Editing IPCC:AR6/SR15/Chapter-1 (section)

==== 1.2.3.1 Pathways remaining below 1.5°C ====

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In this category of 1.5°C pathways, human-induced warming either rises monotonically to stabilise at 1.5°C (Figure 1.4, brown lines) or peaks at or below 1.5°C and then declines (yellow lines). Figure 1.4b demonstrates that pathways remaining below 1.5°C require net annual CO <sub>2</sub> emissions to peak and decline to near zero or below, depending on the long-term adjustment of the carbon cycle and non-CO <sub>2</sub> emissions (Bowerman et al., 2013; Wigley, 2018) <sup>[[#fn:r115|115]]</sup> . Reducing emissions to zero corresponds to stabilizing cumulative CO <sub>2</sub> emissions (Figure 1.4c, solid lines) and falling concentrations of CO <sub>2</sub> in the atmosphere (panel c dashed lines) (Matthews and Caldeira, 2008; Solomon et al., 2009) <sup>[[#fn:r116|116]]</sup> , which is required to stabilize GMST if non-CO <sub>2</sub> climate forcings are constant and positive. Stabilizing atmospheric greenhouse gas concentrations would result in continued warming (see Section 1.2.4).

If emission reductions do not begin until temperatures are close to the proposed limit, pathways remaining below 1.5°C necessarily involve much faster rates of net CO <sub>2</sub> emission reductions (Figure 1.4, green lines), combined with rapid reductions in non-CO <sub>2</sub> forcing and these pathways also reach 1.5°C earlier. Note that the emissions associated with these schematic temperature pathways may not correspond to feasible emission scenarios, but they do illustrate the fact that the timing of net zero emissions does not in itself determine peak warming: what matters is total cumulative emissions up to that time. Hence every year’s delay before initiating emission reductions decreases by approximately two years the remaining time available to reach zero emissions on a pathway still remaining below 1.5°C (Allen and Stocker, 2013; Leach et al., 2018) <sup>[[#fn:r117|117]]</sup> .

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