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 CO2 will be much greater, involving positive feedbacks loops, which can accelerate global warming.74 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 . 7 ’ ' r 5.6. As a result, 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).76 Tipping elements at risk : 1 WC — 3°C O 3 °C — 5°C O > 5° C Greenland .^Arctic winter Ice Sheet Boreal forest Jet Stream sea-ice Arctic summer sea -ice Permafrost Alpine glaciers El Nlfto Southern Oscillation r Thermohaline circulation t ndlan summer Sahel monsoon Amazon rainforests West Antarctic Ice Sheet / Coral reefs East Antarctic Ice Sheet 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/ upIoads/sites/2 /2019/05 /SR 15 _SPM _version _ report _ LR . pdf, ( last accessed on 12 September 2019 ). 4 U.S. Global Change Research Program . Wuebbles, D.J ., et al. 2017: 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 ). 7 SU .S. Global Change Research Program . Wuebbles, D .J ., et al . 2017: Climate Science Special Report , supra note 74. Chapter 15. 76 Steffen , W., et. al . ( 14 August 2019 ), Trajectories of the Earth in the Anthropocene , PNAS 115 (33 ), p. 8354 , available at https:// www.pnas.0 rg/c0 ntent/ l 15/33 /8252 / tab-article- info ( last accessed on 12 September 2019 ).