Two of the world's most fire-prone regions have been heading in opposite directions for decades. Burned forest area in the U.S. Southwest has exploded. Eastern Australia has not seen the same rise. A new study says a long-running shift in the tropical Pacific Ocean helps explain why.
According to Phys.org, the study was published in Environmental Research Letters. Researchers found that the western tropical Pacific has warmed faster than the eastern Pacific since the 1980s, and that this imbalance has been quietly shaping fire conditions on both sides of the globe.
"We found that what's happening in the tropical Pacific over longer timescales, not just during El Niño and La Niña, is another significant piece of the puzzle," said lead author Tess Wei-Ping Jacobson, a postdoctoral fellow at NASA Goddard Institute of Space Studies who recently earned a Ph.D. at the Lamont-Doherty Earth Observatory, which is part of the Columbia Climate School.
The researchers analyzed burned forest area and climate records from 1984 to 2022, combining observational data with a large set of climate model simulations. Their key measurement was vapor pressure deficit, which tracks how strongly the atmosphere pulls moisture from plants and soil. It is one of the strongest predictors of how much forest burns in a given year.
The team used statistical attribution methods to separate two drivers of increased atmospheric dryness: human-caused warming and the long-term Pacific trend. Human-caused warming accounted for most of the drying in both regions. But the Pacific shift added a significant layer on top of that in the Southwest, while doing the opposite in Australia.
The numbers for the U.S. Southwest are stark. Burned forest area increased by more than 3,000% in the interior Southwest and more than 1,000% in a coastal region that includes much of California during the study period. Most of that increase was tied to human-caused warming, but the Pacific trend contributed an additional 22% to the drying-related rise in burned area.
In eastern Australia, the same Pacific pattern worked in reverse. The shift reduced drying relative to what warming alone would have caused, which helps explain why that region has not seen the same surge in burned area despite facing a similarly warming climate.
El Niño and La Niña are the best-known examples of Pacific climate variability, typically influencing weather and fire risk over periods of months to a few years. The pattern examined in this study operates on a longer timescale, persisting across multiple decades. The researchers say this slower-moving signal has been operating in the background alongside more familiar short-term climate cycles.
The findings add a new dimension to how scientists understand regional fire trends. Human-caused warming remains the dominant force behind increasing atmospheric dryness in both regions. But the study shows that the structure of ocean warming matters, not just the overall rise in global temperatures. Where the Pacific warms fastest can tip the balance between drying and relative relief depending on which part of the world you are in.
