Effects of heat extremes on wheat yields in Australia

Many agricultural studies have identified that wheat yield is sensitive to seasonal rainfall and extreme high temperatures. We investigate the impact of extreme heat events, in particular on wheat yields in South-East Australia ( SEA) and South-West Western Australia (SWWA). We define a 'heat-day' as a day where the daily maximum temperature exceeds the 1911-2013 90th percentile for the respective calendar month. We find that the number of heat-days has experienced statistically significant increases across most months across much of Australia, particularly in South Australia, Western Australia, the Northern Territory and Tasmania. The trends are especially marked in winter, including in key wheat-growing regions. The temperatures recorded on these hottest days have also shown a statistically significant increase over the last 100 years. We find that, while wheat yields are more strongly correlated with rainfall than with the number of heat-days, there is substantial evidence to suggest that during drought conditions wheat yields are sensitive to the number of heat-days recorded in August and September in SEA and September and October in SWWA. Extreme heat and rainfall have a stronger association with below-average yields than above-average yields. Extreme temperatures and rainfall in these regions are related to major Australian climate drivers which form the basis of seasonal prediction models and are important for natural variability and long-term climate change. Here we assess the degree to which wheat yields in both regions can be related to the El Nino Southern Oscillation (ENSO), Indian Ocean Dipole (IOD) and Southern Annular Mode (SAM). We find that positive IOD events and El Nino events are both associated with reductions in wheat yields in SEA, but that the co-incidence of these events have no additional wheat yield reductions than would be expected if either a positive IOD or El Nino event occurs. The average annual wheat yield loss associated with El Nino state and/or positive IOD state in SEA is estimated to around sixteen to twenty one per cent. This paper provides insights into the historical relationships between wheat yields, extreme heat and climatic modes of variability in Australia, and discusses the possibilities for changes in wheat yields under a future climate change scenario.