In October 2017 in California, more than 4,850 square kilometers and 10,000 structures were burned by about 9,000 wildfires, according to a 4 August Earth and Space Science study investigating the causes and impacts of the fires.
Yuan Wang, a coauthor on the study, emphasized the urgent need to better understand the drivers and effects of wildfires. Wang is an atmospheric scientist at the California Institute of Technology (Caltech) studying human impacts on weather systems and climate. He was also the 2016 recipient of AGU’s James R. Holton Junior Scientist Award.
A First for California
Wang and his team calculated how much the fires contributed to atmospheric carbon dioxide, a first in the study of California wildfires. Using data from the Orbiting Carbon Observatory-2 (OCO-2) satellite, they determined that after the October 2017 fires, atmospheric carbon dioxide levels increased by 2 parts per million. The environmental effect creates a feedback loop: An increase in atmospheric carbon dioxide contributes to greater drought and wildfire-prone conditions, which in turn lead to more wildfires and subsequent increases in atmospheric carbon dioxide.
Future climate models need to account for how wildfires contribute to carbon dioxide levels, which requires the use of similar satellite measurements, Wang said.
Since this particular satellite was launched in 2014, few wildfire studies have used its data to examine the contribution of wildfires to atmospheric carbon dioxide, according to Jonathan Jiang, supervisor of the aerosol and cloud group for the Earth science section of NASA’s Jet Propulsion Laboratory. (Jiang wasn’t involved with the study, but Wang was previously a postdoctoral researcher working for him.)
“The identified impacts on the atmospheric greenhouse gas concentrations should bring much attention on future research of wildfires, and also impact policy making,” Jiang wrote in an email to Eos.
Probing Factors Contributing to Wildfires
The researchers analyzed a variety of meteorological and atmospheric data—including surface temperature and pressure, liquid and ice water contents of clouds, precipitation, and wind measurements—to determine what caused the fires. In many cases, human factors triggered the fires, but environmental conditions worsened their magnitude, Wang said.
Multiple factors created wildfire-prone conditions. January-to-April precipitation “triggered a massive growth of weeds/vegetation,” which later dried out and fueled the fires, the researchers write.
Increased drought conditions also contributed to the likelihood of wildfires. A 1.7 kelvin increase of California’s mean surface temperature over the past 39 years, along with decreases in precipitation and cloud water path (the total amount of water in a cloud area), worsened droughts, researchers noted. High pressure over the Pacific Ocean and intensification of the Santa Ana winds also intensified drought conditions. The analysis yielded negative correlations between surface temperature, cloud water content, and precipitation.
Researchers didn’t find a clear relationship between El Niño, the Pacific Decadal Oscillation, and summer precipitation values. However, their analysis of atmospheric vertical motion data detected negative velocity anomalies, which suggests the presence of strong sinking air when the fires occurred. This air behavior “enhances the atmospheric stability and does not favor the formation of precipitation.”
The findings are consistent with previous theories, Wang said.
Wang’s coauthor Andy Li, first author on the study, is a senior at Clements High School in Sugar Land, Texas. He participated in the research through Caltech’s Summer Research Connection, a program for science teachers and high school students.
“I consider [the study] to be a very big achievement for a high school student,” Wang said.
—Rachel Crowell (@writesRCrowell), Freelance Science Journalist