Atténuation du changement climatique : mesures concrètes en vue de limiter le réchauffement de la planète. Rapport du GIEC 2014 Working Group III

How are uncertainties handled in this report?

The IPCC uses a very specific language when it comes to expressing the degree of uncertainty or agreement for each statement in the fifth assessment report. For an overview of the specific meaning of each qualifier, you can read the relevant section in our summary of the Working Group I report.

What are the essential previsions of the basis scenarios of climate change?

For scenarios without mitigation efforts beyond those in place today, greenhouse gas concentrations would reach 750 to over 1300 ppm CO₂eq by 2100. Global surface temperature would increase over pre-industrial levels by 2.5 to 7.8 °C (high confidence). Greenhouse gases emissions are expected to continue to grow in all sectors except agriculture, forestry and other land uses. (robust evidence, medium agreement). The emissions from the energy supply sector would be the major source in relation with electricity use in the buildings and industry sectors. Emissions from transport and the building sectors are projected to almost double and even triple for the energy sector by 2050 compared to the level of 2010, unless energy intensity improvements can be significantly accelerated (medium evidence, medium agreement).

Meanwhile, mitigation scenarios to limit CO₂eq concentrations around 450 ppm by 2100, which are necessary to limit the global temperature increase to 2°C relative to pre-industrial levels, show the need of large‐scale global changes in the energy supply sector (robust evidence, high agreement). Scenarios that exceed about 650 ppm CO₂eq by 2100 are unlikely to limit temperature change to below 2°C relative to pre‐industrial levels.

Scenarios reaching concentrations around 450 ppm CO₂eq by 2100 are characterized by global GHG emissions 40% to 70% lower globally in 2050 than in 2010, and emissions levels near zero in 2100. These scenarios are based on models that link many important human systems (e.g., energy, agriculture and land use, economy) with physical processes associated with climate change (e.g., the carbon cycle) in order to approximate cost‐effective solutions that minimize the aggregate economic costs of achieving mitigation outcomes, unless they are specifically constrained to behave otherwise.

These models are of course simplified representations of highly‐complex, real‐world processes, and the scenarios they produce are based on relatively uncertain projections about key events and drivers over often century‐long timescales.

What are the conditions to be met to reach climate impact mitigations objectives?

The design of climate policy is influenced by how individuals and organizations perceive risks and uncertainties and take them into account. People often utilize simplified decision rules such as a preference for the status quo. Effective mitigation will not be achieved if individual agents advance their own interests independently. Climate change has the characteristics of a collective action problem at the global scale, because most greenhouse gases (GHGs) accumulate over time and mix globally, and emissions by any agent (e.g., individual, community, company, country) affect other agents.

Accurately estimating the benefits of mitigation takes into account the full range of possible impacts of climate change, including those with high consequences but a low probability of occurrence.

Social, economic and ethical analyses may be used to inform value judgments and may take into account values of various sorts, including human wellbeing, cultural values and non‐human values.

International cooperation is therefore required to effectively mitigate GHG emissions and address other climate change issues. Furthermore, research and development in support of mitigation creates knowledge spillovers. International cooperation can play a constructive role in the development, diffusion and transfer of knowledge and environmentally sound technologies.

If well‐managed, climate policy intersections with other societal goals creating the possibility of co‐benefits or adverse side-effects can strengthen the basis for undertaking climate action. In particular, economic evaluation is commonly used to inform climate policy design.

Practical tools for economic assessment include cost‐benefit analysis, cost‐effectiveness analysis, multi‐criteria analysis and expected utility theory and the limitations of these tools are well documented.

Delaying mitigation efforts beyond those in place today through 2030 is estimated to substantially increase the difficulty of the transition to low longer‐term emissions levels and narrow the range of options consistent with maintaining temperature change below 2°C relative to pre‐industrial levels (high confidence).

The problem is that infrastructure developments and long‐lived products that lock societies into GHG‐intensive emissions pathways, in particular those related to infrastructure and spatial planning, may be difficult or very costly to change. This reinforces the importance of early action for ambitious mitigation (robust evidence, high agreement). However materials, products and infrastructure with long lifetimes and low lifecycle emissions can facilitate a transition to low‐emission pathways while also reducing emissions through lower levels of material use.

What are the possible mitigation pathways and measures in the context of sustainable development?

Mitigation strategies, when associated with non‐climate policies at all government levels, can help decouple transport GHG emissions from economic growth in all regions (medium confidence).
Major options at the global level that are considered by the scenarios to reach the 450 ppm CO₂eq objective include:

1. Replacing current world average coal‐fired power plants with modern, highly efficient natural gas combined‐cycle power plants or combined heat and power plants, provided that natural gas is available and the fugitive emissions associated with extraction and supply are low or mitigated;
2. More rapid improvements of energy efficiency and a tripling to nearly a quadrupling by the year 2050 of the share of zero‐ and low‐carbon energy supply from renewables, nuclear energy and fossil energy with carbon dioxide capture and storage (CCS), or bioenergy with CCS (BECCS). These scenarios also imply a wide range of changes in land use, reflecting different assumptions about the scale of bioenergy production, afforestation, and reduced deforestation.
3. Technical and behavioural mitigation measures for all transport modes, plus new infrastructure (such as high‐speed rail systems that reduce short‐haul air travel demand) and urban redevelopment investments (urban planning, transit‐oriented development, more compact urban form that supports cycling and walking), could reduce final energy demand in 2050 by around 40% below the baseline (medium evidence, medium agreement).

Would decarbonizing and CO2 storage technologies be efficient mitigation tools?

There is insufficient knowledge to quantify how much CO₂ emissions could be partially offset by these methods as these indeed carry side‐effects and long‐term consequences on a global scale. In most integrated modelling scenarios, decarbonizing (i.e. reducing the carbon intensity) happens more rapidly in electricity generation than in the industry, buildings, and transport sectors (medium evidence, high agreement). While all components of integrated carbon storage systems (CCS) exist and are in use today by the fossil fuel extraction and refining industry, CCS has not yet been applied at scale to a large, operational commercial fossil fuel power plant. This would depend on regulatory incentives and/or if CCS units become competitive with their unabated CO₂ counterparts and if the additional investment and operational costs caused in part by efficiency reductions are compensated by sufficiently high carbon prices (or direct financial support). There are also other barriers to large‐scale deployment of CCS technologies which include concerns about the operational safety and the long‐term integrity of CO₂ storage, as well as transport risks.

Combining bioenergy with CCS (BECCS) offers another prospect of energy supply with large‐scale net negative emissions which plays an important role in many low‐stabilization scenarios, while it entails also challenges and risks (limited evidence, medium agreement).

How can the citizen contribute to the mitigation efforts to reduce CO2 emissions?

Behaviour, lifestyle and culture have a considerable influence on energy use and associated emissions, with high mitigation potential in some sectors, in particular via efficiency enhancements and behavioural changes complemented by technological and structural changes (medium evidence, medium agreement). Changes in consumption patterns (e.g., mobility demand and mode, energy use in households, choice of longer‐lasting products) and dietary change and reduction in food wastes can substantially lower emissions. Technical and behavioural mitigation measures for all transport modes, plus new infrastructure and urban redevelopment investments, could reduce final energy demand in 2050 by around 40 % below the baseline. The cost‐effectiveness of different carbon reduction measures in the transport sector in particular varies significantly with vehicle type and transport mode (high confidence). In this context, depending on transport mode and vehicle type; behavioural changes, such as cycling and walking or the choice of vehicles, may represent projected energy efficiency and vehicle performance improvements ranging from 30–50% in 2030 relative to 2010 (medium evidence, medium agreement).

A number of options including monetary and non‐monetary incentives as well as information measures may facilitate these behavioural changes. For developped counties, lifestyle and behavioural changes could reduce energy demand in buildings by up to 20% in the short term and by up to 50% of present levels by mid‐century. Indeed, a three‐ to five‐fold difference in energy use has been shown for provision of similar building‐related energy service levels in buildings.

What would be the cost of the efforts to mitigate climate change impacts?

Mitigation scenarios that reach atmospheric concentrations of about 450ppm CO₂eq by 2100 entail losses in global consumption—not including benefits of reduced climate change as well as co-benefits and adverse side‐effects of mitigation - of 1 to 4% in 2030, 2 to 6% in 2050, and 3 to 11% in 2100 relative to consumption, in the baseline scenarios, that grows anywhere from 300% to more than 900% over the century.

These assumptions suppose however that all countries of the world begin mitigation measures immediately, that there is a single global carbon price, and that all key technologies are available and have been used as a cost‐effective benchmark for estimating the macroeconomic mitigation costs. Delaying mitigation further would increase mitigation costs in the medium- to long-term.

The distribution of costs across countries can differ from the distribution of the actions themselves (high confidence). In globally cost‐effective scenarios, the majority of mitigation efforts take place in countries with the highest future emissions in baseline scenarios and the distribution of the mitigation effort across sectors is strongly influenced by the availability and performance of key technologies, such as bioenergy, Carbon Dioxide Capture and Storage (CCS) and their combination (BECCS) and large scale afforestation.

Many models could not achieve atmospheric concentration levels of about 450 ppm CO₂eq by 2100 if additional mitigation is considerably delayed or limited by the availability of the key technologies.

Under the absence or limited availability of these technologies, mitigation costs can increase substantially depending on the technology considered and delaying additional mitigation will further increases mitigation costs in the medium to long term.

There is a wide range of possible adverse side‐effects as well as co‐benefits and spillovers from climate policy, which have not been well-quantified and will depend on local circumstances and the scale, scope, and pace of implementation. Important examples include biodiversity conservation, water availability, food security, income distribution, efficiency of the taxation system, labour supply and employment, urban sprawl, and the sustainability of the growth of developing countries.

Mitigation scenarios reaching about 450 or 500 ppm CO₂eq by 2100 show that the costs for achieving air quality and energy security objectives will be reduced, with significant co‐benefits for human health and ecosystem impacts particularly high where currently legislated and planned air pollution controls are weak and for sufficiency of resources and resilience of the energy system; these scenarios did not quantify other co‐benefits or adverse side‐effects (medium confidence).

Overall, the potential for co‐benefits for energy end‐use measures should outweigh the potential for adverse side‐effects, whereas the evidence suggests this may not be the case for all energy supply and AFOLU (Agriculture, Forestry and Other Land Use) measures.

What will be the consequences of mitigation policies for the exploitation of the main energy sources?

• For fossil fuel exporters, mitigation policies could devalue fossil fuel assets and reduce revenues, but differences between regions and fuels exist (high confidence). Most mitigation scenarios are associated with reduced revenues from coal and oil trade for major exporters (high confidence). The effect of mitigation on natural gas export revenues is more uncertain, with some studies showing possible benefits for export revenues in the medium term until about 2050 (medium confidence). The availability of CCS would reduce the adverse effect of mitigation on the value of fossil fuel assets (medium confidence).
• Nuclear energy could make an increasing contribution to low‐carbon energy supply, but a variety of barriers and risks exist (robust evidence, high agreement). Those include: operational risks, and the associated concerns, uranium mining risks, financial and regulatory risks, unresolved waste management issues, nuclear weapon proliferation concerns, and adverse public opinion (robust evidence, high agreement). New fuel cycles and reactor technologies addressing some of these issues are being investigated and progress in research and development has been made concerning safety and waste disposal.
• Methane‐based fuels are already increasing their share for road vehicles and waterborne craft.
• Electricity produced from low‐carbon sources has near‐term potential for electric rail and short‐ to medium‐term potential as electric buses, light duty and 2‐wheel road vehicles are deployed.
• Hydrogen fuels from low‐carbon sources constitute longer term options.
• Liquid and gaseous biofuels already commercially available provide co‐benefits together with mitigation options that can be increased by technology advances.