To Whom It May Concern: 300-Year-Old Letters Reveal Hurricanes’ Long-Term Rise
The number of storms in the south Indian Ocean has spiked since 1940, alongside a local increase in seawater temperatures.
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“We had a hurricane on March 8th.” So starts a letter from 1743 written by the governor of Isle de France, now Mauritius, when the island was a French colony. The brief dispatch to his colonial collaborators at the East India Company details the storm’s devastation: ships smashed on shore, waves crashing onto the storerooms, and the harvest “almost completely destroyed.” He promises to send peas and beans once the port is repaired.
Legumes aside, letters like this one contain priceless clues for climate historians who are piecing together the storm record for Mauritius and the other Mascarene Islands, a small archipelago east of Madagascar. Today, data from satellites, ship-based sensors, and drifting ocean buoys form the backbone of weather records, but prior to the information age, storm details came from eyewitness accounts recorded in paper documents. In a recent study, researchers used stacks of old letters, including the governor’s account from 1743, to determine that the number of hurricanes has quadrupled in the region in the last 80 years.
The Mascarenes were uninhabited when the French showed up in 1715. The Dutch had already failed to start an agricultural colony in the stormy islands but the French dug in, transporting thousands of slaves from places such as Mozambique and Madagascar to harvest crops and build sugar refineries. Representatives of these early trading companies scribbled reports to their anxious investors in Europe, detailing the day-to-day management of ships and sugar. The British continued the record keeping after seizing control of the islands in 1809 and 1810.
Climate historian Emmanuel Garnier tracked down hundreds of years of colonial correspondence in archives in Mauritius, Réunion, England, and France. Then, along with his coauthors, he set about translating the letter writers’ observations into data. To determine past hurricane frequency, the researchers leafed through the yellowed pages to find descriptions of sea state and the damage to people, property, and trade, using those details to rank each storm on the Saffir-Simpson Hurricane Wind Scale.
The letters dwindled over time, and more sophisticated technologies appeared in the storm-tracking records. The British started monitoring wind speed and air pressure in the 19th century. In 1892, photographs began appearing in newspapers, enabling the study authors to see storm damage instead of just reading about it. By incorporating this information into the data, Garnier and his colleagues built a 300-year chronology—the longest tropical cyclone record in the southern hemisphere.
The chronology includes records from every year between 1695 and 2007, meaning it likely references nearly every storm. “Very few, if not no, cyclones have escaped us,” Garnier writes in an email.
Between 1720 and 1940, cyclones that made landfall on the Mascarene Islands were relatively infrequent, with just three hurricanes every 20 years. Since 1940, though, the average has jumped to 14 storms per 20-year interval. Most of the increase has been in low severity Category 1, 2, and 3 storms, which are four times more common since 1940. The most severe storms—Category 4 and 5—have also increased in frequency.
The long chronology allows scientists to spotlight the increasing number of hurricanes in the Mascarenes, says Carlos Loureiro, a marine geoscientist at the University of KwaZulu-Natal in South Africa. “Those two or three hundred years are really important to make sure that the last decades are exceptional,” he says. “I think it’s quite an important study.”
Previous research estimates that the Indian Ocean has warmed about 1 °C since the French first arrived in the islands, with a noticeable uptick since 1940. That coincides with the tipping point in storm frequency that Garnier and his coauthors identified.
The letters provide a much-needed window into a challenging region. In some locales, climate scientists can identify past storms from disturbances in underwater sediments. But the rocky Mascarenes haven’t left much of a sediment record to explore, says Loureiro. “This historical work is another piece of the puzzle.”
While researchers uncover the past, new storms keep coming. Cyclone Freddy skirted the Mascarenes in February 2023, sinking a fishing boat. The crew of 16 is still missing. The storm subsequently pummeled the African coast, killing hundreds of people in Madagascar and mainland Africa. When Freddy finally fizzled over Mozambique and Malawi, it held the title of the longest-lasting tropical cyclone in recorded history. What storm surpasses it remains to be seen.