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

==== 10.6.3.9 Regional Climate Information Distilled from Multiple Lines of Evidence ====

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Above, we presented assessments from observational and model attribution studies of the historical period, followed by future climate projections in global and regional models, and storylines approaches including low-likelihood high impact events. Miscellaneous lines of evidence are considered here.

Our assessment could also be informed by attempting to constrain future projections of the Indian summer monsoon using paleoclimate evidence. In modelling work of the mid-Holocene ( [[#D’Agostino--2019|D’Agostino et al., 2019]] ), the increased obliquity (axial tilt) and altered orbital precession lead to an enhanced monsoon with a stronger dynamic component (strengthening the mean monsoon overturning) controlling the increase in monsoon rainfall. In future climates however, the dynamic contribution decreases ( [[#10.6.3.6|Section 10.6.3.6]] ), yet the increased thermodynamic component (greater moisture availability) overcomes this to cause a wetter monsoon. Monsoon changes under different epochs may not be governed by the same mechanisms ( [[#D’Agostino--2019|D’Agostino et al., 2019]] ; [[#Hill--2019|Hill, 2019]] ), making the mid-Holocene, in particular, unsuitable as a period to compare with.

Finally, the recent national climate-change assessment for India ( [[#Krishnan--2020|Krishnan et al., 2020]] ) has distilled multiple lines of evidence to show declining summer monsoon rainfall over the second half of the 20th century, attributable to emissions of anthropogenic aerosols, while future projections informed by CMIP5 modelling and dominated by GHG forcing show increased mean rainfall by the end of the 21st century.

There is ''very high confidence'' ( ''robust evidence'' , ''high agreement'' ) of a negative trend of summer monsoon rainfall over the second half of the 20th century averaged over all of India. There is ''medium agreement'' over trends at the regional level owing to uncertainty among observational products, which hinders model evaluation, downscaling and assessment of changes to extremes. There is ''high confidence'' ( ''robust evidence'' , ''medium agreement'' ) that anthropogenic aerosol emissions over the Northern Hemisphere and internal variability have contributed to the negative trend, while there is ''high confidence'' ( ''robust evidence'' , ''medium agreement'' ) that Indian summer monsoon rainfall will increase at the end of the 21st century in response to increased GHG forcing, due to the dominance of thermodynamic mechanisms. No contradictory evidence is found from downscaling methods. The contrast between declining rainfall in the observational record and long-term future increases can be explained using multiple lines of evidence. They are not contradictory since they are attributable to different mechanisms (primarily aerosols and greenhouse gases, respectively). The long-term future changes are generally consistent across global (including at high resolution) and regional climate models, and supported by theoretical arguments. Furthermore, while there are subtle differences found in past periods with a climate similar to the future climate (the mid-Holocene), different physical mechanisms at play suggest that paleoclimate evidence does not reduce confidence in the future projections. In the near term, there is ''high confidence'' that internal variability will dominate.

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