Editing IPCC:AR6/WGII/Chapter-5 (section)

==== 5.4.4.1 Adaptation options for major crops ====

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Crop management practices are the most commonly studied adaptation measures ( [[#Shaffril--2018|Shaffril et al., 2018]] ; [[#Hansen--2019a|Hansen et al., 2019a]] ; [[#Muchuru--2019|Muchuru and Nhamo, 2019]] ), but quantitative assessments are mostly limited to existing agronomic options such as changes in planting schedules, cultivars and irrigation ( [[#Beveridge--2018a|Beveridge et al., 2018a]] ; [[#Aggarwal--2019|Aggarwal et al., 2019]] ). This section draws on the global data set used in [[#5.4.3.2|Section 5.4.3.2]] ( [[#Hasegawa--2021b|Hasegawa et al., 2021b]] ) to estimate adaptation potential, defined as the difference in simulated yields with and without adaptations. A caveat to the analysis is that the data set includes management options if the literature treats them as adaptation. They include intensification measures such as fertilizer and water management, not allowing for physical and economic feasibility.

The overall adaptation potential of existing farm management practices to reduce yield losses averaged 8% in mid-century and 11% in end-century (Figure 5.9), which is insufficient to offset the negative impacts from climate change, particularly in currently warmer regions ( [[#5.4.3.2|Section 5.4.3.2]] ). Emission scenarios, crop species, regions and adaptation options do not show discernible differences. Combinations of two or more options do not necessarily have greater adaptation potential than a single option, though a fair comparison is difficult in the data set from independent studies. One regional study in West Africa found that currently promising management would no longer be effective under future climate, suggesting the need to evaluate effectiveness under projected climate change.

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'''Figure 5.9 |''' '''Adaptation potential, defined as the difference between yield impacts with and without adaptation in projected impacts (Hasegawa et al''' '''.''' ''', 2021b).''' '''(a)''' Projections under three RCP scenarios by regions and '''(b)''' by options at mid-century (MC, 2040–2069) and end-century (EC, 2070–2100). ''n'' is the number of simulations. See Figure 5.6 for legends.

A global-scale meta-analysis estimated a 3–7% yield loss per degree Celsius increase in temperature ( [[#Zhao--2017|Zhao et al., 2017]] ). Two global-scale studies using multiple global gridded crop models found that growing-season adaptation through cultivar changes offsets global production losses up to 2°C of temperature increase ( [[#Minoli--2019|Minoli et al., 2019]] ; [[#Zabel--2021|Zabel et al., 2021]] ). While these studies do not account for CO 2 fertilisation effects, another global-scale study with the CO 2 fertilisation effects ( [[#Iizumi--2020|Iizumi et al., 2020]] ) showed that residual damage (climate change impacts after adaptation) would start to increase almost exponentially from 2040 towards the end of the century under RCP8.5. The cost required for adaptation and due to residual damage is projected to rise from USD 63 billion at 1.5°C to USD 80 billion at 2°C and to USD 128 billion at 3°C ( [[#Iizumi--2020|Iizumi et al., 2020]] ). All these global studies project that risks and damages are greater in tropical and arid regions, where crops are exposed to heat and drought stresses more often than in temperate regions ( [[#Sun--2019|Sun et al., 2019]] ; [[#Kummu--2021|Kummu et al., 2021]] ; SM5.4). There are still large uncertainties in the crop model projections ( [[#Müller--2021a|Müller et al., 2021a]] ), but these multiple lines of evidence suggest that warming beyond +2°C (projected to be reached by mid-century under high-emission scenarios) will substantially increase the cost of adaptation and the residual damage to major crops ( ''high confidence'' ). The residual damage will prevail much sooner in currently warmer regions, where the effect of even a modest temperature increase is greater ( [[#5.4.3.2|Section 5.4.3.2]] ).

Most crop modelling studies on adaptation are still limited to a handful of options for each crop type ( [[#Beveridge--2018a|Beveridge et al., 2018a]] ). A range of other options are possible not just to reduce yield losses but to diversify risks to livelihoods, which are partially assessed in Sections 5.4.4.4 and 5.14.1. Current modelling approaches are not suited for the assessment of multiple dimensions of adaptation options. New studies are emerging that evaluate multiple options for productivity, sustainability and GHG emission ( [[#Xin--2019|Xin and Tao, 2019]] ; [[#Smith--2020b|Smith et al., 2020b]] ), but local- and household-scale assessment, taking account of future climatic variability, needs to be enhanced ( [[#Beveridge--2018a|Beveridge et al., 2018a]] ).

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