Desert locusts live for about three months. After a generation matures, the adults lay their eggs which, under the right conditions, can hatch to form a new generation up to 20 times larger than the previous one. In this way, desert locusts can increase their population size exponentially over successive generations, Cressman says. Ultimately, these two 2018 cyclones enabled three generations of wildly successful locust breeding in just nine months, increasing the number of insects buzzing over the Arabian desert roughly 8,000-fold.

Then the locusts started to migrate. By the summer of 2019, swarms were leapfrogging over the Red Sea and the Gulf of Aden into Ethiopia and Somalia, where they enjoyed another bout of successful breeding in subsequent months, Cressman says. This might have been as far as the locusts got were it not for the fact that last October, East Africa experienced unusually widespread and intense autumn rains, which were capped in December by a rare late season cyclone that made landfall in Somalia. These events triggered yet another reproductive spasm.

As the locusts continue to multiply, they’re invading new areas. By late December, the first swarms were starting to arrive in Kenya, moving quickly throughout the country’s northern and central areas; by January, the country was experiencing its worst infestation in 70 years. Djibouti and Eritrea also started developing locust infestations, and on February 9, swarms of the insects started arriving in northeast Uganda and northern Tanzania.

Bracing for the worst

The worst of the outbreak may be yet to come. The fall rains, Cressman says, “tipped the situation into something not only very usual but very dangerous” by effectively enabling at least another two generations of locust breeding.

By June, he fears the desert locusts will have increased their numbers 400-fold compared with today, triggering widespread devastation to crops and pastures in a region that’s already extremely vulnerable to famine. Over 13 million people in Djibouti, Eritrea, Ethiopia, Kenya, and Somalia experience “severe acute food insecurity,” according to the FAO, while another 20 million are on the brink.

“It’s all about timing,” Cressman says, explaining that most crops are planted at the beginning of East Africa’s first rainy season, in March or April. “When that rainy season starts and farmers are ready to plant, that will coincide with this new generation of swarms.”

A confluence of unusual weather and climate conditions have helped stoke the outbreak.

Each of the 2018 cyclones that fuelled locust breeding in the Arabian Peninsula was unusual. As NASA notes, the Arabian Sea can go years without a single cyclone forming. But while 2018 was a fairly stormy year, 2019 was extreme, with the North Indian Ocean shattering many records, including most hurricane days and most “accumulated cyclone energy,” a measure of the season’s destructive power. The rare December storm was but one symptom.

This storminess, particularly in 2019, was tied to the Indian Ocean Dipole, which fluctuates between positive, negative and neutral states as ocean temperatures seesaw across the Indian Ocean. When the Indian Ocean Dipole is negative, westerly winds push warm waters close to Australia, bringing additional rain to the southern part of the continent. When it’s positive, the westerlies weaken, allowing that warm water—and rainfall—to shift toward East Africa.