Also in this chapter:


In Figure 4, a set of 126 IAM emission pathways (see Box 2a) have been assembled that give rise to a range of likely future temperatures from below 2° C to more than 5° C. Since the emission pathways have all been generated by IAM models, the rates of decline in annual emissions in each of these scenarios are constrained by assumptions about technological and/or economic feasibility embedded in these models. Superimposed on these pathways is a bar representing the range of 2020 expected emissions derived from the pledge cases in Chapter 3.

Figure 4 shows that the range of 2020 emission levels resulting from the pledges tends to be consistent with the IAM pathways that have a likely temperature increase ranging from 2.5° C to 5° C. This is consistent with the findings in chapters 2, 3 and 4. This broad range of temperatures results from a variety of assumptions about post 2020 policy, technological and economic development.

As discussed in previous chapters, this does not mean that current pledges preclude meeting the 2° C limit. However, achieving this goal from the level of emissions resulting from the pledges would involve faster rates of decline, or greater negative emissions than included in most of the scenarios in Chapter 2. This could involve factors not assumed in the IAM scenarios considered in this report such as development of new technologies or higher economic expenditures.

One clear implication of Figure 4 is that a “likely” chance of meeting a 2° C or 1.5° C limit will require attention to two factors:

  • Implementing and strengthening 2020 emissions pledges: Implementation of the “conditional pledges, strict rules” case would bring emissions in 2020 to about 49 GtCO2e (range of 47-51 GtCO2e) compared with the 44 GtCO2e (range of 39-44 GtCO2e) that would give a “likely” chance of meeting the 2° C limit. Hence, strengthening the pledges would be needed in order to close the gap when considering the majority of results.
  • Laying the policy and investment groundwork for faster and deeper reductions in post 2020 emissions: Since all the pathways that have a “likely” chance of achieving temperature limits show strong declines in emissions after 2020 it will be important to achieve faster and deeper emission reductions post 2020.

These conclusions also hold for a “medium” chance of meeting the 2° C limit.

Figure 4: Temperature increases associated with emission pathways and compared to the expected
emissions from the pledges

(a) Coloured bands show IAM emission pathways over the twenty-first century. The pathways were
grouped based on ranges of “likely” avoided temperature increase in the twenty-first century. Emission
corridors were defined by, at each year, identifying the 20th to 80th percentile range of emissions and
drawing the corresponding coloured bands across the range. Wide gaps are visible between the coloured
bands because most of these scenarios aim for low greenhouse gas emission targets and because only
the 20th to 80th percentile of results are shown. The small black bar represents the range of the median
estimates of the pledge cases from Chapter 3 in 2020. The thin blue bar represents the range from the
80th percentile of the “unconditional pledges, lenient rules” case to the 20th percentile of the
“conditional pledges, strict rules” case.

(b) The coloured bars on the left hand side of this panel show the range (20th to 80th percentile) of 2020
emission levels from the IAM pathways consistent with a “likely” chance of avoiding
different temperature increases—as shown in panel (a). The right hand side of panel (b) compares these
emissions corridors with the 20th to 80th percentile ranges of expected emissions resulting from the four
pledge cases developed in Chapter 3.