Increasing heat is driving off clouds that dampen California wildfires

  • Urbanization and climate change combine to heighten danger
  • Cloud cover is decreasing in Southern CA and as clouds decrease, that increases the chance of bigger and more intense fires.
  • Scientists found that periods of less cloud cover during the summer are correlated neatly with lower vegetation moisture, and thus more danger of fire.
May 30, 2018 Lamont-Doherty Earth Observatory, Columbia University Read full ScienceDaily article here
“Cloud cover is plummeting in southern coastal California,” said Park Williams, a bioclimatologist at Columbia University’s Lamont-Doherty Earth Observatory and lead author of the research. “And as clouds decrease, that increases the chance of bigger and more intense fires.” Williams said the decrease is driven mainly by urban sprawl, which increases near-surface temperatures, but that overall warming climate is contributing, too. Increasing heat drives away clouds, which admits more sunlight, which heats the ground further, leading to dryer vegetation, and higher fire risk, said Williams. The study appears this week in the journal Geophysical Research Letters…

…The catastrophic California-wide fires that consumed over 550,000 acres in fall of 2017 were probably not strongly affected by the reductions in summer cloud cover, said Williams. Although he did find that vegetation is drier in fall seasons that follow summers with few clouds, the fall 2017 fires were driven mainly by extreme winds and a late onset of the fall rainy season. And ironically, part of this record wildfire wave resulted not from a recent record four-year drought driven in part by climate change, but rather from record rains that followed the drought, which produced a surfeit of flammable vegetation…

A. Park Williams, Pierre Gentine, Max A. Moritz, Dar A. Roberts, John T. Abatzoglou. Effect of reduced summer cloud shading on evaporative demand and wildfire in coastal southern California. Geophysical Research Letters, 2018; DOI: 10.1029/2018GL077319