The impact of climate extreme events on global agricultural yields

© 2018 Dr. Elisabeth VogelClimate extreme events, such as droughts, heat waves, cold spells or heavy precipitation events, pose significant risks to food production and the livelihoods of farmers. Understanding the effect of such events on crop yields is crucial to predict the response of agricultural production to climate change and inform adaptation processes. This PhD research investigated the effect of temperature and precipitation extremes on the yields of four major crops — maize, rice, soybeans, and wheat — over the years 1961–2008, using a global, high-resolution yield dataset and global, gridded data on past weather conditions and climate extremes. First, a bibliographic network-type literature review was conducted to provide an overview of the research landscape. The number of publications focusing on climate and agriculture significantly increased after 2005, reflecting an increased awareness of the challenges related to climate change impacts as well as effects of extreme events on food production. An overview of the most influential publications and research clusters is presented and the main challenges faced by the agricultural sector, as identified in the literature, are summarised. The second part of this thesis analysed the effects of growing season climate variability and extremes (warm and cold temperature extremes, drought and heavy precipitation) on yield anomalies of maize, rice, soybeans, and spring wheat at the global scale, using random forests, a machine learning algorithm. The findings suggest that globally, 20–49% (range over all crops) of the variability of yield anomalies is explained by variations in growing season climate, including climate extremes. Excluding climate extreme indicators from the statistical models decreases the explained variance by 18–43% (range over all crops). This decrease represents more than half of the explained variance for maize, rice, and soybeans, highlighting the importance of climate extremes for understanding yield fluctuations. Yield anomalies are more strongly associated with temperature-related climate variables than with precipitation-related predictors. However, irrigation modulates the impacts of high temperatures on yield anomalies, reflecting the convoluted effects of water and temperature stress. By developing a composite indicator, regions were identified that are most strongly influenced by climate extremes and at the same time major contributors to global crop production, and hence may be the focus of adaptation efforts. The final analysis presented in this study focused on the effects of temperature extremes on maize yields more specifically. The first part assessed the effect of growing degree days above 30°C (GDD30+) — a commonly used, plant-critical temperature threshold — on maize yields across major producing regions. Most detrimental effects are found for rainfed yields in Europe, North America and Oceania, i.e. regions with highly industrialised agricultural systems. The strength of the negative relationship between GDD30+ and maize yield depends on a region’s mean climate and climate variability, with most negative yield effects found in colder and drier regions and regions with low temperature and precipitation variability. The second part determined region- and irrigation-specific temperature thresholds above which adverse yield effects accumulate. These thresholds range from 20°C in Europe to 26°C in Asia and North America. In summary, this study shows that the effects of high temperatures on maize production differ considerably between producing regions, with important implications for the adaptation of maize production systems to climate change and temperature extremes