17 - A quick look under the global climate bonnet
Professor Ross Kingwell, Chief Economist, AEGIC
It would be stating the obvious to say that weather and climate affect grain production. We know all too well that droughts and floods can be ruinous for crop production. But if you look closely under the climate bonnet, it’s possible to see more clearly how changes in key climate variables, like rainfall and temperatures over a growing season can impact on crop yields across the globe.
A recent paper by Ray et al (2015) in Nature Communications investigates how wheat yields have changed and been affected by temperatures and rainfall. At thousands of locations across the globe these researchers examined the variation in wheat yields. First, they measured at each location the upward trends in wheat yields over a 30-year period, 1979 to 2008. Then they measured the variation in yield about those trends. Similarly, they measured temperature and precipitation trends and the variation around those trends. Next, they examined the variation in wheat yields around the yield trends to see whether it could be explained by a variation in detrended growing season average temperatures and rainfall. They also examined rainfall and temperatures in the 12-month period prior to harvest, rather than just examining temperatures and rainfall during the growing season.
These researchers wanted to know what features of rainfall and temperature most influenced yield variability. What they found is summarised in Chart 1.
The wide array of colours in Figure 1 indicates there was no pervasive influence of any one characteristic of rainfall or temperature; although generally speaking, rainfall changes had a comparatively greater impact on wheat yields than temperature changes. This is perhaps not so surprising, as growing season and annual rainfall is inherently more variable than average temperature over a year or growing season.
Extreme temperatures (red patches in Figure 1) affected wheat yields mostly in the eastern prairies of Canada and central Russia. Extreme rainfall (dark blue patches in Figure 1) affected wheat yields in some parts of eastern China, northern India, parts of the Canadian prairies and the grain belt of the USA. In scattered parts of Europe, central Russia, northern India, eastern China, Turkey, Argentina and Western Australia there is often a complex interplay between temperature and rainfall, which has a pronounced effect on wheat yield. Unsurprisingly perhaps, drought often involves low rainfall and above average temperatures.
What is surprising (see Figure 2) is that these simple indicators of climate – namely, rainfall and average temperatures over a growing season (or 12-month period before harvest) - often explain 30 to 80 percent of the variation in wheat yields (relative to trend). When you consider all of the other factors than can contribute to wheat yield, such as the pattern of rainfall and temperature during a production year, the amount of fertiliser and other inputs applied, the time of sowing and the influence of rotations and soils, it is surprising that these simple indicators of rainfall and temperature are such useful explanators of variation in wheat yields.
An implication of the findings of Ray et al (2015) is that as the global climate changes in coming decades – with a strong consensus that average temperatures will increase, how rainfall changes and interacts with those temperature changes – will substantively impact on wheat yields and their variability. With expert consensus indicating that average global temperatures will continue to rise over the coming decades, understanding the implications for wheat yields will become increasingly important. However, as this recent work by Ray et al (2015) suggests, when looking at future wheat yields, there will be an important interplay between rising temperatures and changes to rainfall patterns that is likely to be more instructive than looking at each factor in isolation