account the uncertainty of estimating the level of climate change that will affect ecosystems, and other valued components of the environment 30R J Swart, G J J Kreileman, M Berk, M Jansen, J Bollen, R Leemans and H M S de Vries. ‘The safe landing approach: risks and trade-offs in climate change’, RIVM Report. In preparation

318

Change of average global 2.O”C, rate of temperature d 40cm, rate of emissions Change of average global l.YC, rate of temperature 6 30cm, rate of emissions Change of average global l.O”C, rate of temperature < 20cm, rate of emissions

surface temperature relative to 1990 < change 6 0.20”C/decade, sea level rise reductions d 4%/yr (Figure 5A). surface temperature relative to 1990 < change < O.lS”C/decade, sea level rise reductions d 3%/yr (Figure 5B). surface temperature relative to 1990 d change < O.lO”C/decade, sea level rise reductions < 2%/yr (Figure 5C).

The strictest of these limits (set 3) is based on the data in Table 2. The middle set (set 2) is a factor of 1.5 higher, and the softest set (set 1) is a factor of 2.0 higher. Results of the evaluation are presented in Figure 5. Obviously, when limits are made more strict, many more segments exceed these limits. This includes some of the lower emission scenarios which violate the stricter limit set on rates of emission reductions. Whether or not an emissions scenario is ‘safe’ depends, of course, on the limits assigned to the indicators, in other words, on the desiredlevel of climateprotection. The emission scenarios also shift in thickness over time, sometimes shifting from an unsafe segment to uncertain segment and back again. This indicates that the time path of emissions is important in their complying with climate goals. On the other hand, some scenarios are primarily safe (eg St2000-e) and some primarily unsafe (eg IS92e and Baseline A) during the entire simulation period (Figure 5). The stabilization scenarios also vary greatly in the level of climate protection they afford, which is consistent with the earlier conclusion that stabilization of CO2 does not guarantee a high level of climate protection (see Policy Approach II section). The lowest of the stabilization scenarios (350 ppm) provides the highest level of climate protection of all the scenarios, but consists of mostly unsafe segments in Figure 5C because it violates the 2%/yr limit set on emission reduction rate. We note that there is some evidence that Z%/yr is a reasonable upper limit for the reduction rate of emissions.30 Which indicators are responsible for violating the desired climate goals? In Figure 6 we present the data for temperature change, sea level rise and emission reduction rates that are behind the evaluation presented in Figure 5. From these data it is clear that the rate of temperature change is the limiting climate indicator in the earlier part of the simulation

1990

-

2wO

equivalent

CO,

2050

1990

0;

I-

30-

with

specified

t

2050

1

2025

climate

goals.

r

2075

(Gt C/yr)

1990

5

15

20

25

30

35

2000

Anthropogenic

40

45

B)

CO, emissions

Reduction s 4.0 Wvear

Sea Level Rise zG40 cm

Rate of Temperature Change ZG0.20”Cldecadl

Temperature Chary 5 2.0 c

equivalent

2100

in complying

2000

I

Anthropogenic

x175

(Gt C&r)

of scenarios

emissions

of effectiveness

2025

No limits are exceeded

Anthropogenic

Figure 5. Comparison

(4

2025

0.1 O”C/decad~

2050

---.-,I

CO, emissions

Sea Level Rise

S

equivalent

2075

(Gt C/yr)

2100

1

2

Sea Level Rise 630 cm

Rate of Temperature Change s 0.1 S”C/decadl

Emission scenarios and global climate protection.. J Alcamo and E Kreileman

(A)

Temperature change

3.5 .**.* 2 : --- 3: --44. -e-5: ---6:

1990

w

2000

I S92E Stab Stab Stab Stab

350 450 550 650

2025

2050

2100

2075

Rate of temperature change

0.40 ”

i

...“... ..” .. :..:

_...... ...‘.,2 .........-

..:

, Figure 6. Background data to Figure 5.

1990

2000

2025

2050

2075

2100

period because this is when most emission scenarios have their steepest increase. By comparison, the limits set on accumulated temperature change and sea level rise (relative to 1990) tend to be exceeded towards the end of the simulation period as the effects of climate change accumulate. As already noted, some of the lower emission scenarios exceed the specified limit on rate of emission reductions. The main point is that different indicators (ie climate goals) influence the effectiveness of emission scenarios at different times. Hence the effectiveness of a scenario in

320

Emission scenarios and global climate protection: J Alcamo and E Kreileman

(Cl

Sea level rise -

50.

40. ‘6 5 30.

4

LO.

10.

O-

1

1990

2000

2050

2025

2075

(D) Rate of change

in anthropogenic

equivalent

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CO, emissions

/. i, / . ., -2.0 **... 2 : --- 3: -4: -e-5: -.6:

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i

I

I

I

I

1990

2000

2025

2050

2075

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Figure 6. Contd.

I S92E Stab Stab Stab Stab

2100

mitigating climate change cannot be judged by comparing it to only one indicator. Several goals must be taken into account at the same time. Are

scenarios

that are different

globally

also different

regionally?

To

answer this question we compare regional results from a higher emissions scenario (Baseline A) which is mostly in an unsafe zone, and a lower emissions scenario (Start2000-3) which is mostly in a safe zone (Figure 5). First we compute the climate change of the two scenarios, and then compare climate impacts on crops, natural vegetation, and sea

321

Emission scenarios and global climate protection: J Alcamo and E Kreileman

level rise (Figure 7 and Table 3). Large differences are observed: for the higher emissions scenario (a no policy action case) 32% of the global maize growing area has a decrease in yield between 1990 and 2100, as compared to 13% for the lower emissions scenario (a vigorous climate policy case). We remind the reader that some areas also experience an increase in yield because of climate change (see Table 1). For the higher emissions scenario, 41% of the global area of nature reserves are threatened by climate change between 1990 and 2100, as compared to 13%

Nature mewves atrisk

Other areas at risk

Nature

reserves, risk noteval

Other areas. risk not evaluated

(W

Nature reserves at risk

Other areas at risk

Other areas, risk not evaluated

Figure 7. Threat

322

to natural

vegetation

between

1990 and 2100 under

(a) Baseline

A, (b) Start2000-e.

0.5 0.7 0.8 0.7 0.9 0.8 0.7 0.8 0.3 0.9 0.4

2.6

3.6

3.9 3.7

4.6 4.5 3.9 4.3

1.7 4.1

2.2

Latin

Africa

OECD Europe Eastern Europe

CIS Mlddle East India + S Asia China + C P Asta

East Asia Oceania

Japan

America

1.2

812000-E

5.9

“C

A) emission

USA

1 geO-2100 BaselIne-A

(Baseline

change

of a higher

Temperature

conseguences

0.8 0.9

Regional

3.9 4.5

3.

World total Canada

Table Threat 2100% %

area

7 12 72 1 0 36 2 41 2 7 100

1 3 0 4 6 0 3 0 0 0

22 15 26 25 9 18 14 0 68 0

7 15 17 16 14 13 22 3 10

25 47 42

33

17 34 4

75

1 2

7

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0 20

16 18

41 50 50 62

% Baseline-A

2100

of current

83

Area

0

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growing

16

maize

13

EaHlb8e-A

2100

of current

32

Area

scenario

1

ST2000.2

emission

54

41 42

(St20004

vegetation

and a lower to natural

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scenario

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area

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Emission scenariosand global climate protection: J Alcamo and E Kreileman for the lower scenario. Although the size of the areas affected are quite different from region to region, they all show much smaller area affected under the lower emission scenario than the higher (Table 3). For this example it is clear that the scenario with a higher level of climate protection on the global scale also gives a higher level of protection on the regional scale. Summing up this section l

0

l

0

As expected, when limits are made more strict, many more segments exceed these limits. Whether a scenario is safe or not depends very much on the desired level of climate protection. The emission scenarios shift zones over time. Hence, the time path of emissions is important in their complying with climate protection goals. However, some emission scenarios remain in the safe zone and others in the unsafe zone over the entire simulation period. The stabilization scenarios vary greatly in the level of climate protection they afford, which again indicates that stabilization of CO2 does not guarantee a high level of climate protection. An emission scenario which provides greater climate protection (than other scenarios) on the global level, may indeed provide higher protection on the regional scale. However, some especially vulnerable areas may still be affected by climate change.

Computing the safe emissions corridor Method and assumptions

3’ln this report we select 2010 as a target year for near-term policies. While this is arbitrary, it is nevertheless intermediate to the various target years now being discussed in international negotiations 32We note here that the German Advisory Council on Global Change (WBGU) has recently followed a different procedure to reach a similar aim to ours. See for Advisory example, WBGU (German Council on Global Change). ‘Scenario for the derivation of global CO2 reduction targets and implementation strategies’, c/o A Wegener Institute for Polar and Marine Research, PO Box 12 01 61, D27515, Bremerhaven, Germany, 1995. Using mathematical techniques they have back calculated allowable emissions for a ‘tolerable climate window’. This window is analogous to our setting of limits on climate indicators

324

To this point we have evaluated different types of scenarios, and then compared them in a standardized way against goals for climate protection. In this part of the paper we reverse the procedure and begin with climate goals and then use IMAGE 2 to back calculate allowable emissions. To do so we introduce the concept of ‘safe emission corridor’ If global emissions are too high in year 20103’ there is little chance afterwards to make a course correction which avoids climate related impacts. On the other hand, emissions cannot be too low in 2010 because this would require reduction rates that are infeasible economically and technologically. Between these two extremes lies a Safe Emissions Corridor32 which is the allowable range of emissions over time that complies with specific short- and long-term climate goals. In the following paragraphs we compute this safe corridor between 1990 and 2010. This period is selected because it is the focus of current climate negotiations. The procedure for identifying a safe emissions corridor is described in Appendix B. In this part of the paper we address the following questions: 0 l

a l

What are the safe emission corridors from 1990 to 2010? Which future emission pathways fall within the corridor? What is the effect of future changes of SO2 emissions on corridors? What are acceptable pathways of emissions after 2010?

What are the safe emission corridors from 1990 to 2010? Using the same sets of limits from the previous section, we compute the emission corridors depicted in Figures 8A, 8B and 8C. We note that a new condition has been added here - we permit the limits on

z

!+

Figure

8. Safe emission

LG

afe emission corridor

corridors

for specified

5

E p 15 t

5

; > J

-.

climate goals. Shown

_ 2. Safe emission corridor

___.

equivalents

93

Temperature Chatye 51.0 c

-I-

are anthropogonic

CO2 emissions

of CO2,

(Violationfor B 2 decades)

Rate of Temperature Change 6 0.1OWdecade

CH4 and N20.

Sea Level Rise 530 cm

Sea Level Rise 540 cm

Emission Reduction i 3.0 o/d ear

L-J

(Violation for S 2 decades)

:nt%!?

II

Temperature

(Violationfor 6 2 decades)

Rate of Temperature Change 6 0.20°C/decad

afe emission corridor

6 5’ 3

Emission scenarios and global climate protection: J Alcamo and E Kreileman

33Swart et al. op cif, Ref 30 %We note that corridors should only be calculated for a ‘consistent’ set of climate indicators, because the three climate indicators (change in global average surface temperature relative to 1990, rate of temperature change per decade, and change in global average sea level relative to 1990) are correlated with one another

Table 4.

&h

the rate of temperature change per decade to be violated during two decades. This is because relatively high rates of temperature change occur for a limited number of years in the case of virtually all emission scenarios. For the three sets of limits, increasing in strictness, the computed corridors of global emissions (equivalent COZ) in 2010 are: (1) 7.3 to 14.5 Gt C/yr (Figure 8A); (2) 7.3 to 12.5 Gt C/yr (Figure 8B); (3) 7.6 to 9.3 Gt C/yr (Figure 8C). The top of the corridor is partly determined by the specified shortand long-term climate goals; the stricter the climate goals, the lower the top boundary and the narrower the corridor. From Figure 8A to Figure 8C we halve the limits of the indicators. The top boundary is also determined by restrictions on the maximum rate of emission reductions. The stricter the constraint (eg reducing the maximum from 4 to Z%/year), the lower the top of the corridor. The bottom of the corridor is determined simply by the specified maximum rate of emission reduction. When this rate is high, the bottom boundary of the corridor rapidly declines between 1990 and 2010. When it is lower, the bottom boundary declines more slowly. As noted earlier, there is some evidence that 2%/yr is a reasonable upper limit for the reduction rate of emissions.33 Any location within the aliowable range of emissions in 2010 will be on a path to the specified goals of short- and long-term climate protection. However, as will be noted later in this paper, the pathway of emissions after 2010 is also very important in meeting these climate goals. The size of the corridor depends very much on the assumed limits of the indicators. The stricter the limits, the lower the top and the narrower the width of the corridor. It is also important to note that Figure 8 shows the emissions corridor for only three of many possible sets of limits on climate indicators and emission reduction rates. The corridors for many other sets of limits are given in Table 4.34

emission corridors for eelected climate goals Sea level rlu i ggO-2100 (cm)

Clmnge in temperature 1990-2100 (“C)

Rate of temperature increase (T/decade

VioletIons allowed (number of decade@

21.0

20.10

2

20

>I.0

20.10

2

20

Wv0 2.0

Allowable equlvelent CO, emlulon8 In 2010 (01 C&r) 7.6-9.3


21.0

0.10

2

a30

2.0

21.0

0.10

”

330

1.5

E&9.0

a1.0

0.10

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230

1.0

8.5-6.6

21.0

0.15

2

a30

2.0

7.6-12.3

21.0

0.15

2

230

1.5

&&II.9

310

0.15

2

a30

1.0

M-11.3

>I.0

0.20

2

a30

2.0

7.c14.3

1.0

0.20

2

a30

1.5

8.gl3.9

1.0

0.20

2

a30

1.0

6.S12.4

21.5

0.20

2

a30

1.5

6.fS14.2

21.5

0.20

2

a30

1.0

6 514.0

2.0

7 69.3

21.0

20.15

1

20

21.0

ao.15

1

20

$1.5 s2.0

at.0

0.10

a20

>I.0

0.15

a30

2.0

7.6-9.9

21.0

0.15

a30

1.5

6.0-9.9

>l.O

0.15

a30

1.0

6.S9.7

31.0

0.20

330

2.0

7.6-12.9

21.0

0.20

a30

1.5

6.Cl2.9

1.0 21.5

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1.5

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Emission scenarios and giobaf climate protection: J Akamo and E Kreiieman Summing l

l

up

this section

The size of the corridor depends very much on the assumed limits of the indicators. The stricter the limits, the lower the top and the narrower the width of the corridor. Any location within the allowable range of emissions in 2010 will be on a path to the specified goals of short and long term climate protection. However, the pathway of emissions after 2010 is also very important in meeting these climate goals.

Which future

emission pathways fall within the corridor?

We now examine whether the trend of uncontrolled emissions will fall within the computed corridors. The range of IPCC reference scenarios, all of which assume no controls on greenhouse gas emissions35 span from 10.7 to 12.5 Gt C/yr equivalent CO2 in 2000, and from 11.5 to 15.3 in 2010. For the first and softer set of limits on climate indicators (Figure 8A), the corridor overlaps the IPCC scenarios. For the medium limits (Figure 8B), the corridor is on the low side of the range of IPCC scenarios, and for the strictest limits (Figure 8C) the corridor is far below the entire range of reference emission scenarios of the IPCC. Hence, for somewhat stricter levels of climate protection (and constraints on emission reduction rates) the safe emissions corridor will be far below reference emission scenarios. This implies that this level of long-term climate protection can only be achieved by controlling the expected growth in global emissions up to year 2010. The so-called ‘Berline Mandate’ (adopted at the Climate Convention’s First Conference of Parties meeting in March, 1995) stipulates that nonAnnex I countries will not be required to control emissions under a climate protocol now being negotiated. Hence, we can assume that any agreed upon emission controls between 1990 and 2010 will only take place in Annex I countries. What then are the allowable emissions in 2010 in Annex I countries so that global emissions will fall within a safe emission corridor? For our calculations we must first assume an emissions level of non-Annex I countries for year 2010. For this purpose we use the IPCC emission scenarios36 which have a range of 5.5 to 7.0 Gt C/yr (anthropogenic equivalent CO*) for non-Annex I countries for year 2010, and an intermediate estimate of approximately 6.3 Gt C/ yr. Here we will just use the intermediate estimate, but the reader should keep in mind that because of the range of non-Annex I estimates, all of our estimates for Annex I also have a range of $0.75 Gt C/yr. Since about 6.3 Gt C/yr of the emissions in 2010 will come from the non-Annex I countries this means that global emissions will reach the top of the corridor when Annex I emissions in 2010 are no greater than: l

l

35Leggett et al, op cit. Ref 7 ?bid

l

8.2 Gt C,/yr for the highest and widest corridor (Figure 8A), (141% of 1990 emissions of Annex I countries which are approximately 5.8Gt Ciyr). (Recall, that the range here and in following paragraphs is ~tO.75Gt C/yr caused by the range of emissions from non-Annex I countries in the IPCC reference scenarios.) 6.2GtC/yr for the medium corridor (Figure 8B) (107% of 1990 emissions). and 3.0 Gt C/yr for the lowest and narrowest corridor (Figure 8C) (52% of 1990 emissions).

321

Emission scenarios and global climateprotection: J Alcamo and E Kreileman Allowable Annex I Emissions in 2010 @ Uncontrolled non-Annex I emissions 0 Allowable Annex I emissions

Medium Corridor E 0, 5 ._ $6 v ‘5

Figure 0. Allowable Annex I emissions for the corridors in Figure 8, and according to location in the corridor in 2010.

10

a

4

g g

2

E

0

a

Narrow Corridor

Top

Middle Bottom

Top

Middle Bottom

Top

Middle Bottom

J

To reach the middle of the corridors, their emissions in 2010 must be no greater than: 0 0 0

4.6Gt C/yr for the highest and widest corridor (shown in Figure 8A), (79% of 1990 emissions) 3.6Gt C/yr for the medium corridor (Figure 8B) (62% of 1990 emissions), and 2.2 Gt C/yr for the lowest and narrowest corridor (Figure 8C) (38% of 1990 emissions).

These and other results are summarized in Figure 9. Summing up this section l

l

0

Because of the Berline Mandate, it will be up to the Annex I countries to ensure that global emissions remain within a safe emissions corridor up to year 2010. The amount of action they have to take depends, among other things on: (1) uncontrolled emissions of non-Annex I countries in 2010, (2) the height and width of the corridor (which depends on the selection of climate goals) and, (3) the place within the corridor where global emissions should fall. For global emissions to reach the top of the medium corridor presented in this paper, total emissions in 2010 from Annex I countries must be nearly stabilized relative to their 1990 level (which is about 5.8 Gt C/yr). For global emissions to reach the middle of any corridor examined in this paper, emissions in Annex I countries in 2010 must be decreased relative to their 1990 value.

What are acceptable pathways of emissions after 2010?

From an economics perspective it seems logical to allow emissions to rise to the very top of an emissions corridor. Emissions here require the least amount of controls but can still meet the same climate goals as emissions lower in the corridor. However, a drawback to this approach is that the higher the emissions in 2010, the faster they must be reduced afterwards. For illustration, Figure 10 presents two safe emission corridors after

328

x

Emissionscenarios and global climateprotection: J Alcamo and E Kreileman _ 2. Safe emission corridor ; G

Temperature :h$$

Rate of Temperature Change s 0.1B°Cldecad~ (Violationfor I2 decades)

Sea Level Rise 630 cm

Figure 10. Safe emmision corridors from 2010 to 2100 starting from top and bottom boundaries of the 19902010 corridor.

1

2010, one originating from the top of the 1990-2010 corridor, and the other originating at the bottom. Note that if emissions are at the top of the corridor in 2010, they must follow a narrow path downwards in order to comply with the specified short- and long-term climate goals. By contrast, if emissions are lower down in the corridor in 2010 they have many more possible pathways afterwards. This is because emission pathways’ beginning here have lower cumulative emissions up to this point than those beginning higher up in the corridor. Put another way, the lower the emissions in 2010, the less of a burden put on future generations to control emissions.37 Another point is that lower emissions in 2010 gives future generations more flexibility in deciding on climate goals - higher emissions in 2010 eliminate the option to reach some climate goals after 2010. Summing up this section 37M Grubb, ‘Technologies, energy systems, and the timing of COP emissions abatement: an overview of economic issues’, presented to CGC workshop on: Incorporating technology issues into the consideration of policy responses to human induced climate change, 8-10 February, 1996, University of Maryland, USA. 3BSee for example, P R Jonas, R J Charlson and H Rodhe, ‘Aerosols’, in IPCC, Ciimate Change 1994, Cambridge University Press, 1995 3gSee for example, W Pepper, J Leggett, R Swart, J Wasson, J Edmonds and I Mintzer. Emission scenarios for the IPCC: an update. Assumptions, methodology Prepared for the and results. Intergovernmental Panel on Cli’mate Working Group I, 1992; J Change, Alcamo, M S Krol and M Posch, ‘An integrated analysis of sulfur emissions, acid deposition, and climate change’, Water, Air, and Soil Pollution, Vol 85, 1995, pp1539-1550

The higher the emissions in 2010, the more future generations must control emissions afterwards. The lower the emissions in 2010, the more flexibility future generations will have in selecting future emission pathways and climate goals. What is the effect of future changes of SOI emissions on corridors?

It is known that sulphate (SO,) particles in the atmosphere scatter solar radiation and may mask global warming caused by greenhouse gases.38 The effect of these particles is taken into account in the IMAGE 2 model, but in the calculations presented up to now we have held the atmospheric distribution of SO4 constant at its 1990 level. However, the future level of SO4 is expected to change because of expected changes in one of its principal sources - anthropogenic emissions of SO*. It is typical to assume that these emissions will decrease in most industrialized countries and increase in developing countries, with a net substantial increase in global emissions.39 An increase in global SO* emissions, if it occurs, is likely to increase atmospheric SO4 levels and therefore may compensate somewhat for an increase in greenhouse gas emissions. The net result would be to raise the upper boundary of the safe emission corridors computed earlier. To investigate the future role of SO2 we

Emission scenarios and global climate protection: J Alcamo and E Kreileman

perform a sensitivity analysis, and repeat the calculation of the safe emission corridors, but with new assumptions about SO4 in the atmosphere: instead of assuming that SOa remains constant at its 1990 level, we use the medium SO:! emission scenario from IPCC (IS92a) to compute new future levels of SO4 particles.40 Under this scenario, energy related SOz emissions (the main anthropogenic source of SO2 emissions) globally increase by 90% from 1990 to 2100.4’ For the three sets of limits used, the new allowable ranges of global emissions (equivalent CO*) in 2010 are: (i) 7.3 to 16.3 Gt C/yr; (ii) 7.3 to 15.2 Gt C/yr; (iii) 7.6 to 13.3 Gt C/yr. As expected, changing the SO4 level does not change the lower boundary of the corridors because this boundary is determined by the specified limit on rate of emission reductions. However, it does raise the upper boundaries. The narrowest corridor (iii) experiences the largest relative change ~ its upper boundary increases from 9.3 Gt C/yr to 13.3 Gt C/yr. On the other hand, as was pointed out in the previous section, the upper boundary may be less relevant to policy than the middle or lower boundary of the corridor because emissions at the top boundary will have to be sharply reduced afterwards. This is an important point because the location of the middle of the corridors does not change too much when SOz emissions increase. For example, in the narrowest corridor (iii) the middle value increases from 8.5 Gt C/yr to 10.5 Gt C/yr in our sensitivity analysis with increasing global S02. Another important point is that there is some doubt that developing countries will actually experience the large increases in SO2 emissions assumed in the IPCC scenario - eg in the IS92a scenario, energy related SO;! emissions increase by more than a factor of three in China and Centrally Planned Asia from 1990 to 2100, and a factor of five in other developing countries.42 This may not be realistic because these large emission increases will also cause significant health problems and ecological damage in Asia and other developing regions. For example, the World Health Organization has reported on the already existing health risks due to high SO* levels in large Chinese cities such as Beijing and Shanghai.4’ From the ecological perspective, Posch et al 44 computed that vegetation in 40% of Asia’s terrestrial area could be adversely affected by sulphur deposition if SO* emissions in Asia reaches levels similar to the IPCC scenario in year 2100. Summing

up this section

Increasing SOz emissions according to the medium IPCC scenario could compensate somewhat for increasing greenhouse gas emissions and significantly raise the upper boundaries of the safe emission corridors. However, the middle values of these corridors will not change too much, and furthermore, there is some doubt that this SO2 emissions scenario is realistic.

40Pepper et af, ibid 4’/bid @Ibid 43WH0 (World Health Organization), City Air Trends. Volume 1. WHO PEP 92.1, 1992 “M Posch. J-P Hettelingh, J Alcamo, M Krol. ‘Integrated scenarios of acidification and climate change in Asia and Europe’, in this issue

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Discussion and findings Results presented in this paper have many sources of uncertainties, including the uncertainties inherent in the IMAGE 2 model and all global models. Other important sources of uncertainty in the calculations of this paper are the error ranges of the statistical correlations used to compute the safe emissions corridor (see Figure 1l), and the uncertainties of environmental impacts related to the limits on the climate indicators.

Emission scenarios and global climate protection: J Alcamo and E Kreileman

This last type of uncertainty was taken into account earlier (Figure 5). Recall that we indicated the segments of the emission scenarios that were in an uncertain zone between compliance and violation of climate goals. Although there is no ideal solution to the problem of uncertainty, one way to address it would be to repeat the calcuaitions in this paper using other global models to see if similar results are obtained. Keeping in mind the uncertainties of the analysis, certain messages still stand out that are relevant to policy discussions about emission scenarios and global climate protection: Not acting to mitigate climate change (ie baseline conditions) could result in large increases of greenhouse emissions, which cause a doubling of current atmospheric levels of CO2 by 2100, and important negative impacts on crops, natural vegetation and sea level. The results of several different scenarios indicate that the increase in temperature and some climate impacts (eg on natural vegetation) may be faster in the first half of the next century than before or after. Climate policy making should take this information into account. Control of only CO2 emissions, and in only Annex I countries, will not substantially protect climate. On the other hand, controlling emissions in both Annex I and non-Annex I countries, will signiticantly slow temperature increases. Similarly, controlling non-CO2 emissions along with CO2 emissions is an effective policy to slow temperature increases. Stabilizing global greenhouse gas emissions will not prevent a sharp increase in temperature. But reducing emissions by at least l%/yr over the long run, will stabilize or reverse temperature increases. To stabilize COz in the long run at 3.50 or 450ppm, global emissions cannot substantially increase, and eventually must be signiticantly reduced. Below 450ppm global impacts of climate change are greatly reduced compared to baseline conditions, although some sensitive regions may still be affected. Stabilizing CO2 above 450ppm can have significant climate impacts, which indicates that stabilization of greenhouse gases in the atmosphere will not necessarily provide a high level of climate protection. Emission scenarios will exceed the limits of different climate indicators at different times. Therefore, in order to judge the overall effectiveness of a particular scenario in protecting climate, it is important to take into account several climate indicators at the same time. Whether or not an emissions scenario will be ‘safe’ or ‘unsafe’ depends very much on the limits assigned to climate indicators (ie the desired level of climate protection) and specified constraints on the rate of emission reductions. Differences between emission scenarios observed on the global level also occur on the regional level. An emission scenario which appears to provide greater climate protection (than other scenarios) on the global level, indeed may have substantially lower regional climate impacts. However, some especially vulnerable areas may still be affected by climate change. Any location within a safe emission corridor in 2010 will be on a path to the specified goals of short- and long-term climate protection. Nevertheless, the higher the emissions in 2010. the lower the

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Emission scenarios and global climate protection: J Alcamo and E Kreileman

l

0

0

l

allowable emissions afterwards. Hence, being high in the corridor reduces options of future generations to select climate goals and emission pathways after 2010. The size of the safe emission corridor depends very much on the assumed limits of the indicators. The stricter the limits, the lower the top and the narrower the width of the corridor. For some levels of climate protection (and constraints on emission reduction rates) the safe emissions corridor will be far below IPCC reference scenarios that assume no emission controls. This implies that this level of long-term climate protection can only be achieved by controlling the expected growth in global emissions up to year 2010. For global emissions to fall within the lowest safe emissions corridor examined in this paper, total emissions from Annex I countries must be decreased in 2010 relative to 1990. This assumes that emissions in non-Annex I countries will not be controlled. The increase in global SOz emissions could compensate somewhat for the increase in greenhouse gas emissions and raise the upper boundaries of the safe emission corridors. However, the middle values of these corridors will not change too much, and furthermore, there is some doubt that global SO2 emissions will reach far above their already high levels.

In conclusion, this paper takes a new approach to relate emission scenarios to specified climate goals. Furthermore, it proposes a procedure to identify near-term emission limits that are on the right path to shortand long-term climate protection. Together, these analyses are intended to help close the gap between the long time scales of climate change and the much shorter time frame of climate policy making.

Appendix

A

Annex I of the UN Framework Convention on Climate Change Australia, Austria, Belarus, Belgium, Bulgaria, Canada, Czechoslovakia, Denmark, European Economic Community, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Japan, Latvia, Lithuania, Luxembourg, Netherlands, New Zealand, Norway, Poland, Portugal, Romania, Russian Federation, Spain, Sweden, Switzerland, Turkey, Ukraine, United Kingdom, United States of America.

Appendix

B

Procedure for computing safe emission corridors To compute a safe emissions corridor the following steps are followed: 1.

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The IMAGE 2 model is run separately for a large number of different emission scenarios, including all of the scenarios reported before.

Emission scenurios and global climate protection: J Alcamo and E Kreileman 2.

3.

Using data from step I, a statistical relation is determined cumulative total between equivalent COz emissions and each of three climate indicators noted in the text. The indicators ‘temperature change’ and ‘sea level rise’ in 2100 relative to 1990 are correlated with the total equivalent CO? cumulative emissions over the period 1990.2100 (Figures 11A and C). The rate of indicator ‘decadal change’, is temperature correlated with the cumulative emissions of the preceding 20year period before each decade (Figure 11B) (eg the rate of temperature change in the decade 202@2030 is correlated with the cumulative equivalent CO? emissions over the period 2OlO2030). See Figure II for examples of the correlations. Using the correlations from Step 2, we can now select a climate corresponding to a goal maximum temperature change or sea level rise, and compute the maximum allowable cumulative emissions. However, this does not tell us the allowable pathway of emissions because in theory there are an infinite number of pathways that can achieve the level of cumulative same emissions. Hence. in this step we identify a sub-set of acceptable pathways. By ‘acceptable’ we mean that the pathways do not exhibit unlikely trends, for oscillating behaviour, example, or sharp reversals of direction. We compute and compile a data bank of acceptable pathways by the following specifying constraints on the pathways: l

l

45Leggett

et al. op tit,

Ref 7

Emissions of CFCs and natural sources of N20 and CH4 are taken from Baseline A, described on page 307 of this paper. The absolute rate of change of remaining anthropogenic emissions is bounded by +3% per year (the highest rate of increase of anthropogenic emissions in the IPCC scenar-

ios45) and -4% per year (the highest rate of emissions reduction evaluated for the calculation of the safe emissions corridor). The rate of change of emissions in one decade is within f2% of the rate of change of these emissions in the previous decade (eg if the rate of change in one decade is +l% per year, the rate of change in the next decade is between - 1% and +3% per year). 4.

5.

6.

7.

For the pathways found in Step 3, the cumulative equivalent CO2 emissions between 1990 and 2100 are calculated. The width and magnitude of the emission corridor depends very much on the specified limits of the three climate indicators used in the analysis. Therefore, to compute a corridor, the limits on these climate indicators over the period 1990 to 2100 have to be set. Once these limits are specified, the correlations from Step 2 are used to compute the maximum level of cumulative CO2 equivalent emissions that comply with these limits. Once the maximum allowable cumulative equivalent COz emissions have been calculated in Step 5, and a limit is set on the maximum rate of emission reductions for the CO2 equivalent emissions, the results from Step 3 are used to select the set of allowable emission scenarios. The allowable emission scenarios from Step 6 are then plotted from 1990 to 2010 to indicate the safe emissions corridor. This corridor depicts the allowable range of emissions from 1990 to 2010 that complies with the specified shortand long-term limits of both climate indicators and emission reduction rates. Any point within the corridor will be on the right path to meeting shortand long-term climate goals. But of course, meeting these goals also depends on the path of emissions after 2010, as discussed in the text.

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