In recent months, some of the worst wildfires in U.S. history have devastated communities in Western states, reducing entire neighborhoods to ash and creating walls of flames that turned the sky fiery orange in places like Shaver Lake, California (above).

As of early October, fires had consumed more than 7,800 square miles across California, Oregon, and Washington alone. (That’s an area about the size of Massachusetts.) At least 27 people had died. Tens of thousands of others had fled their homes.

Wildfires are common in the West and even provide benefits to the environment, such as clearing brush from forest floors so new trees can grow. But in the past decade, the blazes have grown deadlier and more destructive. 

Experts say a key reason is climate change. Higher average temperatures and a lack of rainfall are creating the perfect conditions for wildfires: dried-out shrubs, trees, and other vegetation. Then all that’s needed is something to ignite the flames, such as a downed power line or lightning.

Experts say the West will continue to see more intense wildfires unless significant action is taken to stop climate change—for example, by sharply reducing the nation’s carbon emissions. 

“The science is very, very simple,” climate scientist Philip Duffy told The New York Times. “Warmer and drier conditions create drier fuel. What would have been a fire easily extinguished now just grows very quickly and becomes out of control.”