Also in this chapter:
This section explains how the re-analysed IAM pathways relate to 2020 emission levels, and how these levels relate to the subsequent evolution of pathways that are consistent with the 2° C and 1.5° C temperature limits. Findings from stylized pathways are also discussed, because they add to our understanding of emission pathways consistent with temperature limits. It is shown that expected levels of global emissions in 2020 carry important information for policymakers about the feasibility, scale and magnitude of actions required afterwards to limit global temperature increase.
Of all IAM emission pathways that were included in our quantitative assessment, 9 were found to have a “likely” chance (greater than 66 per cent) of limiting warming to less than 2° C above pre-industrial levels. The results of our quantitative assessment (Table 1) show that the majority of emission pathways with a “likely” chance of meeting the 2° C limit show the following characteristics:
A further 18 IAM pathways were found to have a “medium” chance (50-66 per cent) of staying below a temperature increase of 2° C. The 2020 emission levels are similar (median 45 GtCO2e, range 42-46), while the emission reduction rate between 2020 and 2050 is lower (2.5 compared with 3 per cent per year), Half of these “medium” chance pathways involve net negative CO2 emissions from energy and industry, beginning between the mid-2050s and mid-2070s32.
In general, “medium” chance pathways for 2° C differ from “likely” chance pathways either by having higher emission levels in 2020 but the same rates of emission reductions afterwards, or having the same emission levels in 2020 but slower reduction rates afterwards. “Likely” chance pathways also rely more often on negative emissions.
The re-analysed set of stylized pathways (not included in Table 1) shows that, if emissions ranged up to 50 GtCO2e in 2020, average reduction rates of up to 4 per cent per year would be needed in the 2020-2050 period to meet the 2° C limit (Rogeljet al. 2010b, Schaeffer and Hare 2009)33. The high end of these reduction rates is currently not found in the IAM literature. These pathways also require large negative emissions in the second half of this century to meet the temperature limit.
Another important message from analysing IAM emission pathways is that they suggest that it is economically and technologically feasible to achieve substantial emission reductions. This implies that it is possible to reach emission levels consistent with a 2° C target (i.e. approximately 44 GtCO2e in 2020).
To have a higher confidence of staying below a 2° C limit, it seems essential to deploy negative emission technologies (to reduce CO2 from energy and industry) in the second half of the century, that is, unless emission levels are significantly below 44 GtCO2e or below in 2020.
None of the IAM or “stylized” pathways in this assessment lead to temperature increases below 1.5° C throughout this century. One IAM study published by Magné et al. (2010) depicts an emission pathway with a “medium” chance of achieving the 1.5° C target by the end of the century and has 2020 emissions of 41 GtCO2e. These results suggest that after a small (0.1° C) transient overshoot of the temperature limit of about half a century, the temperature increase by the end of the twenty-first century could be brought back to below 1.5° C with a “medium” chance. In general, the IAM pathways that meet the 2° C limit with a “likely” chance also meet the 1.5° C target by 2100 but with a lower probability of 30 per cent (range 27-35 per cent for the 20th-80th percentile) and with a median temperature peak at some point in the twenty-first century of between 1.6° C and 1.7° C.
A few studies have used stylized pathways to explore the achievement of a 1.5° C limit in more detail (Ranger et al. 2010, Schaeffer and Hare 2009). The stylized pathways included in this assessment suggest that limiting warming to 1.5° C by 2100 (with a “medium” to “likely” chance) means 2020 emission levels of 40 to 48 GtCO2e (20th-80th percentile range), and reduction rates of 3 to 5 per cent per year in the 2020-2050 period (Schaeffer and Hare 2009). These pathways would also employ negative CO2 emissions in the second half of this century. As discussed in Section 2.3, the feasibility of achieving such high emission reduction rates is difficult to assess and they are not found in the current literature of IAM results.
A few studies have used stylized pathways to explore the achievement of a 1.5° C limit in more detail (Ranger et al. 2010, Schaeffer and Hare 2009). The stylized pathways included in this assessment suggest that limiting warming to 1.5° C by 2100 (with a “medium” to “likely” chance) means 2020 emission levels of 40 to 48 GtCO2e (20th-80th percentile range), and reduction rates of 3 to 5 per cent per year in the 2020-2050 period (Schaeffer and Hare 2009). These pathways would also employ negative CO2 emissions in the second half of this century. As discussed in Section 2.2, the feasibility of achieving such high emission reduction rates is difficult to assess and they are not found in the current literature of IAM results.
In the text we have focused on the “majority of results” of the re-analysed IAM pathway set (the median and 20th to 80th percentile range). However, results outside this range are also valid and provide useful information.
We first consider the high end of the range of expected emissions in 2020 represented by results from van Vuuren et al. 2007 for a “likely” chance to stay below a 2ºC limit, and O'Neill et al. 2009 for a “medium” chance. At this end of the range emissions are 48 GtCO2e. For a “likely” chance to achieve the temperature target, average reduction rates between 2020 and 2050 (of CO2 emissions from energy and industry) are 3.2 per cent per year, and for a medium chance 3.6 per cent per year34. These set the upper range of emissions and reduction rates.
The low end of the range shows that relatively low emission reduction rates between 2020 and 2050 are sufficient to reach the temperature limit, if 2020 emission levels are at the low end of the range. Some pathways, for example in Barker and Scrieciu (2010) and Clarke et al. (2009), indicate 2020 emission levels of 26-36 GtCO2e. These results suggest that it may be technologically and economically feasible to reduce global emissions by 2020 by substantially more than the majority of IAM pathways assume.