Editing IPCC:AR6/WGI/Chapter-10 (section)

==== 10.6.3.8 Storyline Approaches for India ====

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Formal storyline approaches (see Box 10.2) have been used infrequently for the Indian summer monsoon, representing a knowledge gap. In an expert-elicitation approach ( [[#Dessai--2018|Dessai et al., 2018]] ), physically plausible futures substantiated by climate processes were constructed, focusing on a river basin in southern India. Possible outcomes were framed based on changes in two drivers: availability of moisture from the Arabian Sea and strength of the low-level flow. The narratives identified were able to explain 70% of the variance in monsoon rainfall over 1979–2013, the implication being that climate uncertainties could be easily communicated to stakeholders in the context of present-day variability.

The storylines terminology could be used to loosely describe the interplay between internal variability and forced change (see [[#10.6.3.6|Section 10.6.3.6]] ), such as considering the difference between groups of wettest and driest ensemble members of a SMILE for the near-term future in Figure 10.19f. However, given the interest in low-likelihood high-impact scenarios ( [[#Sutton--2018|Sutton, 2018]] ), we can also consider possible storylines for the Indian monsoon constructed from evidence in paleoclimate records and modelling. For example, a future AMOC collapse could cause reduced monsoon rainfall ( [[IPCC:Wg1:Chapter:Chapter-8#8.6.1|Section 8.6.1]] ; [[#Liu--2017|Liu et al., 2017]] ), offsetting increases expected due to GHG. Large tropical volcanic eruptions are also known to weaken the Asian summer monsoon, in observations and model simulations over the last millennium ( [[IPCC:Wg1:Chapter:Chapter-8#8.5.2.3|Section 8.5.2.3]] ; [[#Zambri--2017|Zambri et al., 2017]] ), although a hemispheric dependence is found, with Southern Hemisphere eruptions even strengthening the monsoon around India ( [[#Zuo--2019|Zuo et al., 2019]] ). Typically, future climate projections do not consider plausible eruption scenarios and their mitigating effects on greenhouse warming (see also Cross-Chapter Box 4.1). A single-model ensemble ( [[#Bethke--2017|Bethke et al., 2017]] ) demonstrates a future drier Indian monsoon relative to conditions in which volcanic eruptions are not considered, although the effects of GHG warming dominate beyond the mid-term.

The few studies on low-likelihood high-impact scenarios, often in single models, together with findings in SR1.5 ( [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ), noting the small radiative forcing in 1.5°C or 2°C scenarios, or the absence of large aerosol emissions at the end of the 21st century in RCPs, give us ''low confidence'' in abrupt changes to the monsoon on this time scale.

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