The relationship between the peaks in international commodity prices observed in 2007/2008 and those observed in 2010/2011 and SWS is complex, as these spikes are a result of a “perfect storm” in the form of mutually reinforcing simultaneous developments in multiple drivers, rather than of a single isolated event [e.g., (7)]. Tadasse et al. (18) distinguished between fundamental drivers (which refer to shocks on the demand and supply side) and macro drivers (which act outside of the agricultural sector). Over time, fundamental drivers can potentially lead to reductions in the stock-to-use ratio. Macro drivers have an indirect impact on prices, for example, through costs due to exchange rate effects or energy prices. Last, the financialization of commodity markets, which refers to the unprecedented flow of capital into commodity markets, has made them subject to speculation. Within this context, our results provide unique insight into shocks on the supply side, as they coherently quantify water scarcity in terms of time and space on the global scale, including future developments of the SWS intensity, extent, and frequency across all key wheat-growing areas. Our results suggest that, even under the ambitious mitigation scenario aimed to stabilize global warming at 2°C compared to preindustrial levels (26), the increase in the frequency and extent of adverse weather extremes and related shocks on the production side would be unprecedented. How much this will affect food prices and food security will depend on the development of other influencing factors.

Reaching ambitious mitigation targets will likely lead to higher energy prices and a greater demand for bioenergy—at least in the medium term (27). These two factors would further reinforce the impacts of SWS on wheat prices, such as the recently observed price peaks. Decoupling future production-side shocks from price spikes will thus require coordinated efforts in stock management to maintain the stock-to-use ratio at safe levels and the control of financial markets to minimize the price reaction beyond the fundamental drivers. Last, liberalized trade has often been advocated as a potentially efficient adaptation measure (28, 29), while unilateral restrictive trade policies contribute to aggravating recent price spikes. A solid framework for the global coordination of trade policies will thus be necessary to allow trade to alleviate rather than exacerbate the effects of regional extremes on global markets.

Studies have argued that some or all the negative global warming impacts on wheat yields might be compensated for by the increasing atmospheric CO 2 concentration in combination with adaptation strategies ( 32 , 34 ). In semiarid environments, wheat growth will be enhanced by the higher water use efficiency under elevated CO 2 ( 35 ). However, the authors of the corresponding study stated that “supplemental irrigation was applied to the entire experiment on occasion during excessively dry periods to prevent crop loss,” indicating that the CO 2 transpiration effect has limitations under extreme drought, as has been confirmed experimentally by Medina et al. ( 36 ). Although water use is reduced under elevated CO 2 ( 37 ) and may alleviate moderate dry spells, recent studies have found that drought stress mediated by severe heat cannot be compensated for by elevated CO 2 ( 38 , 39 ). Long-term studies with elevated CO 2 revealed that intensifying drought for some crops resulted in diminished yield stimulation under an elevated CO 2 concentration ( 40 ). In addition, Dai et al. ( 41 ) compared the future drying in a model simulation with and without considering the plant physiology in response to increasing CO 2 . They found that the plant physiological response to increasing GHGs is secondary, suggesting that the impact of CO 2 fertilization on future drought is small. As explained in Materials and Methods, we calculated the potential evapotranspiration (PET) through the Penman-Monteith method using an approach based on the surface energy budget, and therefore, the ambient CO 2 effects on plant transpiration, vegetation growth, and feed feedback are implicitly considered. Therefore, we postulate that the effects of SWS will likely not be alleviated by enhanced CO 2 and that SWS implementing PET based on the surface energy budget approach represents a reliable indicator of drought irrespective of CO 2 levels.

We assumed that the current wheat-growing areas and their relative weights will remain static during the entire 21st century. Therefore, we tested the potential benefit of shifting WhP to other agricultural land that has a lower SWS probability than that in current wheat-growing areas. Although the incidence of SWS over the current global arable land/agricultural land increased by 7.7%/9.8% per 1°C of global warming, the rate was fairly similar (8.5%/9.3%) over the entire or main wheat-growing areas (figs. S8 and S9). Thus, there is relatively little to be gained globally in terms of decreasing SWS exposure by shifting wheat-producing areas both within and outside the present wheat-growing area. Although we did not consider expanding the wheat-growing area to regions that are not cultivated at present because it would lead inter alia to increased CO 2 emissions ( 33 ), we acknowledge that these options exist.

It is well known that warmer temperatures will accelerate crop development, leading to earlier maturity, which creates a higher water demand per day, but the total demand depends on the length of the growing season. Therefore, projecting seasonal SWS should consider the changes in crop duration under climate change. This analysis is based on the assumption that this change will be limited to 1 month, as found in other studies ( 30 ). Therefore, we provide a sensitivity analysis of shifting the sowing/harvest date by 1 month (fig. S10) to ensure that the effects of changes in the crop duration and the associated water demand resulting from climate change are captured by the calculated SWS. A notable and significant drought risk reduction (P < 0.01 for all RCPs tested) was found in areas affected by SWS when the harvest date was advanced by 1 month, changing from 6.0 ± 2.0% to 5.0 ± 1.8% per 1°C of warming (fig. S10). However, the advancement of the harvest date was insufficient in terms of reducing the areas of SWS to the levels experienced during 1961–1990, and such a change is also likely to decrease the yield potential, unless cultivars are adapted ( 31 ). Postponing the harvest by 1 month will lead to a significant (P < 0.001) increase in the area affected by SWS, changing from 6.0 ± 2.0% to 6.6 ± 2.2% per 1°C of warming. These changes would affect not only wheat but also entire crop rotations ( 32 ).

Adaptation strategies

Even the strongest mitigation efforts assumed in the RCPs will not prevent increasing SWS, and therefore, timely adaptation action is required. Strategies to reduce the impacts of water shortages on WhP could include (i) shifting the wheat growing season; (ii) full or partial irrigation; (iii) increasing the water use efficiency by, among other things, enhancing rainfall infiltration and reducing soil evaporation; and (iv) using wheat varieties with enhanced drought and heat tolerance.

Shifting the sowing and harvest dates has already been documented for adaptation [e.g., (42)] and is likely to be combined with the use of more resilient cultivars and management optimized for the specific environment. Another strategy for coping with water stress has been and will likely continue to be drought avoidance, e.g., shifting the harvest date to earlier (or later) in the season by shifting the sowing date and/or by using early-ripening cultivars to alleviate drought stress. Shifting the harvest time may reduce the yield losses caused by water scarcity in some regions (43). However, in some regions with the highest risks of increasing water scarcity within the wheat season (Fig. 2), such as the Mediterranean, this strategy has a limited scope if the dry period extends into late autumn. In addition, for temperate climates, particularly those at higher latitudes, this avoidance strategy can lead to less global radiation being intercepted by the crop [effective global radiation (EfGr)] (44) and, thus, a lower yield potential. Therefore, although adjusting the harvest date may be a beneficial strategy in several regions, it will most likely reduce production levels, unless the sowing date can be adjusted to maintain the EfGr.

Using drought- and heat-adapted wheat varieties seems to be a promising option; however, the breeding of enhanced drought- or heat-tolerant wheat cultivars depends on which physiological factors cause yield penalties under drought and to what extent these factors are under genetic control. Phenomic and genomic approaches need to be integrated with crop physiological investigations and ecophysiological and genetic modeling to design wheat traits for future climate conditions (45). These approaches should aim to exploit not only genetic variation (providing productivity gains) but also quality traits (45).

Irrigation represents a seemingly attractive option as well; however, dwindling water resources in some regions (23, 46) cast doubt on the feasibility of irrigation being able to increase wheat yield on a global scale without massive investment programs. Therefore, developing management strategies to improve field conditions in a bid to increase drought resilience is crucial. For example, in the Mediterranean, the application of only one supplementary irrigation event during sensitive stages in combination with the selection of an optimized sowing date and cycle duration can maximize the grain filling length while preventing environmental stressors at both the end (at sowing) and the beginning (at grain filling) of the dry summer period (47). Deficit irrigation is a common on-farm water-saving strategy, which involves irrigating crops below the requirements defined by evapotranspiration (48). In general, this approach aims to increase the water use efficiency while minimizing reductions in crop yields and saving available water resources. Although deficit irrigation can improve the water use efficiency and may reduce the total withdrawals for irrigation, the consumptive use of water may also increase, thereby reducing return flows and causing negative groundwater balances [see, e.g., (49)], which may increase the risk of salinization in arid regions.

Another option that should be considered is soil management focused on building up the soil water for the next crop. This is partially possible by conserving water through minimizing tillage and reducing nonproductive water loss (i.e., evaporation) (50) from the unshaded soil surface. The latter can be achieved by covering the soil surface with a mulch of plant residues or plastic, which reduces evaporative water losses (51). Water use can be further improved by water harvesting approaches, where water is captured in the soil or by water reservoirs located on the farm or in the catchment for later use by crops (52). There are many approaches that can be implemented to enhance water harvesting and management, and these methods need to be tailored to the local landscape, soil, and climatic conditions. In addition, possible adaptations usually have other implications (e.g., forcing changes in crop rotation) and thus should not be considered as cost-free strategies, as rightly noted by Lobell (32).

The suitability of a given adaptation measure will vary on the basis of the climate and soil conditions, as well as the type and timing of the drought event. If the variability in drought timing increases (in addition to the frequency and severity, as shown above), then it will further complicate decision-making and the efficient use of adaptation measures. Integrated strategies for entire catchments have rarely been introduced so far because of governance complexities, although this is potentially the most efficient approach.

Our understanding of the impacts of climate change on agricultural prices has relied to a large degree on global agricultural market models. These models also include the potential of incremental adaptations that rely on currently available management systems and crop varieties. Leclère et al. (28) found in a single-model study that, without adaptation, the total crop calorie loss would reach up to 18% by 2050. Incremental adaptation achieved by selecting an appropriate management system, relocating crop production to more suitable or less negatively affected areas, and expanding the crop area could buffer 17, 44, and 22% of the negative climate effect, resulting in a residual calorie availability decrease due to climate change of only 3%. Similar results were found in a multimodel study by Nelson et al. (53), who also considered the impacts on agricultural prices and highlighted the idea that adaptation would lead to an average of 20% increase in crop prices. However, these studies considered only gradual changes in yields due to climate change, and to the best of our knowledge, no studies have assessed the economic potential for adaptation to an increased frequency of extreme weather events.

Although even major mitigation efforts (represented by RCP 2.6) do not prevent the doubled risk of SWS simultaneously affecting wheat-growing areas, it certainly makes the increase more manageable, especially in comparison with the increases projected under RCP 8.5. Limiting global warming by the end of 2100 to +1.5°C instead of the targeted 2.0°C threshold would reduce the mean area affected by SWS by approximately 3% (figs. S8 and S9), which significantly exceeds the potential achieved by all possible shifts in the wheat growing season (fig. S10). Reported changes in SWS levels and the apparent sensitivity of wheat prices to SWS increases should be considered, together with reports on dwindling water resources across many wheat-growing regions (46, 54, 55).

To meet the projected increase in the global food demand, a sustained annual yield increase of 2.4% (56), in contrast to the current rate of 0.9%, will be needed. Although there are estimates that a 71% increase in the potential global yield is feasible through yield gap closure (57), it was thought that this increase would mainly come from the expansion of irrigated areas and from enhancing nitrogen fertilization in some production regions. The increased probability of SWS years reported in this study would, in many regions, constrain efforts to increase irrigated areas but would also lead to a lower nutrient use efficiency. The efficacy and feasibility of available adaptation options should therefore be interpreted with caution and should account for changes in the SWS probability in combination with other adverse factors to avoid what has been coined “adaptation illusion” (32).

Our study has performed analyses to assess the probability of simultaneous large-scale severe and extreme drought across the globe during critical wheat development phases and has shown an increasing risk for global WhP as a whole. Even ambitious climate change mitigation efforts would not fully alleviate the increased risk. To fully quantify the impacts of large-scale water scarcity events on WhP and the effects of potential adaptation strategies, additional factors such as information on soil characteristics, water resources for irrigation, and the capacity of markets to absorb drought-induced production anomalies need to be included. Process-based or statistical modeling approaches can be used, although recent ensemble model studies revealed large uncertainties in the assessment of the climate change impact on crop production (11, 12). Moreover, increasing competition for water use between different sectors must be considered, which has implications not only for crop production but also for regional conflicts concerning water use. The results of our study underline the urgent need for concerted global efforts to limit global warming within the targets of the Paris Agreement.