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Increasing Influence of Heat Stress on French Maize Yields

18 March 2013

Improved crop yield forecasts could enable more effective adaptation to climate variability and change.

Abstract

Here we explore how to combine historical observations of crop yields and weather with climate model simulations to produce crop yield projections for decision relevant timescales. Firstly, the effects on historical crop yields of improved technology,precipitation and daily maximum temperatures are modelled empirically, accounting for anon-linear technology trend and interactions between temperature and precipitation, and applied specifically for a case study of maize in France. The relative importance of precipitation variability for maize yields in France has decreased significantly since the 1960s, likely due to increased irrigation. In addition, heat stress is found to be as important for yield as precipitation since around 2000. A significant reduction in maize yield is found for each day with a maximum temperature above 32o C, in broad agreement with previous estimates.The recent increase in such hot days has likely contributed to the observed yield stagnation. Furthermore, a general method for producing near-term crop yield projections, based on climate model simulations, is developed and utilised. We use projections of future daily maximum temperatures to assess the likely change in yields due to variations in climate.Importantly, we calibrate the climate model projections using observed data to ensure both reliable temperature mean and daily variability characteristics, and demonstrate that these methods work using retrospective predictions. We conclude that, to offset the projected increased daily maximum temperatures over France, improved technology will need to in-crease base level yields by 12% to be con?dent about maintaining current levels of yield for the period 2016-2035; the current rate of yield technology increase is not sufficient to meet this target.

Introduction

The yield of most crops has increased over the past several decades. However, in the most recent decade, yields have stagnated for many crops in several regions, whilst temperatures have generally increased. The reasons for this stagnation are debated, and could include agricultural policy (Finger, 2010), fundamental genetic limits (Calderini and Slafer, 1998), climate (Lobell and Asner, 2003; Brisson et al., 2010), agronomic practice and crop management (Brisson et al.,2010). Here we explore the relative importance of different climatic factors.

Crops are known to be sensitive to various aspects of climate. Persistently elevated temperatures have long been known to accelerate progress towards maturity, and more recently have been shown to have a significant impact on leaf aging (or senescence) (Asseng et al., 2011; Lobell et al., 2012). Crop responses to shorter periods of high temperature, particularly when co-incident with flowering, show yields falling dramatically beyond a threshold temperature (Luo, 2011). This mechanism is observed in both controlled environments and field studies (Ferriset al., 1998; Wheeler et al., 2000). Similar responses to hot days are beginning to be found at the regional scale: maize yields in the U.S. have been found to decrease sharply when exposed to temperatures over around 29-30o C, and this effect outweighs any yield increase due to higher temperatures more generally (Schlenker and Roberts, 2009).

Crop yields are also sensitive to precipitation. Quantifying the relative effect of temperature and precipitation variability is important for understanding impacts and developing adaptation options for future climatic changes. Whilst this relative importance will vary regionally (e.g.Sakurai et al., 2011), some generalisations may be possible through an analysis of mechanisms. For regions where irrigation is increasing, for example, it seems likely that the sensitivity of yield to rainfall will be decreasing. More detailed analyses also indicate that in particular environments (Thornton et al., 2010) or at the regional scale (Lobell and Burke, 2008), temperature may be a more significant driver of future yields than precipitation. Since temperatures are projected to significantly increase over the next few decades due to continuing anthropogenic emissions of greenhouse gases, whereas precipitation changes are far less certain (Meehl et al.,2007; Hawkins and Sutton, 2011), this suggests predictability in future crop yields.

In order to effectively guide adaptation to future changes, perhaps with different crop growing strategies (Rosenzweig and Tubiello, 2007) or selective crop breeding (Cattivelli et al., 2008),there are several key questions to consider. Firstly, can the relative effects of improved technology, precipitation variability and increasing temperatures be quantified? If so, what is the relative size of the effects of rainfall and hot temperatures on yields? And, what level of technology development may be required to overcome any impact of future climatic changes on yield?

In this analysis we develop a methodology to address these questions, focussing on one particular crop (maize) and one country (France) as a case study to better understand the technology trend and the influence of climate on crops. France is chosen specifically for this case study because it has experienced recent extremes of climate. In particular, the heatwavein summer 2003 (Sch¨ar et al., 2004) has previously been linked to a drop in crop yields across Europe (Easterling et al., 2007; Battisti and Naylor, 2009; van der Velde et al., 2012).

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March 2013

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