Also in this chapter:
The ability to assess pathways consistent with specific temperature limits depends on understanding both the climate system and the global energy system, as well as the ways in which each responds to change over time.
Important uncertainties exist in our understanding of the climate system. We have accounted for some of this uncertainty by examining the probability of meeting particular temperature limits. Future shifts in the underlying probability distributions, as a result of improved understanding of parameters and/or feedbacks in the climate system, could change the expected probability with which a certain pathway would meet a specified temperature limit. There is also much uncertainty around the issue of how rapidly temperatures may be reduced after overshooting, and the reversibility of associated climate system changes.
Our understanding of the feasibility of pathways is also incomplete. Many of the pathways assessed here were not designed to specifically investigate the limits to feasible emission reductions, and none of the studies were designed explicitly to explore the full range of emissions in 2020 that would be consistent with long-term temperature limits. Research specifically targeted to address these questions would improve our understanding of which pathways can feasibly achieve temperature targets.
In addition, emission pathways now considered infeasible could become feasible if variables such as population growth rate, consumption of energy, aerosol emissions, economic growth and technological developments turn out to be different from the assumptions used in current studies. Other factors could also make emission pathways feasible such as the willingness of society to take “extreme” action by retiring energy infrastructure before the end of its useful lifetime, or by making significant lifestyle changes. Similarly, the pathways thought to be feasible in this report could in practice be unachievable, if, for example, participation in mitigation efforts was limited across sectors and countries, or if technological and socio-economic barriers were more severe than expected.
Given these uncertainties, it will be crucial over time to re-evaluate the emission pathways consistent with particular temperature limits and to inform the policy community accordingly.