IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group III: Mitigation of Climate Change

D. Mitigation in the long term (after 2030)

18. In order to stabilize the concentration of GHGs in the atmosphere, emissions would need to peak and decline thereafter. The lower the stabilization level, the more quickly this peak and decline would need to occur. Mitigation efforts over the next two to three decades will have a large impact on opportunities to achieve lower stabilization levels (see Table SPM.5, and Figure SPM.8)[26] (high agreement, much evidence).

  • Recent studies using multi-gas reduction have explored lower stabilization levels than reported in TAR [3.3].
  • Assessed studies contain a range of emissions profiles for achieving stabilization of GHG concentrations[27]. Most of these studies used a least cost approach and include both early and delayed emission reductions (Figure SPM.7) [Box SPM.2]. Table SPM.5 summarizes the required emissions levels for different groups of stabilization concentrations and the associated equilibrium global mean temperature increase[28], using the ‘best estimate’ of climate sensitivity (see also Figure SPM.8 for the likely range of uncertainty)[29]. Stabilization at lower concentration and related equilibrium temperature levels advances the date when emissions need to peak, and requires greater emissions reductions by 2050 [3.3].

Table SPM.5: Characteristics of post-TAR stabilization scenarios [Table TS.2, 3.10]a) Errata

Category Radiative forcing (W/m2CO2 concentrationc) (ppm) CO2-eq concentrationc) (ppm) Global mean temperature increase above pre-industrial at equilibrium, using “best estimate” climate sensitivityb), c) (ºC) Peaking year for CO2 emissionsd) Change in global CO2 emissions in 2050 (% of 2000 emissions)d) No. of assessed scenarios 
2.5-3.0 350-400 445-490 2.0-2.4 2000-2015 -85 to -50 
II 3.0-3.5 400-440 490-535 2.4-2.8 2000-2020 -60 to -30 18 
III 3.5-4.0 440-485 535-590 2.8-3.2 2010-2030 -30 to +5 21 
IV 4.0-5.0 485-570 590-710 3.2-4.0 2020-2060 +10 to +60 118 
5.0-6.0 570-660 710-855 4.0-4.9 2050-2080 +25 to +85 
VI 6.0-7.5 660-790 855-1130 4.9-6.1 2060-2090 +90 to +140 
Total 177 

a) The understanding of the climate system response to radiative forcing as well as feedbacks is assessed in detail in the AR4 WGI Report. Feedbacks between the carbon cycle and climate change affect the required mitigation for a particular stabilization level of atmospheric carbon dioxide concentration. These feedbacks are expected to increase the fraction of anthropogenic emissions that remains in the atmosphere as the climate system warms. Therefore, the emission reductions to meet a particular stabilization level reported in the mitigation studies assessed here might be underestimated.

b) The best estimate of climate sensitivity is 3ºC [WG 1 SPM].

c) Note that global mean temperature at equilibrium is different from expected global mean temperature at the time of stabilization of GHG concentrations due to the inertia of the climate system. For the majority of scenarios assessed, stabilisation of GHG concentrations occurs between 2100 and 2150.

d) Ranges correspond to the 15th to 85th percentile of the post-TAR scenario distribution. CO2 emissions are shown so multi-gas scenarios can be compared with CO2-only scenarios.


Figure SPM.7

Figure SPM.7: Emissions pathways of mitigation scenarios for alternative categories of stabilization levels (Category I to VI as defined in the box in each panel). The pathways are for CO2 emissions only. Light brown shaded areas give the CO2 emissions for the post-TAR emissions scenarios. Green shaded and hatched areas depict the range of more than 80 TAR stabilization scenarios. Base year emissions may differ between models due to differences in sector and industry coverage. To reach the lower stabilization levels some scenarios deploy removal of CO2 from the atmosphere (negative emissions) using technologies such as biomass energy production utilizing carbon capture and storage. [Figure 3.17]

Figure SPM.8 Errata

Figure SPM.8: Stabilization scenario categories as reported in Figure SPM.7 (coloured bands) and their relationship to equilibrium global mean temperature change above pre-industrial, using (i) “best estimate” climate sensitivity of 3°C (black line in middle of shaded area), (ii) upper bound of likely range of climate sensitivity of 4.5°C (red line at top of shaded area) (iii) lower bound of likely range of climate sensitivity of 2°C (blue line at bottom of shaded area). Coloured shading shows the concentration bands for stabilization of greenhouse gases in the atmosphere Errata corresponding to the stabilization scenario categories I to VI as indicated in Figure SPM.7. The data are drawn from AR4 WGI, Chapter 10.8. Errata

19. The range of stabilization levels assessed can be achieved by deployment of a portfolio of technologies that are currently available and those that are expected to be commercialised in coming decades. This assumes that appropriate and effective incentives are in place for development, acquisition, deployment and diffusion of technologies and for addressing related barriers (high agreement, much evidence).

  • The contribution of different technologies to emission reductions required for stabilization will vary over time, region and stabilization level.
  • Energy efficiency plays a key role across many scenarios for most regions and timescales.
  • For lower stabilization levels, scenarios put more emphasis on the use of low-carbon energy sources, such as renewable energy and nuclear power, and the use of CO2 capture and storage (CCS). In these scenarios improvements of carbon intensity of energy supply and the whole economy need to be much faster than in the past.
  • Investments in and world-wide deployment of low-GHG emission technologies as well as technology improvements through public and private Research, Development & Demonstration (RD&D) would be required for achieving stabilization targets as well as cost reduction. The lower the stabilization levels, especially those of 550 ppm CO2-eq or lower, the greater the need for more efficient RD&D efforts and investment in new technologies during the next few decades. This requires that barriers to development, acquisition, deployment and diffusion of technologies are effectively addressed.
  • Appropriate incentives could address these barriers and help realize the goals across a wide portfolio of technologies. [2.7, 3.3, 3.4, 3.6, 4.3, 4.4, 4.6].

Figure SPM.9

Figure SPM.9: Cumulative emissions reductions for alternative mitigation measures for 2000 to 2030 (left-hand panel) and for 2000-2100 (right-hand panel). The figure shows illustrative scenarios from four models (AIM, IMAGE, IPAC and MESSAGE) aiming at the stabilization at 490-540 ppm CO2-eq and levels of 650 ppm CO2-eq, respectively. Dark bars denote reductions for a target of 650 ppm CO2-eq and light bars the additional reductions to achieve 490-540 ppm CO2-eq. Note that some models do not consider mitigation through forest sink enhancement (AIM and IPAC) or CCS (AIM) and that the share of low-carbon energy options in total energy supply is also determined by inclusion of these options in the baseline. CCS includes carbon capture and storage from biomass. Forest sinks include reducing emissions from deforestation. [Figure 3.23]

20. In 2050[30] global average macro-economic costs for multi-gas mitigation towards stabilization between 710 and 445 ppm CO2-eq, are between a 1% gain to a 5.5% decrease of global GDP (see Table SPM.6). For specific countries and sectors, costs vary considerably from the global average. (See Box SPM.3 and SPM.4 for the methodologies and assumptions and paragraph 5 for explanation of negative costs) (high agreement, medium evidence).

Table SPM.6: Estimated global macro-economic costs in 2050 relative to the baseline for least-cost trajectories towards different long-term stabilization targetsa) [3.3, 13.3]

Stabilization levels (ppm CO2-eq) Median GDP reductionb) (%) Range of GDP reductionb), c) (%) Reduction of average annual GDP growth ratesb), d) (percentage points) 
590-710 0.5 -1 - 2 <0.05 
535-590 1.3 slightly negative - 4 <0.1 
445-535e) not available <5.5 <0.12 


a) This corresponds to the full literature across all baselines and mitigation scenarios that provide GDP numbers.

b) This is global GDP based market exchange rates.

c) The median and the 10th and 90th percentile range of the analyzed data are given.

d) The calculation of the reduction of the annual growth rate is based on the average reduction during the period until 2050 that would result in the indicated GDP decrease in 2050.

e) The number of studies is relatively small and they generally use low baselines. High emissions baselines generally lead to higher costs.

21. Decision-making about the appropriate level of global mitigation over time involves an iterative risk management process that includes mitigation and adaptation, taking into account actual and avoided climate change damages, co-benefits, sustainability, equity, and attitudes to risk. Choices about the scale and timing of GHG mitigation involve balancing the economic costs of more rapid emission reductions now against the corresponding medium-term and long-term climate risks of delay [high agreement, much evidence].

  • Limited and early analytical results from integrated analyses of the costs and benefits of mitigation indicate that these are broadly comparable in magnitude, but do not as yet permit an unambiguous determination of an emissions pathway or stabilization level where benefits exceed costs [3.5].
  • Integrated assessment of the economic costs and benefits of different mitigation pathways shows that the economically optimal timing and level of mitigation depends upon the uncertain shape and character of the assumed climate change damage cost curve. To illustrate this dependency:
  • if the climate change damage cost curve grows slowly and regularly, and there is good foresight (which increases the potential for timely adaptation), later and less stringent mitigation is economically justified;
  • alternatively if the damage cost curve increases steeply, or contains non-linearities (e.g. vulnerability thresholds or even small probabilities of catastrophic events), earlier and more stringent mitigation is economically justified [3.6].
  • Climate sensitivity is a key uncertainty for mitigation scenarios that aim to meet a specific temperature level. Studies show that if climate sensitivity is high then the timing and level of mitigation is earlier and more stringent than when it is low [3.5, 3.6].
  • Delayed emission reductions lead to investments that lock in more emission-intensive infrastructure and development pathways. This significantly constrains the opportunities to achieve lower stabilization levels (as shown in Table SPM.5) and increases the risk of more severe climate change impacts [3.4, 3.1, 3.5, 3.6]
  • Climate sensitivity is a key uncertainty for mitigation scenarios that aim to meet a specific temperature level. Studies show that if climate sensitivity is high then the timing and level of mitigation is earlier and more stringent than when it is low [3.5, 3.6].
  • Delayed emission reductions lead to investments that lock in more emission-intensive infrastructure and development pathways. This significantly constrains the opportunities to achieve lower stabilization levels (as shown in Table SPM.5) and increases the risk of more severe climate change impacts [3.4, 3.1, 3.5, 3.6]
  1. ^  Paragraph 2 addresses historical GHG emissions since pre-industrial times.
  2. ^  Studies vary in terms of the point in time stabilization is achieved; generally this is around 2100 or later.
  3. ^  The information on global mean temperature is taken from the AR4 WGI report, chapter 10.8. Errata These temperatures are reached well after concentrations are stabilized.
  4. ^  The equilibrium climate sensitivity is a measure of the climate system response to sustained radiative forcing. It is not a projection but is defined as the global average surface warming following a doubling of carbon dioxide concentrations [AR4 WGI SPM].
  5. ^  Cost estimates for 2030 are presented in paragraph 5.