A worker labors at a romaine lettuce farm in Yuma County, Ariz. (Eric Thayer/Reuters)

Unusual weather in the Southwest could cause a nationwide salad shortage later this month. But that’s just the tip of the iceberg (lettuce): Scientists say the weird weather is probably caused by climate change — which means these sorts of problems are likely to happen again.

The shortage, first reported by NPR, is the result of two separate phenomena in Arizona’s Yuma County and California’s Salinas Valley, the two places where the United States grows most of its leafy greens. In Yuma, the lettuce harvest, which usually runs from November to April, wound up early because of unusually warm weather. And in central California, which typically picks up the harvest once Yuma is done, heavy precipitation delayed some plantings.

That could potentially cause a gap at some point between lettuce supplies, said George Frisvold, an agricultural economist at the University of Arizona. Frisvold’s colleague, Jonathan Overpeck, the director of the university’s Institute of the Environment, says we can blame ourselves, in part, for the great salad shortage.

“There’s this old adage in climate science that you can’t attribute any one event to human causes,” Overpeck said. “That’s not really true anymore, because now it’s really been established that humans alter the whole global climate system. Anything related to increased warmth in the atmosphere likely has some element of human causation.”

Both the temperature in Yuma and the rain in Salinas have a link to atmospheric warmth. The case of Yuma is pretty obvious: Temperatures in the Southwest have been increasing for 100 years, and this winter was no different. According to the National Weather Service, February’s average temperature was two degrees warmer than the recorded average in recent decades.

In Salinas, the situation is a bit more complex, Overpeck said. The region has seen an unusual number of storms called “atmospheric rivers” -- you might know them by the name Pineapple Express — which push heavy precipitation to the Pacific coast from around the Hawaiian islands. It’s unclear whether climate change has a role in the increased incidence of atmospheric rivers, Overpeck said. While some early research suggests that is the case, more data is needed to confirm it.

That said, it’s “a basic concept of physics” that when the atmosphere is warmer, it holds more moisture, Overpeck explained. That means that, when storm clouds form, you tend to see more snow and rain.

According to the 2012 Census of Agriculture, Yuma County boasts nearly 70,000 acres of planted lettuce, while Monterey County, the home of the Salinas Valley, has 134,000. The 2015 Salinas harvest was valued at more than $1.65 billion.

Both regions primarily grow iceberg and leaf lettuce, followed by spinach and baby greens.

Incidentally, these sorts of cascading disruptions aren’t just limited to lettuce — or even to the United States. Britain recently suffered a widely publicized shortage of iceberg lettuce, zucchini, broccoli and cabbage, brought on by extreme weather in Europe’s “salad bowl,” Spain.

Spanish farmers say the shortage of lettuce in European retailers will continue into March as a result of bad winter weather conditions. But they say the recent rationing by supermarkets in the UK was not their fault. (Reuters)

Closer to home, fruit growers across the Northeast and Midwest have expressed concern that unusually high and fluctuating temperatures could cause crops like apples, cherries, plums and grapes to develop too early and expose them to spring freezes. The Progressive Farmer recently warned that “almost off the charts” temperatures in Kansas and Oklahoma could put early-growing winter wheat at similar risk, plus expose it to warm-weather pests and diseases.

In 2012, high winter temperatures cost Michigan $220 million in cherry harvests. That same year, unusually hot nighttime temperatures also cut into Corn Belt yields.

The National Climate Assessment estimated that California and Arizona will have gained 70 extra hot nights per year and 12 to 15 additional consecutive days without rain by the end of the century, because of warming.

“Climate change impacts on agriculture,” the report concludes, “will have consequences for food security both in the U.S. and globally.”

Compared with that dire prediction, of course, a few weeks without lettuce probably doesn’t sound so bad. Frisvold cautions that any price spikes resulting from a shortage would probably be short term and that none have materialized just yet.

But in the sense that the looming salad scenario signals things to come, it's worth paying attention.

“Every grower is noting warming,” Overpeck said. “And while we can’t say what percentage is due to humans, we can say humans are putting their foot on the accelerator.”

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