Climate change intensifies the risk of wildfires
On December 10, 2024, a wildfire continued to spread in Malibu, a coastal city in Los Angeles County, southern California, USA, due to strong winds, causing thousands of residents to evacuate and tens of thousands of users to lose power. Climate warming is leading to an increase in wildfires in high-latitude regions, with the affected area, fire intensity and the resulting carbon dioxide emissions all rising
According to the updated statistics from the California Department of Forestry and Fire Services on January 6th, there were 35 wildfires in California in just six days of the New Year. What kind of weather and climate background has led to such a fierce forest fire this time? The reporter interviewed Yang Xiaodan, deputy chief of the Industry Service Office of the Public Meteorological Service Center of the China Meteorological Administration, and Li Fang, a researcher at the Institute of Atmospheric Physics, Chinese Academy of Sciences. Yang Xiaodan said that wildfires do occur frequently in southern California. In 2024, the area affected by wildfires in California was almost four times that of the same period last year. On December 9, 2024, a wildfire hit Malibu, a coastal city in Los Angeles County, California, affecting approximately 22,000 people. In October, a fire spread rapidly in the mountainous area of Oakland, California, due to strong winds, destroying two houses and about 500 people received evacuation orders…
However, historically, the high incidence season for fires in southern California was from late summer to autumn. Winter, being the rainy season in California, had a relatively lower risk of fires. Li Fang said that in recent years, due to climate change, winter precipitation has been delayed and droughts have persisted. The strong monsoon “Santa Ana Wind” has been constantly blowing, and fires have also occurred in winter. This fire is a typical manifestation.
Yang Xiaodan introduced that this fire had the three elements of a forest fire: the source of fire, combustibles and oxidizers. Many residents in California live in forest areas. According to statistics, 90% of wildfires in the United States come from residual sources of human activities such as cigarette butts discarded by people entering the mountains and unextinguished campfires in campsites. In addition, the northern part of California, where it borders Oregon, is covered with dense forests and hills. The eastern part is the dense Sierra Nevada Mountains. In the mountainous areas near cities like Los Angeles and San Francisco, there are also numerous trees and thick shrublands, providing a large amount of combustible materials for the fires and making California almost a natural “firewood stove”. Moreover, California has a typical Mediterranean climate, with hot and dry summers and mild and humid winters. The abundant precipitation in winter and spring causes weeds and shrubs to grow everywhere, while the high temperature in summer takes away the water, making the weeds and shrubs dry and flammable. In 2024, the precipitation in this region was significantly less than the average of previous years, especially since autumn and winter, when a notable drought has occurred. These conditions were the combustibles of this fire.
The fire spread and became a disaster mostly due to superimposition
What are the weather causes of this big fire? Agence France-Presse on the 8th quoted Christina Dahl, vice president of scientific affairs at the independent US agency Climate Center, as saying that the rapid spread of wildfires is related to factors such as higher temperatures, drought and strong winds. “Currently, the southern region of California has all the above conditions,” among which “the most obvious weather signal is temperature.”
NASA, citing an analysis by the National Weather Service, said that strong Santa Ana winds and lack of rainfall provided weather conditions for “extreme” fire risks in southern California. The Santa Ana winds usually occur between October and January of the following year. At this time, a pressure gradient is formed between the Great Basin in the east of the United States and the cool Pacific Ocean in the west, causing strong and dry winds to blow from one side of the inland mountains, through narrow valleys and towards the coast. Li Fang said that the Santa Ana wind, known as the “Devil’s Wind”, is a common monsoon in southern California during autumn and winter. It is dry and strong. This northeasterly wind with the characteristic of descending and warming can reach a speed of 50 to 70 miles per hour. It not only accelerates the rapid spread of fires but also poses a huge challenge to fire-fighting efforts.
Windy weather is the norm in this season, but the lack of rainfall has exacerbated the fire situation even more. Since October 2024, Southern California has received very little precipitation. Climatologist Daniel Swayne said that the region has experienced the driest start of winter on record. Since October 1st, the recorded precipitation at Los Angeles Airport has been only 0.03 inches (0.08 centimeters). Li Fang said that under the control of a powerful high-pressure system, the temperature in the California region is higher than normal, the precipitation is less, and the surface vegetation is extremely dry, further increasing the risk of fire. The superimposition of multiple extreme weather conditions has made the fire situation even more severe.
Apart from weather factors, the terrain was also contributing to this fire. Yang Xiaodan introduced that the terrain of California is higher in the east and lower in the west, with a significant difference in altitude. This makes it very easy for hot and dry winds to form (such as the Great Devil winds affecting Northern California and the Santa Ana winds affecting Southern California). These strong winds not only transport oxygen for fires but also cause the rapid spread of wildfires, which is an important cause of the frequent occurrence of wildfires in recent years. Li Fang said that the California region is surrounded by the SAN Gabriel Mountains and the Santa Monica Mountains. The east-west valleys coincide with the Santa Ana wind path, forming a wind acceleration channel. Meanwhile, steep slopes (with slopes mostly exceeding 30 degrees) produce a “chimney effect”, causing hot air to rise rapidly and accelerate oxygen replenishment, which leads to the rapid upward spread of the fire. This complex terrain also severely restricts the entry of fire vehicles, increasing the difficulty of fire fighting.
Climate change intensifies the risk of wildfires
Looking at the overall climate background, is the influence of El Nino boosting the wildfires in California? Li Fang said that the strong El Nino event might intensify the fire risk in the California region. This is mainly because El Nino usually brings about a “wet to dry” weather pattern: the abundant precipitation in the early stage will promote the dense growth of vegetation and form a large amount of potential combustibles; Subsequently, the weather turned abnormally dry with little rain, and the plants dried up rapidly to become flammable fuels. This combined effect of “high yield – drought”, coupled with the strong Santa Ana winds, makes large-scale wildfires more likely to occur. The current wildfires in California are a typical manifestation of the impact of the strong El Nino event in 2023-2024.
So, in what ways is climate change affecting the occurrence and development of wildfires?
Li Fang introduced that climate change intensifies the wildfire risk in North America through multiple channels: climate warming leads to frequent high temperatures and droughts, increasing the flammability of fuels; Climate change has led to a more extreme distribution of water, causing vegetation to grow rapidly during the wet season but then dry up due to water shortage and accumulate a large amount of flammable materials. As the temperature rises, atmospheric convection intensifies, and the frequency of thunderstorm weather increases. Especially “dry thunderstorms”, the lightning they produce is prone to cause forest fires. Against this backdrop, the frequency of wildfires during the North American wildfire season has continued to rise, with the fire season starting earlier and lasting longer, even frequently occurring in traditional non-wildfire seasons. Meanwhile, the intensity and destructive power of wildfires have significantly increased, and extreme fire incidents have become more common. The large amount of smoke pollution produced by wildfires spreads widely, posing a serious threat to air quality, ecosystems and human health.