December 11, 2023

This week, farmers across the midwest are preparing for temperatures to reach 115F (46C) as a heat dome covers the region. After a tricky growing season – that seesawed between drought and unseasonably heavy rains – many midwestern farmers worry the extreme heat will scorch, or at least stunt, their already struggling crops.

To say it’s been a hot summer would be an understatement. According to Nasa scientists, July was the hottest month ever recorded. Off the coast of Florida, surface ocean temperatures soared over 101F, bleaching coral reefs. In Arizona, Phoenix residents sweated through a record 31 consecutive days above 110F. Even animals that spend much of their time in the sky, like birds, struggled to keep cool in the sweltering heat.

Across much of the country, the food system also struggled. In Texas, farmers reported smaller yields as their corn and cotton crops struggled to survive soaring summer temperatures. In Arizona, beekeepers spotted dead honeybees outside hives. Even underwater, off the coast of Long Island, kelp farmers recorded another year of shrinking yields.

As extreme heat scorched the oceans, land and skies, it affected our food system underwater, on the ground and in the air. But the ways that it did so were unpredictable – not every crop suffered, though many certainly struggled, and not every region was affected equally. If there was any takeaway, it may have been that previous scientific models for extreme heat failed to capture the full scope of what heatwaves of this magnitude could cause – and that farmers, fishers and pollinators should be prepared to adapt to an uncertain future.

“It’s really problematic if we use past disasters as the basis for which we plan for the future,” says Erin Coughlan de Perez, a professor focused on climate risk management at Tufts University. “The future is not going to be the same as the past.”

Land: wilting wheat and heat-stressed cattle

Extreme heat can stress many plants, including common crops, causing them to go dormant or to seed, stunting growth and disrupting pollen (and therefore fruit) development. In a study published in Nature’s climate and atmospheric science journal this June, researchers found that extreme heat is now more likely to affect wheat yields in the US and China – a major wheat exporter – than historical precedents suggest.

Extreme heat “that used to happen every 100 years, used to be a really rare, unusual disaster event, is now something that would happen every six years” in the United States, said Coughlan de Perez, the lead author on the paper. When temperatures get too hot (above 27.8C, or about 82F), wheat photosynthesizes more slowly, and when it gets very hot (above 32.8C, or about 91F) enzymes in the plant can start to break down.

Although extreme heatwaves are becoming more common, that doesn’t mean every region has become uninhabitable for every crop. Rather, the types of crops that can survive in different regions are changing. Coughlan de Perez points to data published by the Intergovernmental Panel on Climate Change that shows how different crops are faring across the globe. In Texas and across much of the midwest this year, extreme heat threatened corn, cotton, sorghum and soybean crops, which were only saved by heavy rains late in the growing season. As the climate changes, farmers are considering ways to plant or harvest earlier to take advantage of milder temperatures.

Heat stress can affect livestock’s milk yield and fertility.
Heat stress can affect livestock’s milk yield and fertility. Photograph: Richard Hamilton Smith/Design Pics/Getty Images/Design Pics RF

Some experimental initiatives in the US are looking to develop crops with heat-resistant genes or grow plants that haven’t traditionally been eaten as prolifically. At the Wheat & Rice Center for Heat Resilience, for example, researchers are studying the genes that help rice tolerate heat stress. Meanwhile, at the Experimental Farm Network, growers from across the country are breeding new crops and testing the range of certain plants to see what might grow best in a changing climate.

Extreme heat doesn’t only pose a risk to crops, but to livestock as well. In late August, 22 cattle died in a Nebraska heatwave and Oklahoma ranchers reported cattle drinking twice the amount of water to survive a “heat danger” advisory. Even when it isn’t fatal, heat stress can affect livestock’s milk yield and fertility – and can also harm the people who care for those animals and farms.

Oceans: anchovies and algal blooms

Scientists who study the oceans are witnessing that kind of unpredictability underwater as well. According to the National Oceanic and Atmospheric Administration (Noaa), the ocean absorbs 90% of the excess heat associated with global warming – meaning that heatwaves that happen on land are also happening underwater.

“The question has become not really so much whether there’s a marine heatwave or not, in the north-east Pacific Ocean, but how close it comes to shore,” said Andrew Thompson, a research fisheries biologist at Noaa’s Fisheries Service. When heatwaves, like an unprecedented marine heatwave known as “the Blob” that dominated the US west coast from 2013 to 2016, come too close to shore, they start to affect crab, squid, salmon and other fisheries that supply much of our seafood.

But just as heat can be devastating for some crops and beneficial for others on land, so can it be for ocean life. In a paper published earlier this year about the impact of “the Blob” on west coast fisheries, Thompson and his co-authors found that the effects of extreme heat weren’t always clearcut.

“We’re seeing increased warmth unlike we’ve ever seen before,” said Thompson. As a result, “the system is responding in ways that we really wouldn’t have expected based upon our past observations.”

For example, Thompson is part of a team that studies anchovies, sardines and hundreds of other fishes through the California Cooperative Oceanic Fisheries Investigations program. Although his team had previously seen anchovies thrive in cooler waters, they saw the anchovy population explode during the 2013-2016 marine heatwave. Today the anchovy population off the west coast is probably higher than it’s been since 1950.

“The thing that’s interesting to me as a fisheries scientist is the often unexpected results that are coming from the marine heatwaves. The results have been very, very nuanced,” Thompson added. That includes “a couple of really big, unexpected positive things that happened”.

While cod, abalone, Chinook salmon and Dungeness crab struggled during the 2013-2016 marine heatwave, market squid, shrimp and bluefin tuna fisheries did surprisingly well. On one hand, the heatwave created toxic algal blooms that forced the closure of California’s Dungeness crab fishery. But on the other, it increased the range where cool-water-loving market squid could survive, creating new fishery opportunities in Oregon and Alaska.

Air: pollinators on pause

Although much of the research on climate change and food systems has so far focused on the direct effects of heat – for example, how heat exposure changes plants – Jenna Walters, a doctoral candidate at Michigan State University’s Pollination Ecology Lab, says it’s important to consider the indirect effects of heat too – like how changing crops will affect pollinators.

Bees pollinate about 100 commercial nut, fruit and vegetable crops. But extreme heat can affect the formation of pollen tubes, diminishing the nutritional value of flowers for the bees that would pollinate them.

Buckfast honey bees fly near a beehive in Merango, Illinois.
Buckfast honey bees fly near a beehive in Merango, Illinois. Photograph: Bloomberg/Getty Images

“Those nutrients that the plants require for pollination and for their reproductive processes are actually the same nutrients that bees are visiting the plants for,” said Walters, who has published papers on the effects of extreme heat on pollinators and pollen tube formation. “The bee is going to be indirectly stressed because the plant is enduring these stressors.” Without sufficient nutrition, bees won’t develop properly and may even die before reaching adulthood.

This summer, entomologists in Arizona reported a growing number of dead honeybees outside hives – a sign that the bees had closed off the hive during the worst of the heat. Scientists worry that more bees will die as temperatures rise – or focus their energy on staying cool rather than pollinating.

Walters notes that, as social species, honeybees and bumblebees have developed techniques like wing fanning, where pollinators return to the hive during heatwaves to flap their wings together and ventilate their home. But solitary bees, which make up the vast majority of bees in the world, are by definition not social, so they don’t have similar measures to cope with the heat. And because they don’t care for their larvae after laying their eggs, their young are especially vulnerable to extreme temperatures.

In the face of rapidly rising temperatures, Walters emphasizes the importance of deeper research on alternative pollinators. “If you are trying to make a resilient system in agriculture or otherwise, you need to have backup plans,” she said. “You need to have a diversity of different pollinators present in a field so that if we’re in a situation where it’s too hot for mason bees, who are more effective pollinators of a certain crop, then maybe honeybees can come in and help out.”

That idea of having a backup plan may be key to supporting the entire food system through extreme heat.

“I would suggest a full basket of approaches,” says Coughlan de Perez. While some scientists breed new crops with more heat-resilient genes, farmers can evaluate what to plant when and policymakers can adjust insurance options for farmers trying to weather the heat. Ultimately, she hopes everyone uses models, like the one she and her colleagues designed, “to encourage thoughtful adjustments to our behavior”.

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