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

=== 4.7.2 Projections of Future Effectiveness of Adaptation Responses ===

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Several adaptation options have been shown to have beneficial effects on societally relevant outcomes under current climate conditions ( [[#4.7.1.2|Section 4.7.1.2]] ) and will remain critical to adapt to future climate change. However, there is limited quantitative information on the future viability of available responses to reduce projected climate impacts effectively. However, the context-specific nature of adaptation on the ground and the uncertainties associated with future climate outcomes, both in terms of policy decisions around mitigation and model-inherent uncertainties, make long-term projections of adaptation effectiveness of limited use for decision-making on the ground. However, such projections are still needed to understand the efficacy of current technical and managerial solutions to reduce climate risk. Consequently, an increasing body of literature focuses on the effectiveness of specific interventions to reduce projected climate risks in a local to regional setting.

This section provides a quantitative aggregate assessment of effectiveness of projected water-related climate adaptations at different levels of GWLs (SM4.2). Effectiveness is defined as the potential of a given adaptation measure to address projected changes in climate and return the system under analysis to baseline conditions. If the measure cannot fully compensate for the projected climate risk, residual risks remain, defined as the fraction of risk remaining after adaptation. For example, in many regions, projected temperature-driven yield loss can be reduced by shifting to or increasing irrigation. However, yields often do not always fully return to baseline conditions without climate change, leaving residual risk after adaptation. Assessed options are limited to technical solutions, which have quantitative entry points to global climate impact models.

Most adaptation projections focus on water-related interventions in the agricultural sector, including irrigation-related responses, shifting planting dates, changing crops and cultivars, and water and soil conservation. Sectoral projections of adaptation effectiveness are limited in forestry- and agroforestry-related responses, flood protection measures (excluding here options that are solely related to effects of sea level rise), urban water-related adaptation and energy-related responses. The majority of assessed studies focus on comparing different variations of one or several response options in terms of timing or duration, for example, a shift in planting dates of 10 d and 20 d, relative to present-day practice and provide results for a range of scenarios and (or) timeframes.

A total of 45 studies were identified for this assessment, based on their quantitative assessment of the effects of adaptation on projected impacts (SM4.2 for the method of future projected effectiveness assessment). From each study, the distinct combinations of specific variations of adaptations, scenarios and timeframes assessed were considered as individual data points, providing a total of 450 unique data points for the assessment (Table SM4.6). The study-specific temperature increase was classified relative to the 1850–1900 baseline for each data point, based on the model and scenario specifications provided and grouped into outcomes at 1.5°C, 2°C, 3°C and 4°C. The effectiveness is assessed based on the fraction of risk that an option can reduce. Co-benefits are defined as a situation where outcomes improve relative to baseline conditions, whereas maladaptive outcomes describe a situation where risks increase after adaptation has been implemented.

Several studies assess the future effectiveness of improved cultivars and agronomic practices, such as changing fertiliser application or switching to drought-resistant crops (five studies; 85 data points). Results show a range of effectiveness levels across regions and warming levels and vary depending on the tested response options ( [[#Qin--2018|Qin et al., 2018]] ) (Figure 4.29), with moderate to small effectiveness, large residual impacts or potential maladaptive outcomes, as well as decreasing effectiveness with increasing warming (Figure 4.28) ( ''high confidence'' ). For studies testing results across a range of scenarios, approaches show increasingly mixed ( [[#Qin--2018|Qin et al., 2018]] ) and limited effects ( [[#Amouzou--2019|Amouzou et al., 2019]] ) with higher warming, with overall reductions across warming levels for most tested responses ( [[#Qin--2018|Qin et al., 2018]] ).

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'''Figure 4.28 |''' '''Projected effectiveness of adaptation options in returning the system to a study-specific baseline state relative to the projected climate impact; and level of residual risk retained after adaptation, relative to baseline conditions.''' Regional summaries are based on IPCC regions. Warming levels refer to the global mean temperature (GMT) increase relative to an 1850–1900 baseline. For each data point, the study-specific GMT increase was calculated to show effectiveness at 1.5°C, 2°C, 3°C and 4°C. Based on the ability of an implemented option to return the system to its baseline state, the effectiveness is classified based on the share of risk the option can reduce: large (&gt;80%); moderate (80–50%); small (&lt;50–30%); insufficient (&lt;30%). Where the system state is improved relative to baseline, co-benefits are identified. Residual impacts show the share of remaining impacts after adaptation has been implemented: negligible (&lt;5%); small (5 to &lt;20%); moderate (20 to &lt;50); large (≥50%). Where risks increase after adaptation, data points are shown as maladaptation. All underlying data is provided in Table SM4.6.

Changes in cropping patterns and crop systems (Figure 4.28) (five studies; 31 data points) indicate limited potential to reduce projected climate risks, with the majority of studies providing results of up to 1.5°C of warming and limited evidence for higher warming levels. At 1.5°C, effectiveness in Africa is mostly insufficient, with substantial maladaptive potential ( [[#Brouziyne--2018|Brouziyne et al., 2018]] ). Over Asia, effectiveness is mostly small at 1.5°C with substantial residual impacts, further reducing to insufficient effectiveness at large residual risks at 4°C (Figure 4.28 Projected effectiveness) ( ''robust evidence; medium agreement'' ) ( [[#Boonwichai--2019|Boonwichai et al., 2019]] ; [[#Dai--2020|Dai et al., 2020]] ; [[#Mehrazar--2020|Mehrazar et al., 2020]] ). Amongst the options related to changes in cropping patterns and crop systems, shifting planting dates is projected to retain moderate to high residual risks under some specifications in Iran ( [[#Paymard--2018|Paymard et al., 2018]] ) and Morocco ( [[#Brouziyne--2018|Brouziyne et al., 2018]] ), while high effectiveness is reported for similar specifications in Thailand ( [[#Boonwichai--2019|Boonwichai et al., 2019]] ), Australia ( [[#Luo--2016|Luo et al., 2016]] ), Morocco (( [[#Brouziyne--2018|Brouziyne et al., 2018]] ) and Iran ( [[#Mehrazar--2020|Mehrazar et al., 2020]] ). Of the assessed adaptation options, changes in cropping patterns and cropping systems appear least effective in reducing climate risk, with decreasing effectiveness at higher levels of warming.

Studies assessing the future effectiveness of irrigation-related responses (Figure 4.28) focus on a range of specific approaches, including increasing irrigation efficiency, deficit irrigation, irrigated area expansion or shifting from rain-fed to irrigated agriculture, as well as specific types of irrigation (21 studies; 103 data points). As a frequently implemented option with direct entry points to agricultural models, this option provides the most robust set of data points across regions and warming levels. For all regions, a reduction in effectiveness is apparent from 1.5°C to higher levels of warming, leading to increased residual risk with increasing warming ( ''high confidence'' ). Irrigation can increase yield relative to present day, showing co-benefits for some regions, though the share of co-benefits decreases with higher warming ( ''high confidence'' ) (Figure 4.28). However, since many of these studies rely on global agricultural models which do not fully represent the actual availability of water, further expansion of irrigation at the scale assumed in those studies may not be realistic (Sections 4.3.1.2. 4.3.1.3) ( [[#Elliott--2014|Elliott et al., 2014]] ).

A wide range of water and soil management-related options (Figure 4.28), including mulching, no tilling or contour farming, has been assessed for future effectiveness (eight studies; 49 data points). Results underline the context-specific nature and need to carefully adjust the specific options to a regional setting, with variations of options leading to effective outcomes or residual impacts within individual studies ( [[#Qiu--2019|Qiu et al., 2019]] ) and across regions and warming levels.

Similar to observed adaptation, studies assessing combinations of the agricultural adaptation options outlined above (11 studies; 36 data points) show the highest effectiveness across agricultural adaptation outcomes and generally project moderate to high effectiveness with the potential for co-benefits (Figure 4.28). Though maladaptive outcomes are also documented, residual risks are limited, also at higher levels of warming. Therefore, developing integrated plans of synergistic options linked to adequate monitoring and evaluation approaches and designed to adjust to changing conditions continuously is desirable to minimise climate risk and ensure food security ( [[#Babaeian--2021|Babaeian et al., 2021]] ).

Globally, agroforestry-related adaptation (four studies; 18 data points) is moderately to highly effective, with the potential for substantial co-benefits at 1.5° and 2°C of warming, with a sharp decline in effectiveness at 3°C and 4°C and a substantial increase in residual risk and maladaptive outcomes (Figure 4.28).

Flood risk-related adaptation (four studies; 47 data points) is associated with the potential for substantial co-benefits relative to present-day flood risk, indicating a current adaptation gap larger than for other impact areas. These co-benefits decline with increasing warming. Limits to the tested options become increasingly apparent at 3°C and 4°C of warming, where residual risks increase for most assessed cases (Figure 4.28).

Adaptation projections for urban water risks as well as the energy sector are limited to one study each, with one data point for urban adaptation ( [[#Rosenberger--2021|Rosenberger et al., 2021]] ) and 80 data points for different variations of adaptation outcomes across regions and scenarios for the energy sector ( [[#van%20Vliet--2016c|van Vliet et al., 2016c]] ). Sustainable stormwater management, focusing on a combination of nature-based solutions, is shown to be highly effective and yields co-benefits at 3°C. However, these results were gained in a specific case study setting in a European city with limited generalizability (Figure 4.28).

The assessment of adaptation in the hydropower and thermoelectric power-generation sector indicates high effectiveness and co-benefits across all regions for 1.5°C, with decreasing effectiveness and increasing residual risks for 2°C and 3°C of warming and highest reductions in effectiveness for Central and South America (Figure 4.28).

Quantitative projections of future adaptation depend on available impact models to analyse the effect of specific adaptation interventions. However, since not all possible future adaptation responses can be incorporated in climate impact models, this is a major limitation to assessing the full scope of options available in the future. For example, many frequently implemented measures showing effective outcomes, such as behavioural and capacity building-focused responses or migration and off-farm diversification ( [[#4.7.1.2|Section 4.7.1.2]] ), are not incorporated in quantitative water-related climate impact projection models. In addition, projections of future adaptation depend on currently available technologies or approaches, but new methods and technologies will probably emerge. Thus, improving the representation of adaptation in future projections is a significant knowledge gap that remains to be addressed.

Whether specific adaptation responses are shown to be effective and even lead to co-benefits or are associated with residual impacts is highly contextually, location- and crop-specific. In addition, the specific climate-impact-scenario combinations play an important role in determining assessed outcomes.

In practice, responding to increasing climate risk will need to be context-specific and sufficiently agile to respond to ever-changing realities on the ground. The adaptive pathways approach underline that a sequence of different options responding to climate change over time may be most effective ( [[#Babaeian--2021|Babaeian et al., 2021]] ). In addition, impact models generally underestimate or underrepresent climate extremes ( [[#Schewe--2019|Schewe et al., 2019]] ), limiting the ability of the present analysis to reflect adaptation requirements to extremes, which are likely to push systems to their limits ( [[#4.7.4|Section 4.7.4]] ). While currently known structural adaptation responses can reduce some of the projected risks across sectors and regions, residual impacts remain at all levels of warming, and effectiveness decreases at higher levels of warming. Adaptation generally performs more effectively at 1.5°C, though residual damages are projected at this warming level across sectors and regions ( ''high confidence'' ). A range of options also shows the potential for further increasing negative effects (maladaptation) across sectors, regions and warming levels, further underlining the need for contextualised approaches.

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