Impacts of climate change on drought: changes to drier conditions at the beginning of the crop growing season in southern Brazil

ABSTRACT The intensification of drought incidence is one of the most important threats of the 21st century with significant effects on food security. Accordingly, there is a need to improve the understanding of the regional impacts of climate change on this hazard. This study assessed long-term trends in probability-based drought indices (Standardized Precipitation Index and Standardized Evapotranspiration Index) in the State of São Paulo, Brazil. Owing to the multi-scalar nature of both indices, the analyses were performed at 1 to 12-month time scales. The indices were calculated by means of a relativist approach that allowed us to compare drought conditions from different periods. The years 1961-1990 were used as the referential period. To the authors’ best knowledge, this is the first time that such relativist approach is used in historical trend analysis. The results suggest that the evapotranspiration rates have intensified the regional drought conditions. The time scale used to calculate the indices significantly affected the outcomes of drought trend assessments. The reason behind this feature is that the significant changes in the monthly regional patterns are limited to a specific period of the year. More specifically, virtually all significant changes have been observed during the first trimester of the rainy season (October, November and December). Considering that this period corresponds to critical plant growth stages (flowering/regrowth/sprouting) of several major crops (e.g. Sugarcane and Citrus), we may conclude that these significant changes have increased the risk of crop yield reductions due to agricultural drought.

摘要：干旱事件的频发与加剧是21世纪最严峻的全球性威胁之一，对粮食安全造成了显著影响。因此，亟需深化对气候变化给该类灾害带来的区域影响的认知。本研究以巴西圣保罗州为研究区域，评估了两类基于概率的干旱指数——标准化降水指数（Standardized Precipitation Index）与标准化蒸散指数（Standardized Evapotranspiration Index）的长期变化趋势。鉴于两类指数均具有多尺度特性，本研究分别在1至12个月的时间尺度上开展分析。上述指数通过相对化方法计算，该方法可实现不同时期干旱状况的对比分析。研究以1961-1990年作为参考基准期。据笔者所知，该相对化方法在历史趋势分析中的应用尚属首次。研究结果显示，区域蒸散速率的提升加剧了当地干旱状况。计算指数所采用的时间尺度，对干旱趋势评估结果具有显著影响。该现象的成因在于：区域月度气候格局的显著变化仅集中于一年中的特定时段。具体而言，几乎所有显著变化均发生在雨季的前三个月（10月、11月及12月）。考虑到该时段恰好对应甘蔗、柑橘等主要农作物的关键生育期（开花/返青/萌芽阶段），本研究可得出结论：上述显著变化提升了农业干旱引发作物减产的风险。