Large parts of the US Midwest have seen average temperatures cool slightly since the 1950s, due in part to farmers growing more crops and pumping more water for irrigation. But climate change may eventually overwhelm this “warming hole”, with implications for crop yields in one of the world’s major breadbaskets.
For decades, climate researchers have puzzled over since the late 1950s. Average temperatures in the region saw a small decline, even as global average temperatures rose due to human-caused climate change. The trend is not captured in climate models.
Researchers have pointed to a host of factors behind the warming hole, including reforestation and the influence of on the jet stream. A during the Midwest growing season has also been linked to an increase in the intensity of farming. This is because growing crops densely – and irrigating them – adds water vapour to the local atmosphere, which has a cooling effect.
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To explore further, and at Columbia University in New York looked at this cooling effect specifically in the heavily irrigated areas above the Ogallala aquifer, which supplies groundwater to counties across much of Nebraska and Kansas, as well as to parts of several other states. In their they compare weather records between 1959 and 2017 with county-level data on irrigation throughout the region.
They found a substantial cooling effect across the growing season in the most heavily irrigated counties, with temperatures in August reduced by as much as 3°C Strikingly, this cooling effect also held for counties adjacent to irrigated areas. “It doesn’t just help yourself, it also helps people downwind of you,” says Schlenker.
The cooling effect influenced corn yields, which are sensitive to heat extremes. The researchers estimated that an additional 1.2 billion bushels of corn were grown between 1959 and 2017, around 1.5 per cent of total production.
However, the warming hole and its benefits won’t last forever, says at the US Geological Survey in Colorado. “The overall global warming trend will surely overwhelm any regional cooling,” he says. In a separate study published last month, Partridge and his colleagues found that, under a worst-case emissions scenario, there will be significant declines in corn yields across the region by the even with continued irrigation.
Moreover, depletion of the Ogallala aquifer means it may not be possible to irrigate at current levels in the longer term. “Irrigation might give you a temporary reprieve,” says Schlenker. “But you don’t have enough water to sustain this forever.”
at the University of Minnesota says it is unclear when warming at the global level might overwhelm the Midwest warming hole, but that “there are a lot of trends that do not appear sustainable, certainly in the long run”. He points out that other areas of the world that increasingly rely on irrigation to adapt to rising temperatures – such as northern India – could also face shocks if groundwater resources run out and the cooling ends.
