'Indian Niño' drove record heat in 2023 and 2024, new UMD study finds

In 2023 and 2024, Earth’s average global surface temperature spiked nearly 0.3 degrees Celsius above what was already expected from climate change. Each year was declared the hottest on record and coincided with deadly wildfires, heat waves and historic numbers of climate-related disasters. 

Scientists have struggled to explain what caused these anomalously warm years. In a new study, published May 6, 2026, in the journal Earth System Dynamics, University of Maryland researchers propose a novel answer: They attribute part of the temperature surge to the Indian Ocean Dipole (IOD), a climate cycle similar to El Niño. It’s the first time the IOD has been linked to these two unusually hot years.

The researchers built a climate model that predicts global temperatures based on an extensive list of natural and manmade factors. The variables they considered could explain 93% of the global surface temperature anomaly in 2023 and 92% in 2024—the closest scientists have gotten to explaining the record-breaking heat. The IOD was among the most important predictors. When the researchers omitted the IOD from the model, they could only account for 69% of the temperature spike in 2023 and 77% in 2024. The authors say that understanding how such natural factors influence global temperatures will help global decision-makers isolate and potentially mitigate the climate impact of human activity. 

“This is probably the most comprehensive attribution that’s out there right now,” said study lead author Endre Farago, who conducted this research as part of his chemistry Ph.D. research at UMD. “Being able to explain 92% to 93% of the anomaly—that’s basically spot on.”

The IOD, sometimes called “Indian Niňo,” refers to the difference in sea surface temperatures in the Indian Ocean’s western and eastern sides. Depending on the year, the western side can be warmer or cooler than the eastern side by varying degrees. This temperature gradient affects everything from rainfall patterns in India to bushfires in Australia.  

The IOD was only discovered in the late 1990s and is still not fully understood. Until the 2010s, scientists debated whether it was an independent cycle or a consequence of the more well-known El Niño Southern Oscillation (ENSO) in the Pacific Ocean. And because the Indian Ocean is the smallest of the three major ocean basins, many people didn’t think it could strongly affect the global climate, said study co-author and Atmospheric and Oceanic Science Professor Ross Salawitch. 

“There was a view in the climate community that the Pacific Ocean was the dog, and the Indian Ocean was the tail,” said Salawitch, who also holds joint appointments in the Department of Chemistry and Biochemistry and the Earth System Science Interdisciplinary Center. 

Even Salawitch was skeptical when Farago, his former graduate student who is now a postdoctoral researcher at Cornell University, first told him about the IOD result. But Salawitch was convinced by the regional patterns of warming in South America and southern Australia in 2023 that were uniquely associated with the IOD, and how much worse the model performed when they removed this climate cycle from consideration. 

Farago was impressed that the model could explain nearly all of the temperature anomaly in 2023 and 2024. Leading atmospheric scientists had been racing to identify the causes. But those studies, including one by the World Meteorological Organization, didn’t include the IOD and could not fully explain the temperature spike—especially in 2023, which was one of the two most extreme IOD events since modern recordings began around 1850.

In addition to the IOD, the research team estimated contributions from other natural cycles, including the other two major ocean basins: high sea surface temperatures in the North Atlantic and an ENSO event in the Pacific. The IOD affected global temperatures almost as much as ENSO in 2023 and nearly half as much in 2024. 

Although natural climate cycles partly explained the hot temperatures, the researchers emphasize that human activity was a key factor as well. The new study found that in 2023 and 2024, anthropogenic greenhouse gas emissions increased temperatures by about 0.022 degrees Celsius per year—an acceleration compared with the late 20th century.

Additionally, recent efforts to clean up the shipping industry affected temperatures in subtle but noticeable ways. To improve public health, the International Maritime Organization implemented a regulation in 2020 that limited the allowable amount of sulfur in ship fuel. The new rule reduced toxic sulfur oxide emissions. Although these pollutants are unhealthy to breathe, they reflect solar radiation away from Earth, so reducing their presence in the atmosphere unintentionally allowed more radiation to heat the planet. In 2023 and 2024, that caused heating equivalent to up to 25% to 30% of that caused by the recent rise in greenhouse gases, per the new study. It’s also possible that these maritime regulations contributed significantly to the unusually warm conditions in the North Atlantic in 2023 and 2024, which were also a major driver of the record global surface temperatures, the study states.

Although the extent of the temperature anomaly (0.3 degrees Celsius) seems small, it is meaningful. It represents about 20% of the roughly 1.5 degrees Celsius Earth has warmed since 1850 and manifests as prolonged heat waves, stronger precipitation and other climate extremes that harm people and the economy, Farago said. He says it’s crucial to understand the natural and manmade factors that lead to such drastic temperature swings.

Thus, Salawitch hopes that other researchers will include the IOD in their climate models moving forward. 

“After our paper comes out, colleagues will have serious conversations about whether they should include IOD in future global warming attribution studies. It's our hope that they'll find some importance and truth—some veritas—to what we did and what we found,” Salawitch said. “Doing so will hopefully lead to better quantification of the various natural and human factors that drive global warming, with meaningful implications for future policies to address climate change.” 

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UMD AOSC graduate student Brian Bennett and Associate Professor Timothy Canty co-authored this article with McLane and Dodson.

The paper, “Quantification of the influence of anthropogenic and natural factors on the record-high temperatures in 2023 and 2024,” was published in Earth System Dynamics on May 6, 2026.

This research was supported by the NASA Climate Indicators and Data Products for Future National Climate Assessments program. This article does not necessarily reflect the views of this organization.