Figure 3.1
The relationship betweenby 2050.
- risks from climate change [across Reasons For Concern],
- temperature change,
- cumulative CO2 emissions, and
- changes in annual GHG emissions
(p 35)
Effective decision making to limit climate change and its effects can be informed by a wide range of analytical approaches for evaluating expected risks and benefits, recognizing the importance of governance, ethical dimensions, equity, value judgments, economic assessments and diverse perceptions and responses to risk and uncertainty. …
Adaptation and mitigation are complementary strategies for reducing and managing the risks of climate change.
Substantial emissions reductions over the next few decades can(p 32)
- reduce climate risks in the 21st century and beyond,
- increase prospects for effective adaptation,
- reduce the costs and challenges of mitigation in the longer term, and
- contribute to climate-resilient pathways for sustainable development. …
Without additional mitigation efforts beyond those in place today, and even with adaptation, warming by the end of the 21st century will lead to high to very high risk of severe, widespread, and irreversible impacts globally (high confidence).
Mitigation involves some level of co-benefits and of risks due to adverse side-effects, but these risks do not involve the same possibility of severe, widespread, and irreversible impacts as risks from climate change, increasing the benefits from near-term mitigation efforts.
(p 33)
Adaptation can reduce the risks of climate change impacts, but there are limits to its effectiveness, especially with greater magnitudes and rates of climate change.
Taking a longer-term perspective, in the context of sustainable development, increases the likelihood that more immediate adaptation actions will also enhance future options and preparedness. …
(p 36)
There are multiple mitigation pathways that are likely to limit warming to below 2°C relative to pre-industrial levels.
Limiting warming to 2.5°C or 3°C involves similar challenges, but less quickly.
These pathways would require substantial emissions reductions over the next few decades, and near zero emissions of CO2 and other long-lived GHGs over by the end of the century.
Implementing such reductions poses substantial technological, economic, social, and institutional challenges, which increase with delays in additional mitigation and if key technologies are not available.
Limiting warming to lower or higher levels involves similar challenges, but on different timescales.
(p 37)
Climate change is a threat to equitable and sustainable development.
Adaptation, mitigation, and sustainable development are closely related, with potential for synergies and trade-offs.
(p 44)
(AR5 Synthesis Report — Longer Report, 2014)
Contents
Foundations of decision-making about climate change
Climate change risks reduced by adaptation and mitigation
Characteristics of adaptation pathways
Characteristics of mitigation pathways
Interaction among mitigation, adaptation, and sustainable development
Intergovernmental Panel On Climate Change
- AR5 Synthesis Report — Longer Report, 1 November 2014.
Future Pathways for Adaptation, Mitigation and Sustainable Development
Foundations of decision-making about climate change
Sustainable development and equity provide a basis for assessing climate policies.
Limiting the effects of climate change is necessary to achieve sustainable development and equity, including poverty eradication. …
Delaying mitigation shifts burdens from the present to the future, and insufficient adaptation responses to emerging impacts are already eroding the basis for sustainable development.
(p 32)
Effective mitigation will not be achieved if individual agents advance their own interests independently. …
Cooperative responses, including international cooperation, are therefore required to effectively mitigate GHG emissions and address other climate change issues. …
The evidence suggests that outcomes seen as equitable can lead to more effective cooperation. …
Decision-making about climate change involves valuation and mediation among diverse values, and may be aided by the analytic methods of several normative disciplines. …
Analytical methods of valuation cannot identify a single best balance between mitigation, adaptation and residual climate impacts. …
Effective decision-making and risk management in the complex environment of climate change may be iterative: strategies can often be adjusted as new information and understanding develops during implementation. …
Opportunities to take advantage of positive synergies between adaptation and mitigation may decrease with time, particularly if mitigation is delayed too long. …
Climate change risks reduced by adaptation and mitigation
The risks of climate change, adaptation, and mitigation differ in nature, timescale, magnitude, and persistence (high confidence).
(p 33)
Climate change risks may persist for millennia and can involve very high risk of severe impacts and the presence of significant irreversibilities combined with limited adaptive capacity. …
Mitigation and adaptation are complementary approaches for reducing risks of climate change impacts.
They interact with one another and reduce risks over different timescales (high confidence). …
Without additional mitigation efforts beyond those in place today, and even with adaptation, warming by the end of the 21st century will lead to high to very high risk of severe, widespread and irreversible impacts, globally (high confidence).
Estimates of warming in 2100 without additional climate mitigation efforts are from 3.7°C to 4.8°C compared with pre-industrial levels …
[The] range is 2.5 °C to 7.8 °C when using the 5th to 95th percentile range of [Transient Climate Response].
The risks associated with temperatures at or above 4°C include- severe and widespread impacts on unique and threatened systems,
- substantial species extinction,
- large risks to global and regional food security,
- consequential constraints on common human activities,
- increased likelihood of triggering tipping points (critical thresholds), and
- limited potential for adaptation in some cases
Substantial cuts in greenhouse gas emissions over the next few decades can substantially reduce risks of climate change by limiting warming in the second half of the 21st century and beyond (high confidence). …
Mitigation involves some levels of co-benefits and risks, but these risks do not involve the same possibility of severe, widespread, and irreversible impacts as risks from climate change (high confidence).
Scenarios that are likely to limit warming to below 2°C or even 3°C compared with pre-industrial temperatures involve large-scale changes in energy systems and potentially land-use over the coming decades.
(p 34)
Inertia in the economic and climate systems and the possibility of irreversible impacts from climate change increase the benefits of near-term mitigation efforts (high confidence). …
Lock-ins and irreversibilities occur in the climate system due to large inertia in some of its components …
Delays in additional mitigation or constraints on technological options limit the mitigation options and increase the long-term mitigation costs …
The limits of the economic assessment of climate change risks
A subset of climate change risks and impacts are often measured using aggregate economic indicators, such as GDP or aggregate income.
Estimates, however, are partial and affected by important conceptual and empirical limitations.
Characteristics of adaptation pathways
Adaptation can contribute to- the well-being of current and future populations,
- the security of assets and
- the maintenance of ecosystem goods, functions and services now and in the future.
Adaptation planning and implementation at all levels of governance are contingent on societal values, objectives, and risk perceptions (high confidence). …
Adaptation planning and implementation can be enhanced through complementary actions across levels, from individuals to governments (high confidence). …
A first step towards adaptation to future climate change is reducing vulnerability and exposure to present climate variability (high confidence), but some near-term responses to climate change may also limit future choices.
Integration of adaptation into planning … and decision making can promote synergies with development and disaster risk reduction.
However,- poor planning or implementation,
- overemphasizing short-term outcomes, or
- failing to sufficiently anticipate consequences,
Numerous interacting constraints can impede adaptation planning and implementation (high confidence).
Common constraints on implementation arise from the following:- limited financial and human resources;
- limited integration or coordination of governance;
- uncertainties about projected impacts;
- different perceptions of risks;
- competing values;
- absence of key adaptation leaders and advocates; and
- limited tools to monitor adaptation effectiveness.
Greater rates and magnitude of climate change increase the likelihood of exceeding adaptation limits (high confidence). …
Transformations in economic, social, technological, and political decisions and actions can enhance adaptation and promote sustainable development (high confidence).
Restricting adaptation responses to incremental changes to existing systems and structures without considering transformational change, may increase costs and losses, and miss opportunities. …
Transformational adaptation can include- introduction of new technologies or practices,
- formation of new financial structures or systems of governance,
- adaptation at greater scales or magnitudes, and
- shifts in the location of activities. …
Building adaptive capacity is crucial for effective selection and implementation of adaptation options (high agreement, robust evidence). …
Significant co-benefits, synergies, and trade-offs exist between mitigation and adaptation and among different adaptation responses; interactions occur both within and across regions (very high confidence).
Characteristics of mitigation pathways
Without additional efforts to reduce GHG emissions beyond those in place today, global emission growth is expected to persist driven by growth in global population and economic activities (high confidence).
Global GHG emissions under most scenarios without additional mitigation (baseline scenarios) [fall between] the RCP 6.0 and RCP 8.5 pathways. …
[RCP 6.0 is likely to maintain warming below 4°C.]
Baseline scenarios exceed 450 parts per million (ppm) CO2eq by 2030 …
Many different combinations of technological, behavioural, and policy options can be used to reduce emissions and limit temperature change (high confidence). …
Scenarios leading to CO2-eq concentrations in 2100 of about 450 ppm or lower are likely to maintain warming below 2°C over the 21st century relative to pre-industrial levels (high confidence). …
Mitigation scenarios reaching about 450 ppm CO2eq in 2100 (consistent with a likely chance to keep warming below 2°C relative to pre-industrial level) typically involve temporary overshoot of atmospheric concentrations, as do many scenarios reaching about 500 ppm CO2eq to about 550 ppm CO2eq by 2100.
Depending on the level of overshoot, overshoot scenarios typically rely on the availability and widespread deployment of bioenergy with carbon dioxide capture and storage (BECCS) and afforestation in the second half of the century (high confidence).
(p 38)
Limiting warming with a likely chance to less than 2 °C relative to pre-industrial levels would require substantial cuts in anthropogenic GHG emissions by mid-century through large-scale changes in energy systems and possibly land use.
Limiting warming to higher levels would require similar changes, but less quickly.
Limiting warming to lower levels would require these changes more quickly (high confidence).
Scenarios that are likely to maintain warming at below 2°C [~450 ppm CO2-eq] are characterized by a 40% to 70% reduction in GHG emissions by 2050, relative to 2010 levels, and emissions level near zero or below in 2100. …Table 3.1:
Reducing emissions of non-CO2 climate forcing agents can be an important element of mitigation strategies.
The CO2 equivalent concentration in 2011 is estimated to be 430 ppm …
[Global] 2010 emissions are 31% above the 1990 emissions …
Emissions of non-CO2 gases (methane, nitrous oxide, and fluorinated gases) contributed about 27% to the total emissions of Kyoto gasses in 2010.
For most non-CO2 gases, near-term, low-cost options are available to reduce their emissions.
(p 39)
All current GHG emissions and other climate forcing agents affect the rate and magnitude of climate change over the next few decades. …
Delaying additional mitigation to 2030 will substantially increase the challenges associated with limiting warming over the 21st century to below 2°C relative to pre-industrial levels (high confidence). …
Estimated global emission levels by 2020 based on the Cancún Pledges are not consistent with cost-effective long-term mitigation trajectories that are at least about as likely as not to limit warming to below 2 °C relative to pre-industrial levels (2100 concentration levels of about 500 ppm CO2eq or below), but they do not preclude the option to meet this goal (high confidence).
The Cancún Pledges are broadly consistent with cost-effective scenarios that are likely to limit temperature change to below 3°C relative to pre-industrial levels. …
Estimates of the aggregate economic costs of mitigation vary widely depending on methodologies and assumptions, but increase with the stringency of mitigation (high confidence).
(p 40)
Many models could not limit likely warming to below 2°C over the 21st century relative to pre-industrial levels,- if additional mitigation is considerably delayed, or
- if availability of key technologies, such as bioenergy [BE], CCS, and their combination (BECCS) are limited
Mitigation efforts and associated cost are expected to vary across countries.
The distribution of costs can differ from the distribution of the actions themselves (high confidence).
Greenhouse gas metrics and mitigation pathways
(p 41–2)
Emission metrics facilitate multi-component climate policies by allowing emissions of different GHGs and other climate forcing agents to be expressed in a common unit (so-called ‘CO2 equivalent emissions’). …
The choice of emission metric and time horizon depends on type of application and policy context; hence, no single metric is optimal for all policy goals. …
The weight assigned to non-CO2 climate forcing agents relative to CO2 depends strongly on the choice of metric and time horizon (high agreement, robust evidence). …
The choice of emission metric affects the timing and emphasis placed on abating short- and long-lived climate forcing agents.
For most metrics, global cost differences are small under scenarios of global participation and cost-minimizing mitigation pathways, but implications for some individual countries and sectors could be more significant (high agreement, medium evidence).
Carbon Dioxide Removal [CDR] and Solar Radiation Management [SRM] geoengineering technologies — possible roles, options, risks and status
(p 43)
CDR plays a major role in many mitigation scenarios. …
Several CDR techniques could potentially reduce atmospheric GHG levels.
However, there are biogeochemical, technical and societal limitations that, to varying degrees, make it difficult to provide quantitative estimates of the potential for CDR. …
SRM is untested, and is not included in any of the mitigation scenarios, but, if realisable, could to some degree offset global temperature rise and some of its effects. …
If it were deployed, SRM would entail numerous uncertainties, side effects, risks and shortcomings. …
SRM technologies raise questions about costs, risks, governance, and ethical implications of development and deployment.
There are special challenges emerging for international institutions and mechanisms that could coordinate research and possibly restrain testing and deployment.
Interaction among mitigation, adaptation, and sustainable development
Climate change poses an increasing threat to equitable and sustainable development (high confidence). …
Climate change is a threat multiplier.
It exacerbates other threats to social and natural systems, placing additional burdens particularly on the poor and constraining possible development paths for all. …
Aligning climate policy with sustainable development requires attention to both adaptation and mitigation (high confidence). …
Both adaptation and mitigation can bring substantial co-benefits (medium confidence). …
- improved air quality [ie reduced black carbon and SO2];
- enhanced energy security,
- reduced energy and water consumption in urban areas through greening cities and recycling water;
- sustainable agriculture and forestry; and
- protection of ecosystems for carbon storage and other ecosystem services. …
Strategies and actions can be pursued now that will move towards climate-resilient pathways for sustainable development, while at the same time helping to improve livelihoods, social and economic well-being, and effective environmental management (high confidence).
Co-benefits and adverse side effects
(pp 44–5)
A government policy or a measure intended to achieve one objective often affects other objectives, either positively or negatively. …
Comprehensive strategies in response to climate change that are consistent with sustainable development take into account the co-benefits, adverse side-effects and risks that may arise from both adaptation and mitigation options.
