CA, USA — Researchers predict a decades-long increase in widespread fires across the Western United States in the coming years, based on a new study that reviews the link between sea-surface temperatures in the North Atlantic Ocean and the ferocity of wildfire seasons in the West.
Although scientists often cite the regional impacts of fluctuating Pacific Ocean temperatures — the El Niño/La Niña effect — a new look back across 500 years’ worth of wildfire history shows a different trend: Warmer surface temperatures in the North Atlantic translated to worse wildfire seasons on the West Coast.
The Atlantic Ocean is entering its next warm phase, scientists note. And global warming will exacerbate the trend.
The findings are being published this week in the online edition of the Proceedings of the National Academies of Science. The research was led by Thomas Kitzberger of the Universidad Nacional del Comahue in Argentina and scientists at the University of Colorado and the University of Arizona.
“More than any other study, this really shows how the El Niño/La Niña cycle influences where fires occur,” said Christopher Field, biology professor at Stanford University who managed the article’s peer review for the journal.
“What they show that’s really interesting is that there are quite long periods where you get fires everywhere,” he added. “And there are other long periods where you get this strong alternation of fires in the north and fires in the south.”
The Pacific Ocean regularly cycles between warmer and cooler surface temperatures. In El Niño years, when a large swath of the ocean warms, larger and wetter storms hit the southern half of the Western United States, leaving the northern woods drier.
This year is considered a weak El Niño year, with above-normal rain expected for Southern California across into Texas.
La Niña does the opposite: A cooler Pacific sends moisture north, leaving the southern half primed for fire.
But when scientists examine fire scars in ancient trees throughout western North America, they see evidence of long periods when the north and south seem to burn simultaneously.
To explain that, they have to look across the globe, to the Atlantic.
Unlike the El Niño/La Niña oscillations and its two- to three-year cycle, the Atlantic seesaws between warmer and colder surface temperatures every 60 to 70 years.
It is a poorly understood phenomenon, Field said, largely because ocean records extend only 150 years back — less than three cycles. But Kitzberger and colleagues reconstructed past cycles from tree-ring growth in forests in Finland, Italy, France, Jordan, Turkey, the United States and elsewhere.
They then dated almost 34,000 fire scars in trees from the Black Hills of South Dakota to Mexico’s Sierra Madre Occidental, finding wildfire most widespread in years when the Atlantic was warmest.
The records extended back to the 16th century and included almost 7,000 scars from 57 sites in California’s Sierra Nevada. Kitzberger, in Argentina, deferred questions about the research to colleagues in the United States, who could not be reached for comment.
“It does a lot to explain the historical relationship among fires in different places,” said Field, who also directs the Stanford-based Carnegie Institution’s Department of Global Ecology. “It clarifies what kind of situations you need to get fires everywhere. Those are the ones that result in a large loss of life and property and that dominate the political spectrum.”
The Atlantic is thought to be entering its warm phase. Scientists aren’t sure where in the cycle it is, but hurricane watchers speculate there’s more warming on the way. (A warmer Atlantic also births more and stronger hurricanes.)
Scientists say the Atlantic is already warmer as a result of global warming, with more of that warming on the way — purchased, so to speak, but not yet delivered because of the lag between industrial emissions and climate change.
So a key question facing those watching the West is how bad fire conditions will get in the coming decades.
Field said: “We’ve seen a number of large wildfire years recently. Our projections call for more warming, and more (fire) intensity.”