Increasing Heat Is Driving Off Clouds That Dampen California Wildfires

Recently an article in the State of the Planet, a publication of the Earth Institute at Columbia University, caught my eye. The article noted that urbanization and climate change are combining to make wildfires in Southern California’s coastal cities more intense because it is “driving off low-lying clouds”, aka Stratus clouds as well as fog, increasing the risk of forest fires along with the ability of nature to fight them. 

Kevin Krajik writes:

“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 research follows a 2015 study in which Williams first documented a decrease in cloud cover around the sprawling Los Angeles and San Diego areas. Urban pavement and infrastructure absorb more solar energy than does the countryside, and that heat gets radiated back out into the air–a major part of the so-called heat-island effect, which makes cities generally hotter than the rural areas. At the same time, overall temperatures have been rising in California due to global warming, and this has boosted the effect. In the new study, Williams and his colleagues have found a 25 to 50 percent decrease in low-lying summer clouds since the 1970s in the greater Los Angeles area.

Normally, stratus clouds form over coastal southern California during early morning within a thin layer of cool, moist ocean air sandwiched between the land and higher air masses that are too dry for cloud formation. The stratus zone’s altitude varies with weather, but sits at roughly 1,000 to 3,000 feet. But heat causes clouds to dissipate, and decades of intense urban growth plus global warming have been gnawing away at the stratus layer’s base, causing the layer to thin and clouds to burn off earlier in the day or disappear altogether. Cloud bases have risen 150 to 300 feet since the 1970s, says the study. “Clouds that used to burn off by noon or 1 o’clock are now gone by 10 or 11, if they form at all,” said Williams.

We know that wildfires are increasing in strength, and that wildfire season starts earlier and later than usual across the western United States. But in urbanized California the link is more subtle. The author notes that few scientists have looked at clouds as an amplifier of wildfire. The data that Coulmbia University was able to access came from airports which have been collecting cloud observations since the 1970’s for navigational safety. Comparing airport data with the U.S. Wildland Fire Assessment System, that measures the moisture of vegetation,  they were able to conclude “that periods of less cloud cover during the summer correlated neatly with lower vegetation moisture, and thus more danger of fire”.

However, the study did not find that total area burned in summer has increased as a result of decreases in cloud shading. There are too many other factors at play, said Williams. These include yearly variations in rainfall, winds, locations where fires start, and perhaps most of all, decreases in burnable area as urban areas have expanded, and the increased effectiveness of fire-fighting. “Even though the danger has increased, people in these areas are very good at putting out fires, so the area burned hasn’t gone up,” he said. “But the dice are now loaded, and in areas where clouds have decreased, the fires should be getting more intense and harder to contain. At some point, we’ll see if people can continue to keep up.”