Abstract. The US agriculture system supplies more than one-third of
globally traded soybean, and with 90 % of US soybean produced under rainfed
agriculture, soybean trade is particularly sensitive to weather and climate
variability. Average growing season climate conditions can explain about
one-third of US soybean yield variability. Additionally, crops can be
sensitive to specific short-term weather extremes, occurring in isolation or
compounding at key moments throughout crop development. Here, we identify
the dominant within-season climate drivers that can explain soybean yield
variability in the US, and we explore the synergistic effects between drivers that
can lead to severe impacts. The study combines weather data from reanalysis
and satellite-informed root zone soil moisture fields with subnational crop
yields using statistical methods that account for interaction effects. On average, our
models can explain about two-thirds of the year-to-year yield
variability (70 % for all years and 60 % for out-of-sample predictions).
The largest negative influence on soybean yields is driven by high
temperature and low soil moisture during the summer crop reproductive
period. Moreover, due to synergistic effects, heat is considerably more
damaging to soybean crops during dry conditions and is less problematic during wet
conditions. Compounding and interacting hot and dry (hot–dry) summer conditions (defined
by the 95th and 5th percentiles of temperature and soil moisture
respectively) reduce yields by 2 standard deviations. This sensitivity is 4 and 3 times larger than the sensitivity to hot or dry
conditions alone respectively. Other relevant drivers of negative yield responses are
lower temperatures early and late in the season, excessive precipitation in
the early season, and dry conditions in the late season. We note that the sensitivity to the identified drivers varies across the spatial domain. Higher latitudes, and thus colder regions, are positively affected by high temperatures during the summer period. On the other hand, warmer southeastern regions are positively affected by low temperatures during the late season. Historic trends in
identified drivers indicate that US soybean production has generally benefited from
recent shifts in weather except for increasing rainfall in the early season.
Overall, warming conditions have reduced the risk of frost in the early and
late seasons and have potentially allowed for earlier sowing dates. More
importantly, summers have been getting cooler and wetter over the eastern US.
Nevertheless, despite these positive changes, we show that the frequency of
compound hot–dry summer events has remained unchanged over the 1946–2016 period. In the
longer term, climate models project substantially warmer summers for the
continental US, although uncertainty remains as to whether this will be accompanied by
drier conditions. This highlights a critical element to explore in future
studies focused on US agricultural production risk under climate change.
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