Editing IPCC:AR6/WGII/Chapter-4 (section)

=== 4.7.3 Comparing Current and Future Water-Related Adaptation Responses ===

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Water-related adaptation is being observed across sectors and regions ( [[#4.6|Section 4.6]] ), and beneficial outcomes are documented across different dimensions ( [[#4.7.1|Section 4.7.1]] ). A limited set of frequently documented adaptation responses is also represented in quantitative projections of adaptation effectiveness ( [[#4.7.2|Section 4.7.2]] , Figure 4.29). However, due to the largely different assessment methodologies for measuring beneficial outcomes for current adaptations and effectiveness to reduce impacts for future adaptations, comparing current and future adaptation outcomes is not straightforward. For current adaptation responses, beneficial outcomes may or may not translate to climate risk reduction, making risk reduction potential of observed adaptation a significant gap in our current understanding. The large diversity of outcomes across regions and assessed options becomes apparent for future adaptation options, with the group of ‘inconclusive’ outcomes indicating a large spread of results across regions. This underlines the contextual nature of adaptation and boundary conditions for implementation that can determine the success of adaptation outcomes, now or in the future.

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'''Figure 4.29 |''' '''The panel on the left side shows observed benefits of adaptation.''' Observed outcomes are reported across five dimensions of benefits, co-benefits and maladaptation outcomes. Benefits are measured across five dimensions. Strength of evidence is high if &gt;80% of adaptation responses in that category have at least one beneficial outcome; medium if between 50 and 80% of adaptation responses in that category have at least one beneficial outcome, and low if &lt;50% of adaptation responses have at least one beneficial outcome. Confidence in evidence relates to the way the article links outcomes of adaptation with the adaptation response. Category 1: studies causally link adaptation outcomes to the adaptation response by constructing credible counterfactuals; category 2: studies correlate responses and outcomes without causal attribution; category 3: studies describe adaptation outcomes without making any causal or correlation claims between adaptation outcomes and adaptation responses. ''High confidence'' : more than 67% of the studies fall in categories 1 and 2; ''medium confidence'' : 50–67% of the studies are in categories 1 and 2; ''low confidence'' is less than 50% of studies are in categories 1 and 2. The panel on the right-hand side shows the effectiveness of future adaptations. Future outcomes are assessed in terms of their effectiveness to reduce climate impacts at 1.5°C, 2°C, 3°C and 4°C of global temperature increase relative to 1850–1900. Effectiveness is defined as the fraction of adaptation that the option is able to reduce; residual risk is the fraction of risk remaining after adaptation. If &gt;66% of assessed data points agree on the effectiveness class, a response–temperature combination is shown as belonging to that class. Where results diverge, the result is inconclusive, with studies showing high and low effectiveness across regions and studies. Confidence is based on the number of data points available for each response–temperature combination with ''high confidence'' : 5 or more data points; ''medium confidence'' : 2–4 data points; ''low confidence'' : 1 data point. Also, see Figure 4.28 for further explanations, and Tables SM4.5 and SM4.6 provide underlying data.

Documented implemented adaptations show several beneficial outcomes, with most studies (319 of 356) documenting positive rather than negative outcomes. However, there may be a positive reporting bias in the literature, as positive outcomes are more likely to be reported than negative ones. Also, positive outcome in one parameter does not preclude negative outcomes in others, so maladaptation is still possible even when an adaption has some positive benefits ( [[#4.7.1.2|Section 4.7.1.2]] ). In addition, much of the adaptation happening on the ground may not be published in peer-reviewed publications and, therefore, not covered by the literature assessed in this report. Further, there is limited knowledge about the effectiveness of current adaptation in reducing climate-related risks due to documentation and methodological challenges elaborated in [[#4.7.1.2|Section 4.7.1.2]] (SM4.2).

In contrast, evaluating the effectiveness for future projected adaptations is methodologically possible ( [[#4.7.2|Section 4.7.2]] , and SM4.2), but every adaptation that is happening now cannot be modelled for the future. Therefore, projections of future adaptation effectiveness are limited to those options that can be incorporated into (global) quantitative climate impact models. Unfortunately, an extensive range of options, such as capacity building or training, migration and employment, which are essential building blocks in the portfolio of available (water-related) adaptation options, are currently not quantitatively represented in adaptation projections. In addition, the future will probably bring further development in technical solutions, which are currently also not modelled. While implementing the modelled technical options may be feasible in general, several barriers and constraints ( [[#4.7.4|Section 4.7.4]] ) and enabling conditions, which influence adaptation action in practice, are not included in current modelling studies. Therefore, the modelling studies may present optimistic assessments of adaptation effectiveness for the future.

Adaptations that are beneficial now (e.g., crop- and water-related ones) are also projected to be effective to varying extents in reducing future risks, with the degree of effectiveness strongly depending on future GWLs. For example, beyond a certain level of warming (2°C and upwards), the effectiveness of most options is projected to reduce, and residual impacts are projected to increase. Reduction in the effectiveness of future adaptation at higher global warming levels emphasises the need for limiting warming to 1.5°C, as space for adaptation solution starts to shrink beyond that for most options for which future projections exists ( ''high confidence'' ).

To sum up, there are two significant knowledge gaps in our understanding of water-related adaptations. First, the nature of literature on current adaptation makes it challenging to infer their effectiveness in reducing climate risks, even though the benefits of adaptation are clear ( ''high confidence'' ). Second, not all adaptation responses that are possible in the future can be modelled because of inherent limitations to what can be modelled. Thus, advancement in tools and metrics for measuring the effectiveness of current adaptation in reducing climate risks and suitable downscaled climate and impact models that incorporate economic, social, cultural and management aspects for an extensive range of future adaptation options is needed.

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