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Switching to renewable sources of energy. Solar, biomass, wind, 2100 (see Figure 6). Cumulative CO2 emissions, from 1990 to 2100, hydro and geothermal technologies already are widely used. In would range from about 450 to about 470 GTC in the alternative 1990, renewable sources of energy contributed about 20% of the LESS constructions. world's primary energy consumption, most of it fuelwood and • Higher energy efficiency is underscored for achieving deep reductions hydropower. Technological advances offer new opportunities and in CO2 emissions, for increasing the flexibility of supply side combideclining costs for energy from these sources. In the longer term, nations, and for reducing overall energy system costs. renewable sources of energy could meet a major part of the world's • Interregional trade in energy grows in the LESS constructions demand for energy. Power systems can easily accommodate compared to today's levels, expanding sustainable development limited fractions of intermittent generation, and with the addition options for Africa, Latin America and the Middle East during the of fast-responding backup and storage units, also higher fractions. next century Where biomass is sustainably regrown and used to displace fossil fuels in energy production, net carbon emissions are avoided as Costs for energy services in each LESS variant relative to costs for the CO2 released in converting the biomass to energy is again fixed conventional energy depend on relative future energy prices, which in biomass through photosynthesis. If the development of biomass are uncertain within a wide range, and on the performance and cost energy can be carried out in ways that effectively address concerns characteristics assumed for alternative technologies. However, about other environmental issues and competition with other land within the wide range of future energy prices, one or more of the uses, biomass could make major contributions in both the variants would plausibly be capable of providing the demanded electricity and fuels markets, as well as offering prospects of energy services at estimated costs that are approximately the same increasing rural employment and income.

as estimated future costs for current conventional energy. It is not

possible to identify a least-cost future energy system for the longer 4.1.4 Integration of energy system mitigation options

term, as the relative costs of options depend on resource constraints

and technological opportunities that are imperfectly known, and on To assess the potential impact of combinations of individual actions by governments and the private sector. measures at the energy system level, in contrast to the level of individual technologies, variants of a Low CO2-Emitting Energy Supply The literature provides strong support for the feasibility of achieving System (LESS) are described. The LESS constructions are "thought the performance and cost characteristics assumed for energy techexperiments" exploring possible global energy systems.

nologies in the LESS constructions, within the next two decades,

though it is impossible to be certain until the research and develThe following assumptions were made: World population grows opment is complete and the technologies have been tested in the from 5.3 billion in 1990 to 9.5 billion by 2050 and 10.5 billio by market. Moreover, these performance and cost characteristics 2100. GDP grows 7-fold by 2050 (5-fold and 14-fold in industrial- cannot be achieved without a strong and sustained investment in ized and developing countries, respectively) and 25-fold by 2100 research, development and demonstration (RD&D). Many of the (13-fold and 70-fold in industrialized and developing countries, technologies being developed would need initial support to enter respectively), relative to 1990. Because of emphasis on energy effi- the market, and to reach sufficient volume to lower costs to become ciency, primary energy consumption rises much more slowly than competitive. GDP. The energy supply constructions were made to meet energy demand in: (1) projections developed for the IPCC's First Market penetration and continued acceptability of different energy Assessment Report (1990) in a low energy demand variant, where technologies ultimately depends on their relative cost, performance global primary commercial energy use approximately doubles, with (including environmental performance), institutional arrangements, no net change for industrialized countries but a 4.4-fold increase for and regulations and policies. Because costs vary by location and applideveloping countries from 1990 to 2100; and (ii) a higher energy cation, the wide variety of circumstances creates initial opportunities demand variant, developed in the IPCC IS92a scenario where for new technologies to enter the market. Deeper understanding of the energy demand quadruples from 1990 to 2100. The energy demand opportunities for emissions reductions would require more detailed levels of the LESS constructions are consistent with the energy analysis of options, taking into account local conditions. demand mitigation chapters of this Second Assessment Report.

Because of the large number of options, there is flexibility as to how Figure 5 shows combinations of different energy sources to meet the energy supply system could evolve, and paths of energy system changing levels of demand over the next century. The analysis of development could be influenced by considerations other than these variants leads to the following conclusions:

climate change, including political, environmental (especially

indoor and urban air pollution, acidification and land restoration) • Deep reductions of CO2 emissions from energy supply systems and socio-economic circumstances.

are technically possible within 50 to 100 years, using alternative strategies.

4.2 Agriculture, rangelands and forestry • Many combinations of the options identified in this assessment

could reduce global CO2 emissions from fossil fuels from about 6 Beyond the use of biomass fuels to displace fossil fuels, the manageGtC in 1990 to about 4 GtC/yr by 2050 and to about 2 GtC/yr by ment of forests, agricultural lands and rangelands can play an


important role in reducing current emissions of CO2, CH, and N20 • Regenerating natural forests and in enhancing carbon sinks. A number of measures could conserve • Establishing tree plantations and sequester substantial amounts of carbon (approximately 60-90 • Promoting agroforestry GTC in the forestry sector alone) over the next 50 years. In the forestry • Altering management of agricultural soils and rangelands sector, costs for conserving and sequestering carbon In biomass and soil • Improving efficiency of fertilizer use are estimated to range widely but can be competitive with other miti- • Restoring degraded agricultural lands and rangelands gation options. Factors affecting costs include opportunity costs of • Recovering CH, from stored manure land; initial costs of planting and establishment; costs of nurseries; the • Improving the diet quality of ruminants. cost of annual maintenance and monitoring and transaction costs. Direct and indirect benefits will vary with national circumstances and The net amount of carbon per unit area conserved or sequestered in could offset the costs. Other practices in the agriculture sector could living biomass under a particular forest management practice and reduce emissions of other greenhouse gases such as CH, and N20. present climate is relatively well understood. The most important Land-use and management measures include:

uncertainties associated with estimating a global value are: (1) the

amount of land suitable and available for forestation, regeneration • Sustaining existing forest cover

and/or restoration programmes; (ii) the rate at which tropical defor. • Slowing deforestation

estation can actually be reduced; (iii) the long-term use (security) of

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BI = Biomass-Intensive Variant; NI = Nuclear-Intensive Variant; NGI = Natural Gas-Intensive Variant;

CI = Coal-Intensive Variant; HD = High-Demand Variant

Figure 5. Global primary energy use for alternative Low CO2-Emitting Energy Supply System (LESS) constructions: Alternatives for meeting different energy demand levels over time, using various fuel mixes.



these lands; and (iv) the continued suitability of some practices for countries would increase availability of land for production of particular locations given the possibility of changes in temperature,

biomass energy water availability and so forth under climate change.

• Geoengineering options. Some geoengineering approaches to

counterbalance greenhouse gas-induced climate change have been 4.3 Cross-sectoral issues

suggested (e.g., putting solar radiation reflectors in space or inject

ing sulfate aerosols into the atmosphere to mimic the cooling Cross-sectoral assessment of different combinations of mitigation influence of volcanic eruptions). Such approaches generally are options focuses on the interactions of the full range of technologies likely to be ineffective, expensive to sustain and/or to have serious and practices that are potentially capable of reducing emissions of environmental and other effects that are in many cases poorly greenhouse gases or sequestering carbon. Current analysis suggests understood. the following:

4.4 Policy instruments • Competing uses of land, water and other natural resources. A Mitigation depends on reducing barriers to the diffusion and transfer

growing population and expanding economy will increase the of technology, mobilizing financial resources, supporting capacity demand for land and other natural resources needed to provide, inter building in developing countries, and other approaches to assist in the alia, food, fibre, forest products and recreation services. Climate implementation of behavioral changes and technological opportunichange will interact with the resulting intensified patterns of ties in all regions of the globe. The optimum mix of policies will vary resource use. Land and other resources could also be required for from country to country, depending upon political structure and mitigation of greenhouse gas emissions. Agricultural productivity societal receptiveness. The leadership of national governments in improvements throughout the world and especially in developing applying these policies will contribute to responding to adverse

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Pigure 6. Annual CO2 emissions from fossil fuels for alternative LESS constructions, with comparison to the IPCC IS92a-l scenarios (see Figure S for acronym definitions).


consequences of climate change. Governments can choose policies that facilitate the penetration of less greenhouse gas-intensive technologies and modified consumption patterns. Indeed, many countries have extensive experience with a variety of policies that can accelerate the adoption of such technologies. This experience comes from efforts over the past 20 to 30 years to achieve improved energy effi. ciency, reduce the environmental impacts of agricultural policies, and meet conservation and environmental goals unrelated to climate change. Policies to reduce net greenhouse gas emissions appear more easily implemented when they are designed to address other concerns that impede sustainable development (e.g., air pollution and soil erosion). A number of policies, some of which may need regional or international agreement, can facilitate the penetration of less greenhouse gas-intensive technologies and modified consumption patterns, including:

• Tradable emissions permits • Voluntary programmes and negotiated agreements with industry • Utility demand-side management programmes • Regulatory programmes, including minimum energy efficiency

standards (e.g., for appliances and fuel economy) • Stimulating RD&D to make new technologies available • Market pull and demonstration programmes that stimulate the

development and application of advanced technologies • Renewable energy incentives during market build-up

Incentives such as provisions for accelerated depreciation and

reduced costs for consumers • Education and training; Information and advisory measures • Options that also support other economic and environmental


• Putting in place appropriate institutional and structural frameworks • Energy pricing strategies (e.g., carbon or energy taxes and reduced

energy subsidies) • Reducing or removing other subsidies (e.g., agricultural and

transport subsidies) that increase greenhouse gas emissions

Accelerated development of technologies that will reduce greenhouse gas emissions and enhance greenhouse gas sinks - as well as understanding the barriers that inhibit their diffusion into the marketplace – requires intensified research and development by governments and the private sector.




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