IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group III: Mitigation of Climate Change Emission reductions and timing

Figure 3.17 shows the projected CO2 emissions associated with the new mitigation scenarios. In addition, the figure depicts the range of the TAR stabilization scenarios (more than 80 scenarios) (Morita et al., 2001). Independent of the stabilization level, scenarios show that the scale of the emissions reductions, relative to the reference scenario, increases over time. Higher stabilization targets do push back the timing of most reductions, even beyond 2100.

An increasing body of literature assesses the attainability of very low targets of below 450 ppmv CO2 (e.g. Van Vuuren et al., 2007; Riahi et al., 2006). These scenarios from class I and II extend the lower boundary beyond the range of the TAR stabilization scenarios of 450 ppmv CO2 (see upper panels of Figure 3.17). The attainability of such low targets is shown to depend on: 1) using a wide range of different reduction options; and 2) the technology ‘readiness’ of advanced technologies, in particular the combination of bio-energy, carbon capture and geologic storage (BECCS). If biomass is grown sustainably, this combination may lead to negative emissions (Williams, 1998; Herzog et al., 2005), Rao and Riahi (2006), Azar et al. (2006) and Van Vuuren et al. (2007) all find that such negative emissions technologies might be essential for achieving very stringent targets.

Figure 3.17

Figure 3.17: Emissions pathways of mitigation scenarios for alternative groups of stabilization targets (Categories I to VI, see Table 3.5). The pink area gives the projected CO2 emissions for the recent mitigation scenarios developed post-TAR. Green shaded areas depict the range of more than 80 TAR stabilization scenarios (Morita et al., 2001). Category I and II scenarios explore stabilization targets below the lowest target of the TAR.

Source: After Nakicenovic et al., 2006, and Hanaoka et al., 2006

The emission range for the scenarios with low and intermediate targets between 3.5 and 5 W/m2 (scenarios in categories III and IV) are consistent with the range of the 450 and 550 ppmv CO2 scenarios in the TAR. Emissions in this category tend to show peak emissions around 2040 – with emissions in 2100 similar to, or slightly below, emissions today. Although for these categories less rapid and forceful reductions are required than for the more stringent targets, studies focusing on these stabilization categories find that a wide portfolio of reduction measures would be needed to achieve such emission pathways in a cost-effective way.

The two highest categories of stabilization scenarios (V and VI) overlap with low-medium category baseline scenarios (see Section 3.2). This partly explains the relatively small number of new studies on these categories. The emission profiles of these scenarios are found to be consistent with the emissions ranges as published in the TAR.

There is a relatively strong relationship between the cumulative CO2 emissions in the 2000–2100 period and the stringency of climate targets (see Figure 3.18). The uncertainties associated with individual stabilization levels (shown by the different percentiles[9]) are primarily due to the ranges associated with individual stabilization categories, substitutability of CO2 and non-CO2-emissions, different model parameterizations of the carbon cycle, but they are also partly due to differences in emissions pathways (delayed reduction pathways can allow for somewhat higher cumulative emissions). In general, scenarios aiming for targets below 3 W/m2 require cumulative CO2 emissions of around 1100 GtCO2 (range of 800–1500 GtCO2). The cumulative emissions increase for subsequently less stringent targets. The middle category (4–5 W/m2) requires emissions to be in the order of 3000 GtCO2 (range of 2270–3920 GtCO2). The highest category (>6 W/m2) exhibits emissions, on average, around 5020 GtCO2 (range of 4400–6600 GtCO2).

Figure 3.18

Figure 3.18: Relationship between the scenario’s cumulative carbon dioxide emissions (2000–2100) and the stabilization target (stabilization categories I to VI, of Table 3.5).

Data source: After Nakicenovic et al., 2006, and Hanaoka et al., 2006

The timing of emission reductions also depends on the stringency of the stabilization target. Timing of climate policy has always been an important topic in the scenario literature. While some studies argue for early action for smooth transitions and stimulating technology development (e.g. Azar and Dowlatabadi, 1999, Van Vuuren and De Vries, 2001), others emphasize delayed response to benefit from better technology and higher CO2 fertilization rates from natural systems at later points in time (e.g. Wigley et al., 1996; Tol, 2000; for a more elaborate discussion on timing see also Section 3.6). This implies that a given stabilization target can be consistent with a range of interim targets. Nevertheless, stringent targets require an earlier peak of CO2 emissions (see Figure 3.19 and Table 3.5). In the majority of the scenarios concerning the most stringent group (< 3 W/m2), emissions start to decline before 2015, and are further reduced to less than 50% of today’s emissions by 2050 (Table 3.5). The emissions profiles of these scenarios indicate the need for short-term infrastructure investments for a comparatively early decarbonization of the energy system. Achieving these low-emission trajectories requires a comprehensive global mitigation effort, including a further tightening of existing climate policies in Annex I countries, and simultaneous emission mitigation in developing countries, where most of the increase in emissions is expected in the coming decades. For the medium stringency group (4-5 W/m2) the peak of global emissions generally occurs around 2010 to 2030; followed by a return to 2000 levels, on average, around 2040 (with the majority of these scenarios returning to 2000 emissions levels between 2020 and 2060). For targets between 5–6 W/m2, the median emissions peak around 2070. The figure also indicates that the uncertainty range is relatively small for the more stringent targets, illustrating the reduced flexibility of the emissions path and the requirement for early mitigation. The less stringent categories allow more flexibility in timing. Most of the stringent stabilization scenarios of category I (and some II scenarios) assume a temporal overshoot of the stabilization target (GHG concentration, radiative forcing, or temperature change) before the eventual date of stabilization between 2100 and 2150. Recent studies indicate that while such ‘overshoot’ strategies might be inevitable for very low targets (given the climate system and socio-economic inertia), they might also provide important economic benefits. At the same time, however, studies note that the associated rate of warming from large overshoots might significantly increase the risk of exceeding critical climate thresholds. (For further discussion, see Section 3.3.4.)

The right-hand panel of Figure 3.19 illustrates the time at which CO2 emissions will have to return to present levels. For stringent stabilization targets (below 4 W/m2; category I, II and III) emissions return to present levels, on average, before the middle of this century, that is about one to two decades after the year in which emissions peak. In most of the scenarios for the highest stabilization category (above 6 W/m2; category VI) emissions could stay above present levels throughout the century.

Figure 3.19

Figure 3.19: Relationship between the stringency of the stabilization target (category I to VI) and 1) the time at which CO2 emissions have to peak (left-hand panel), and 2) the year when emissions return to present (2000) levels.

Data source: After Nakicenovic et al., 2006, and Hanaoka et al., 2006.

The absolute level of the required emissions reduction does not only depend on the stabilization target, but also on the baseline emissions (see Hourcade and Shukla, 2001). This is clearly shown in the right-hand panel of Figure 3.20, which illustrates the relationship between the cumulative baseline emissions and the cumulative emissions reductions for the stabilization scenarios (by 2100). In general, scenarios with high baseline emissions require a higher reduction rate to reach the same reduction target: this implies that the different reduction categories need to show up as diagonals in figure 3.20. This is indeed the case for the range of studies and the ‘category averages’ (large triangles). As indicated in the figure, a scenario with high baseline emissions requires much deeper emission reduction in order to reach a medium stabilization target (sometimes more than 3600 GtCO2) than a scenario with low baseline emissions to reach the most stringent targets (in some cases less than 1800 GtCO2). For the same target (e.g. category IV) reduction may differ from 370 to 5500 GtCO2. This comes from the large spread of emissions in the baseline scenarios. While scenarios for both stringent and less-stringent targets have been developed from low and high baseline scenarios, the data suggests that, on average, mitigation scenarios aimed at the most stringent targets start from the lowest baseline scenarios.

In the short-term (2030), the relationship between emission reduction and baseline is less clear, given the flexibility in the timing of emission reductions (left-hand panel in Figure 3.20). While the averages of the various stabilization categories are aligned in a similar way to those discussed for 2100 (with exception of category I, for which the scenario sample is smaller than for the other categories); the uncertainty ranges here are very large.

Figure 3.20

Figure 3.20: Relationship between required cumulative emissions reduction and carbon emissions in the baseline by 2030 (left-hand panel) and 2100 (right-hand panel).

Notes: Coloured rectangles denote individual scenarios for alternative stabilization targets (categories I to VI). The large triangles indicate the averages for each category.

Data source: After Nakicenovic et al., 2006, and Hanaoka et al., 2006

  1. ^  Note that the percentiles are used to illustrate the statistical properties of the scenario distributions, and should not be interpreted as likelihoods in any probabilistic context.