Historical data reveal how climate affects Swedish cereal yields

Faranak Tootoonchi, a researcher at the Swedish University of Agricultural Sciences (SLU), analysed the relationship between climate variables and yields of winter wheat, spring wheat, spring barley and oats. The study shows that higher temperatures alone are not enough to increase crop yields.

– It is the combination of temperature and precipitation that determines the outcome. If precipitation does not increase along with rising temperatures during the growing season, the additional heat instead leads to crop stress and lower yields, she explains.

Five decades of data explain yield variation

Faranak Tootoonchi compiled crop yield data from 21 Swedish counties covering the period 1965 to 2020.

She combined these with daily meteorological observations of temperature and precipitation from the same period, deriving variables such as the number of dry days, frost days and mean temperatures.

– Sweden has unique, open-access data on crop yields and climate variables extending far back in time. This makes it possible to carry out analyses like this and to understand changes over time, says Faranak Tootoonchi.

To link weather conditions to crop development, Faranak and her co-authors also modelled the timing of sowing, flowering and maturity for each crop and year. This enabled them to analyse the effects of climate during different stages of the growing season.

The results show, however, that climate conditions throughout the entire growing season explain yield variation better than weather during individual growth stages.

– Temperature and precipitation over the whole growing season were the most important factors. For winter wheat and spring barley, mean temperature and total precipitation had the greatest influence, while spring wheat and oats were more strongly affected by prolonged dry periods, says Faranak Tootoonchi.

The study also shows that extended dry spells have a major negative impact on yields. By contrast, short-lived extreme events such as heavy rainfall or heatwaves had relatively little influence on the historical yields, likely reflecting their low frequency in the historical record. As climate change is expected to increase the occurrence of these events, their impacts on crop yields can become more pronounced in the future. 

Another important finding is that soil moisture conditions before the start of the growing season can affect crop yields later in the year.

How agriculture can adapt to a warmer climate

Faranak Tootoonchi and her co-authors conclude that future climate change is likely to pose significant challenges for Swedish cereal production unless precipitation increases alongside rising temperatures.

There is a common perception that a warmer climate will benefit agriculture in Northern Europe by extending the growing season and creating new opportunities for cultivation. This study presents a more nuanced picture.

Faranak Tootoonchi, an engineer specialising in water resources, agricultural production and modelling, emphasises that interactions between climate factors will become increasingly important in future analyses.

She points to several possible adaptation strategies for agriculture, including developing more heat-tolerant crop varieties, adjusting sowing dates, increasing the use of autumn-sown crops and promoting greater crop diversity.

The study has been published in the scientific journal Biogeosciences under the title Warmer growing seasons improve cereal yields in Northern Europe only with increasing precipitation.