Mitigation efforts will not fully alleviate the increase in water scarcity occurrence probability in wheat-producing areas

Abstract

Global warming is expected to increase the frequency and intensity of severe water scarcity (SWS) events, which negatively affect rain-fed crops such as wheat, a key source of calories and protein for humans. Here, we develop a method to simultaneously quantify SWS over the world's entire wheat-growing area and calculate the probabilities of multiple/sequential SWS events for baseline and future climates. Our projections show that, without climate change mitigation (representative concentration pathway 8.5), up to 60% of the current wheat-growing area will face simultaneous SWS events by the end of this century, compared to 15% today. Climate change stabilization in line with the Paris Agreement would substantially reduce the negative effects, but they would still double between 2041 and 2070 compared to current conditions. Future assessments of production shocks in food security should explicitly include the risk of severe, prolonged, and near-simultaneous droughts across key world wheat-producing areas.

Fig. 1. Most important wheat-growing areas and the effect of SWS on wheat yields.

(A) Colors mark the spatial distributions of the wheat-growing area and the top 10 wheat exporters during 2009–2012 in descending order, with light gray showing arable land without wheat cultivation. (B and C) Comparison of wheat yield deviations during years with and without severe water scarcity (SWS) occurrence, combining the 10 main wheat exporters [European Union (EU), Russia, Canada, United States, Ukraine, Australia, Kazakhstan, Argentina, Turkey, and Brazil]. SWS and yield data over the period 1991–2016 were used. (B) Frequency of yield deviation expressed as the Z score and smoothed by Gaussian filter for years with no SWS occurrence (n = 136) versus years when at least 1% of the exporter’s wheat-growing area was affected by SWS (n = 78) during the year of harvest. (C) Yield differences at the exporter entity level relative to the previous year for years with no SWS occurrence (n = 136) versus years when at least 10% of the exporter’s wheat-growing area was affected by SWS (n = 5).

Fig. 2. Areas that are most and least at risk of an increased probability of SWS during the wheat season.

The hot spots depict the 10% (or 25%) of grids with the highest SWS occurrence, which are also important wheat-producing areas. The good spots represent the 10% or 25% of wheat-producing grids with the lowest probability of SWS. The estimates are based on the analysis of the entire set of projections of 27 GCMs (table S2) for RCP 2.6, RCP 4.5, and RCP 8.5 for the period 2041–2070, which was compared with the SWS occurrence from 1961 to 1990, based on the control run using the same set of GCMs. The grids where wheat is being grown outside hot/good spots are depicted by light yellow, and light gray depicts the remaining agricultural land.

Fig. 3. Estimated proportion of global wheat-growing area affected by SWS between 1861 and 2100.

(A) Box plots of the proportions of the global WhA affected by SWS during the harvest year or in one of the two preceding seasons, based on observed data (12) and GCM data (table S2) for two controls and three future time slices. (B) Annual values of areas affected by SWS during the harvest year or two preceding seasons using CRU-based observed data (1911–2016) and control run data (i.e., 1860–2010) (24) and GCM data for three RCP scenarios during the period 2011–2100.