This week, the largest wildfire in its history is scorching New Mexico. Nine Western states, suffering from last winter’s drought, are at high risk for fire.
The U.S. Forest Service created Smokey Bear during the last long dry cycle in the West. SOURCE: USDA
Giant forest fires are nothing new to the West. Scientists are learning that these fires come in cycles that are connected to patterns in the Pacific. We are in a cycle that is reducing snowfall in the Western mountains. During the previous dry cycle, which peaked in the 1940s and 50s, the US Forest Service invented Smokey Bear and began to teach people, “Only you can prevent forest fires”.
Unfortunately, the Pacific also plays a huge role in creating or preventing forest fires. If the water off the West Coast is cool, it cools the air above it and holds less moisture. The prevailing winds blow this dry air inland and there is little snow or rain.
Two ocean patterns shift cooler waters off the North American coast.
La Niña—The La Niña is when the tropical Pacific warms up. It shapes global wind and water patterns, which includes shifting cold water off the coasts of Mexico and California. This cooling normally lasts between 6 and 18 months.
Left: The Warm PDO (1925–1946 and 1977–2006). Right: The Cool PDO (1890-1924, 1947-1976 and now). Cool PDOs create drier conditions in the West. SOURCE: USGCRP
The cool phase of the PDO—The Pacific Decadal Oscillation is a longer cycle that shapes water temperatures in the North Pacific. It usually lasts about 50 years with cooler water shifting from the Western Pacific for twenty to thirty years, over to the Eastern Pacific, and back again. A cool phase of the PDO creates decades of reduced Western snowfall. Scientists disagree whether the cool phase began in 1999 or 2006, but it is cool and the West has had years of drier weather.
During the wet years of the PDO, (1977–2006), more trees grew which created dense forests. Now, the West is drier and the forests have more trees than the water supply can support.
Following last winter’s La Niña, the Western wilderness has seen multiple wildfires. SOURCE: U.S. Forest Service
The good news is that fires are part of the natural cycle. In the 1940s and early 1950s, the era of Smokey Bear, fires naturally thinned the forests and the following two decades, were less dangerous. The fires of today are shaping a quieter, less fiery tomorrow.
Evelyn Browning Garriss, historical climatologist, is a longtime writer for The Old Farmer's Almanac. She is also editor of The Browning World Climate Bulletin  and has advised farmers, businesses, and investors worldwide on upcoming climate events and their economic and social impact for the past 21 years.