Large parts of Australia can expect hotter, drier seasons like those that preceded this year's extreme fire season, scientists have warned. This is due to changes in the Indian Ocean's surface temperature.
Researchers writing in Nature studied the changes in the Indian Ocean Dipole (IOD) since 1240 CE. The phenomenon refers to differences in temperature between the Indian Ocean's two "poles"—the Arabian Sea in the west and the Indian Ocean south of Indonesia in the east.
The IOD cycles through three phases: "positive," "negative," and "neutral." Positive phases are caused by warmer water in the west and cooler water in the east. Negative phases are the reverse. A neutral phase is when temperatures are close to average on all sides of the Indian Ocean.
Because warmer waters attract rainfall, countries toward the western side will experience wetter weather during positive events, while those in the east will experience less. The strong positive IOD that took place in 2019 brought floods to East Africa and drought and searing temperatures to Australia.
Thee 2019 positive Indian Ocean Dipole event "was one of the strongest on record" and was likely "a key player" in the drought Australia experienced, Caroline Ummenhofer, physical oceanographer at the Woods Hole Oceanographic Institution in Massachusetts, told Newsweek.
"We have known about Pacific Ocean conditions associated with the well-known El Nino-Southern Oscillation affecting regional and global climate for nearly a century," she said. "However, it is only in the past 20 years that we have recognized that the Indian Ocean can similarly lead to shifts in ocean temperatures, winds, and rainfall patterns."
Now, scientists have shown these events became more frequent and more intense over the twentieth century—and are likely to increase and intensify in the future.
"What was surprising was to see that the extreme events that we've seen in recent decades aren't the worst that is possible," lead researcher Professor Nerilie Abram, a climate scientist at Australian National University (ANU), told Newsweek. "That is a worry when were are pushing this system so that we expect events to get stronger and happen more often in the future."
Using precisely-dated coral records collected from the eastern Indian Ocean, an international team of researchers were able to measure variations in IDO over five (non-consecutive) centuries, starting in 1240 CE. This allowed them to compare changes in IOD to fluctuations in climate.
According to their data, extreme positive events were rare before 1960 — there were just six on record. Since then, extreme positive events have taken place at much smaller intervals and have been recorded in 1961, 1994, 1997, and 2019.
"The IOD was part of the recipe that produced the 2019 bushfire crisis in Australia," said Abram. "When the extreme 2019 IOD event developed in winter and spring, that was a red flag that we had a high bushfire risk coming during the summer. But it wasn't just the IOD."
A multi-year drought, stratospheric warming that caused a succession of hot, dry winds and long-term climate warming that is causing the fire season to start earlier and last longer also played a role, she explained.
The study's authors warn the Indian Ocean is capable of triggering events even more severe than those of 2019, which contributed to the Australian bushfires and East African locust plagues. This is not just a possibility but an expectation if greenhouse gas emissions continue to rise.
According to the scientists' calculations, even if warming is limited to 1.5 degrees celsius, the goal set in the Paris Agreement, the frequency of extreme positive IOD events will be twice as high as they were in pre-industrial times. If warming continues on its current trend and that goal is not met, the number may triple.
The data shows the Indian Ocean is capable of events more extreme than that of 1997, which the researchers say is the strongest on instrumental record.
An extreme positive IOD in the seventeenth century, caused by natural events, was found to be up to 42 percent stronger than 1997.
The 1675 event coincided with historical text detailing extreme drought and famine across Indonesia, as well as rice crop failures in India and Thailand, and war in Java between 1674 and 1679, suggesting changes in the Indian Ocean can have a major societal impact.
"The seventeenth century is known as an interval of crisis in tropical Asia, involving widespread mortality and economic and political disruption," the study's authors write.
"This upheaval has been attributed to strong El Niño variability and a weak Asian summer monsoon but our coral-based reconstruction suggests that unusually strong variability of the IOD was also a factor in causing societal disruption across Asia during the seventeenth century."
This article has been updated to include comments from Caroline Ummenhofer and Nerilie Abram.
