1. This report assesses the scientific, technological,
environmental, economic and social aspects of the mitigation of climate change.
Research in climate change mitigation 1
has continued since the publication of the IPCC Second Assessment Report (SAR),
taking into account political changes such as the agreement on the Kyoto Protocol
to the United Nations Framework Convention on Climate Change (UNFCCC) in 1997,
and is reported on here. The Report also draws on a number of IPCC Special Reports,
notably the Special Report on Aviation and the Global Atmosphere, the Special
Report on Methodological and Technological Issues in Technology Transfer (SRTT),
the Special Report on Emissions Scenarios (SRES), and the Special Report on
Land Use, Land Use Change and Forestry (SRLULUCF).
2. Climate change 2 is a problem with unique characteristics. It is global, long-term (up to several centuries), and involves complex interactions between climatic, environmental, economic, political, institutional, social and technological processes. This may have significant international and intergenerational implications in the context of broader societal goals such as equity and sustainable development. Developing a response to climate change is characterized by decision-making under uncertainty and risk, including the possibility of non-linear and/or irreversible changes (Sections 1.2.5, 1.3, 10.1.2, 10.1.4, 10.4.5). 3
Figure SPM.1: Comparison of reference and stabilization scenarios. The figure is divided into six parts, one for each of the reference scenario groups from the Special Report on Emissions Scenarios (SRES, see Box SPM.1). Each part of the figure shows the range of total global CO2 emissions (gigatonnes of carbon (GtC)) from all anthropogenic sources for the SRES reference scenario group (shaded in grey) and the ranges for the various mitigation scenarios assessed in the TAR leading to stabilization of CO2 concentrations at various levels (shaded in colour). Scenarios are presented for the A1 family subdivided into three groups (the balanced A1B group (Figure SPM.1a), non-fossil fuel A1T (Figure SPM.1b) and the fossil intensive A1FI (Figure SPM.1c)) with stabilization of CO2 concentrations at 450, 550, 650 and 750 ppmv; for the A2 group with stabilization at 550 and 750 ppmv in Figure SPM.1d, the B1 group with stabilization at 450 and 550 ppmv in Figure SPM.1e, and the B2 group with stabilization at 450, 550 and 650 ppmv in Figure SPM.1f. The literature is not available to assess 1000 ppmv stabilization scenarios. The figure illustrates that the lower the stabilization level and the higher the baseline emissions, the wider the gap. The difference between emissions in different scenario groups can be as large as the gap between reference and stabilization scenarios within one scenario group. The dotted lines depict the boundaries of the ranges where they overlap.
3. Alternative development paths 4 can result in very different greenhouse gas emissions. The SRES and the mitigation scenarios assessed in this report suggest that the type, magnitude, timing and costs of mitigation depend on different national circumstances and socio-economic, and technological development paths and the desired level of greenhouse gas concentration stabilization in the atmosphere (see Figure SPM.1 for an example for total CO2 emissions). Development paths leading to low emissions depend on a wide range of policy choices and require major policy changes in areas other than climate change (Sections 2.2.2, 2.3.2, 2.4.4, 2.5).
4. Climate change mitigation will both be affected by, and have impacts on, broader socio-economic policies and trends, such as those relating to development, sustainability and equity. Climate mitigation policies may promote sustainable development when they are consistent with such broader societal objectives. Some mitigation actions may yield extensive benefits in areas outside of climate change: for example, they may reduce health problems; increase employment; reduce negative environmental impacts (like air pollution); protect and enhance forests, soils and watersheds; reduce those subsidies and taxes which enhance greenhouse gas emissions; and induce technological change and diffusion, contributing to wider goals of sustainable development. Similarly, development paths that meet sustainable development objectives may result in lower levels of greenhouse gas emissions (Sections 1.3, 1.4, 2.2.3, 2.4.4, 2.5, 7.2.2, 8.2.4).
5. Differences in the distribution of technological, natural and financial resources among and within nations and regions, and between generations, as well as differences in mitigation costs, are often key considerations in the analysis of climate change mitigation options. Much of the debate about the future differentiation of contributions of countries to mitigation and related equity issues also considers these circumstances 5 . The challenge of addressing climate change raises an important issue of equity, namely the extent to which the impacts of climate change or mitigation policies create or exacerbate inequities both within and across nations and regions. Greenhouse gas stabilization scenarios assessed in this report (except those where stabilization occurs without new climate policies, e.g. B1) assume that developed countries and countries with economies in transition limit and reduce their greenhouse gas emissions first.6
6. Lower emissions scenarios require different patterns
of energy resource development. Figure SPM.2
compares the cumulative carbon emissions between 1990 and 2100 for various SRES
scenarios to carbon contained in global fossil fuel reserves and resources 7
. This figure shows that there are abundant fossil fuel resources that will
not limit carbon emissions during the 21st century. However, different
from the relatively large coal and unconventional oil and gas deposits, the
carbon in proven conventional oil and gas reserves, or in conventional oil resources,
is much less than the cumulative carbon emissions associated with stabilization
of carbon dioxide at levels of 450 ppmv or higher (the reference to a particular
concentration level does not imply an agreed-upon desirability of stabilization
at this level). These resource data may imply a change in the energy mix and
the introduction of new sources of energy during the 21st century.
The choice of energy mix and associated investment will determine whether, and
if so, at what level and cost, greenhouse concentrations can be stabilized.
Currently most such investment is directed towards discovering and developing
more conventional and unconventional fossil resources (Sections 2.5.1,
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