IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Synthesis Report

5.4 Emission trajectories for stabilisation

In order to stabilise the concentration of GHGs in the atmosphere, emissions would need to peak and decline thereafter.[28] The lower the stabilisation level, the more quickly this peak and decline would need to occur (Figure 5.1).[29] {WGIII 3.3, 3.5, SPM}

Advances in modelling since the TAR permit the assessment of multi-gas mitigation strategies for exploring the attainability and costs for achieving stabilisation of GHG concentrations. These scenarios explore a wider range of future scenarios, including lower levels of stabilisation, than reported in the TAR. {WGIII 3.3, 3.5, SPM}

Mitigation efforts over the next two to three decades will have a large impact on opportunities to achieve lower stabilisation levels (Table 5.1 and Figure 5.1). {WGIII 3.5, SPM}

CO2 emissions and equilibrium temperature increases for a range of stabilisation levels

Figure 5.1 Errata

Figure 5.1. Global CO2 emissions for 1940 to 2000 and emissions ranges for categories of stabilisation scenarios from 2000 to 2100 (left-hand panel); and the corresponding relationship between the stabilisation target and the likely equilibrium global average temperature increase above pre-industrial (right-hand panel). Approaching equilibrium can take several centuries, especially for scenarios with higher levels of stabilisation. Coloured shadings show stabilisation scenarios grouped according to different targets (stabilisation category I to VI). The right-hand panel shows ranges of global average 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). Black dashed lines in the left panel give the emissions range of recent baseline scenarios published since the SRES (2000). Emissions ranges of the stabilisation scenarios comprise CO2-only and multigas scenarios and correspond to the 10th to 90th percentile of the full scenario distribution. Note: CO2 emissions in most models do not include emissions from decay of above ground biomass that remains after logging and deforestation, and from peat fires and drained peat soils. {WGIII Figures SPM.7 and SPM.8}

Table 5.1. Characteristics of post-TAR stabilisation scenarios and resulting long-term equilibrium global average temperature and the sea level rise component from thermal expansion only.a {WGI 10.7; WGIII Table TS.2, Table 3.10, Table SPM.5}

Category CO2 concentration at stabilisation (2005 = 379 ppm) b CO2-equivalent concentration at stabilisation including GHGs and aerosols (2005 = 375 ppm)b Peaking year for CO2 emissionsa,c Change in global CO2 emissions in 2050 (percent of 2000 emissions) a,c Global average temperature increase above pre-industrial at equilibrium, using ‘best estimate’ climate sensitivityd, e Global average sea level rise above pre-industrial at equilibrium from thermal expansion onlyf Number of assessed scenarios 
  ppm  ppm  year  percent  °C  metres   
I  350 – 400  445 – 490  2000 – 2015  -85 to -50  2.0 – 2.4  0.4 – 1.4  
II  400 – 440  490 – 535  2000 – 2020  -60 to -30  2.4 – 2.8  0.5 – 1.7  18 
III  440 – 485  535 – 590  2010 – 2030  -30 to +5  2.8 – 3.2  0.6 – 1.9  21 
IV  485 – 570  590 – 710  2020 – 2060  +10 to +60  3.2 – 4.0  0.6 – 2.4  118 
V  570 – 660  710 – 855  2050 – 2080  +25 to +85  4.0 – 4.9  0.8 – 2.9  
VI  660 – 790  855 – 1130  2060 – 2090  +90 to +140  4.9 – 6.1  1.0 – 3.7  


a) The emission reductions to meet a particular stabilisation level reported in the mitigation studies assessed here might be underestimated due to missing carbon cycle feedbacks (see also Topic 2.3).

b) Atmospheric CO2 concentrations were 379ppm in 2005. The best estimate of total CO2-eq concentration in 2005 for all long-lived GHGs is about 455ppm, while the corresponding value including the net effect of all anthropogenic forcing agents is 375ppm CO2-eq.

c) 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 (see Figure 2.1).

d) The best estimate of climate sensitivity is 3°C.

e) Note that global average temperature at equilibrium is different from expected global average temperature at the time of stabilisation 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 (see also Footnote 30).

f) Equilibrium sea level rise is for the contribution from ocean thermal expansion only and does not reach equilibrium for at least many centuries. These values have been estimated using relatively simple climate models (one low-resolution AOGCM and several EMICs based on the best estimate of 3°C climate sensitivity) and do not include contributions from melting ice sheets, glaciers and ice caps. Long-term thermal expansion is projected to result in 0.2 to 0.6m per degree Celsius of global average warming above pre-industrial. (AOGCM refers to Atmosphere-Ocean General Circulation Model and EMICs to Earth System Models of Intermediate Complexity.)

Table 5.1 summarises the required emission levels for different groups of stabilisation concentrations and the resulting equilibrium global average temperature increases, using the ‘best estimate’ of climate sensitivity (see Figure 5.1 for the likely range of uncertainty). Stabilisation at lower concentration and related equilibrium temperature levels advances the date when emissions need to peak and requires greater emissions reductions by 2050.[30] Climate sensitivity is a key uncertainty for mitigation scenarios that aim to meet specific temperature levels. The timing and level of mitigation to reach a given temperature stabilisation level is earlier and more stringent if climate sensitivity is high than if it is low. {WGIII 3.3, 3.4, 3.5, 3.6, SPM}

Sea level rise under warming is inevitable. Thermal expansion would continue for many centuries after GHG concentrations have stabilised, for any of the stabilisation levels assessed, causing an eventual sea level rise much larger than projected for the 21st century (Table 5.1). If GHG and aerosol concentrations had been stabilised at year 2000 levels, thermal expansion alone would be expected to lead to further sea level rise of 0.3 to 0.8m. The eventual contributions from Greenland ice sheet loss could be several metres, and larger than from thermal expansion, should warming in excess of 1.9 to 4.6°C above pre-industrial be sustained over many centuries. These long-term consequences would have major implications for world coastlines. The long time scale of thermal expansion and ice sheet response to warming imply that mitigation strategies that seek to stabilise GHG concentrations (or radiative forcing) at or above present levels do not stabilise sea level for many centuries. {WGI 10.7}

Feedbacks between the carbon cycle and climate change affect the required mitigation and adaptation response to climate change. Climate-carbon cycle coupling is expected to increase the fraction of anthropogenic emissions that remains in the atmosphere as the climate system warms (see Topics 2.3 and 3.2.1), but mitigation studies have not yet incorporated the full range of these feedbacks. As a consequence, the emission reductions to meet a particular stabilisation level reported in the mitigation studies assessed in Table 5.1 might be underestimated. Based on current understanding of climate-carbon cycle feedbacks, model studies suggest that stabilising CO2 concentrations at, for example, 450ppm[31] could require cumulative emissions over the 21st century to be less than 1800 [1370 to 2200] GtCO2, which is about 27% less than the 2460 [2310 to 2600] GtCO2 determined without consideration of carbon cycle feedbacks. {SYR 2.3, 3.2.1; WGI 7.3, 10.4, SPM}

  1. ^  Peaking means that the emissions need to reach a maximum before they decline later.
  2. ^  For the lowest mitigation scenario category assessed, emissions would need to peak by 2015 and for the highest by 2090 (see Table 5.1). Scenarios that use alternative emission pathways show substantial differences on the rate of global climate change. {WGII 19.4}
  3. ^  Estimates for the evolution of temperature over the course of this century are not available in the AR4 for the stabilisation scenarios. For most stabilisation levels global average temperature is approaching the equilibrium level over a few centuries. For the much lower stabilisation scenarios (category I and II, Figure 5.1), the equilibrium temperature may be reached earlier.
  4. ^  To stabilise at 1000ppm CO2, this feedback could require that cumulative emissions be reduced from a model average of approximately 5190 [4910 to 5460] GtCO2 to approximately 4030 [3590 to 4580] GtCO2. {WGI 7.3, 10.4, SPM}