Also in this chapter:

5. Twenty-first century temperature projections associated with the pledges

Lead authors: William Hare, Jason Lowe, Joeri Rogelj, Elizabeth Sawin, Detlef van Vuuren
Contributing authors: Valentina Bosetti, Tatsuya Hanaoka, Jiang Kejun, Ben Matthews, Brian O’Neill, Nicola Ranger, Keywan Riahi

5.1 Introduction

In the previous chapter (Chapter 4), it has been shown that, in the majority of cases, there is a gap between the 2020 emission levels expected as a result of the current pledges and the emission levels that would be consistent with either a 2° C or 1.5° C limit. For a “likely” chance of meeting the 2° C limit, the size of the gap can range between 5 and 9 GtCO2e (range of 3-18 GtCO2e) depending on the pledge case under consideration.

There is also widespread interest in the implications of 2020 pledges for long-term temperature change. Because future temperature increase is highly dependent upon cumulative emissions after 2020, it is not possible to link unambiguously current pledges with a future temperature outcome or likelihood without making assumptions about post 2020 emission levels. However, it is possible to compare 2020 emissions with IAM scenarios associated with different levels of future warming. Each of these IAM scenarios result in an emission pathway consistent with assumptions about technological and economic development. These emission pathways then lead to different levels of temperature increase in the twenty-first century. Superposition of the 2020 pledge estimates on the IAM pathways provides insight into possible long term temperature trends consistent with the pledges.