Memorandum for the Petitioners | 26 5.4. Over the last 150 years, the atmospheric levels of CO2 have increased exponentially. Global temperature is rising, but depending on the emissions pathway followed, it has only reached a fraction of its eventual maximum. Ultimately, global mean temperature change in response to emitted COz will be much greater, involving positive feedbacks loops, which can accelerate global warming.”4 5.5. The accelerating and self-reinforcing effects of positive feedback loops can lead to irreversible climate tipping points being reached. Climate tipping points refer to critical thresholds that, once reached, create non-linear, unpredictable, and irreversible changes to the climate system.” 5.6. As aresult, a small change within the global climate system can transform relatively stable climate system to a new, but also relatively stable, climatic state. Tipping elements describe the large-scale components of the Earth System which may be subject to tipping points. Figure 1 by Steffen et al. below shows the identified possible tipping elements, together with their potential to activate other tipping elements, in a domino-like cascade that can take the Earth System to even higher temperatures (Tipping Cascades).” Tipping elements at risk: @ 1°C-3°C @ 3°c-5°C @ >s°c Figure 1: “Global map of potential tipping cascades. The individual tipping elements are color-coded according to estimated thresholds in global average surface temperature (tipping points) (12, 34). Arrows https://www. ipcc.ch/site/assets/uploads/sites/2/2019/05/SR1 5_SPM_version_report_LR.pdf, (last accessed on 12 September 2019). ™ US. Global Change Research Program, Wuebbles, D.J., et al. 20/7: Climate Science Special Report: Fourth National Climate Assessment, Volume I., U.S. Global Change Research Program, Washington, DC, USA, p. 470, available at https://science2017.globalchange.gov/chapter/2/ (last accessed on 12 September 2019); See also: NASA, Global Climate Change, The Study of Earth as an Integrated System, available at https://climate.nasa.gov/nasa_science/science/ (last accessed on 12 September 2019). 5U.S. Global Change Research Program, Wuebbles, D.J., et al. 20/7: Climate Science Special Report, supra note 74, Chapter 15. ’° Steffen, W., et. al. (14 August 2019), Trajectories of the Earth in the Anthropocene, PNAS 115 (33), p. 8354, available at https://www.pnas.org/content/1 15/33/8252/tab-article-info (last accessed on 12 September 2019).