Wildfires in the western United States are getting bigger, more intense and more destructive because of the drought and unprecedented heat caused by climate change.
However, their impact is not limited to the West. New research shows that the fires can affect the weather far beyond their borders. Heat, ash, gases and other small particles released into the air affect large-scale weather patterns. Ultimately, they can intensify rain and hail several hundreds of kilometers with the wind at 38 percent.
It clearly shows how closely the different areas affected by climate change are interconnected. So says Jiwen Fan, a scientist at the Pacific Northwest National Laboratory and one of the lead authors of the study. It was published on Monday Proceedings of the National Academy of Sciences. “It’s really dramatic how much more damage can be caused with that increase,” she says.
Fires change the weather near and far
Forest fires have increased in intensity in recent years. In response, scientists began to map the ways in which the mass events affect the weather, but so far only on a massive or relatively small scale.
Plumes of smoke reaching the upper atmosphere can affect the temperature hemispherically or even globally. For example, the huge wildfires of 2019 and 2020 in Australia spewed so much smoke into the stratosphere that it looked like a volcanic eruption. Sunlight was blocked and the Southern Hemisphere eventually cooled in the following years.
Other scientists studied how fires cause dramatic, sometimes blinding, weather phenomena in the area. With truckloads of instruments, research teams drove up to fire pits rising from intense flames to understand why they were created. They also flew airplanes through high-altitude pyrocumulonimbus clouds. Burning embers and lightning often shoot from it.
But during California’s devastating 2018 wildfire season, which was the worst on record at the time, Jiwen Fan began to wonder. Could the increasingly frequent and intense fires in the western United States affect not only the weather nearby, but even up to 1,500 miles downwind?
Major US weather patterns tend to move from west to east with prevailing winds. In mid-July, shockingly early in the expected wildfire season, the Carr fire broke out in California. Fan noted that a few days later, a massive, day-long storm swept through the High Plains region. States such as Wyoming and Colorado experienced flooding from heavy rainfall, baseball-sized hail and wind gusts of 140 kilometers per hour. The storm caused more than $100 million in damage. Was it possible that the two were related?
Her team had just the right tools to explore that question. First, they sifted through ten years of meteorological and wildfire data, looking for other large fires that occurred just before major storms. That combination was actually quite rare. That’s because the storm season in the central United States is at its peak around the beginning of summer. Previously, that season ended when the forest fire season in August and September began. But forest fires are starting earlier and earlier under the influence of drought and heat as a result of climate change. Beginning in 2010, the team found large storms in the central United States that coincided with large fires in the West.
They focused on a storm from 2018. Using a weather model to which the effects of heat and smoke from the fires were added, they simulated a day-long storm in different ways. The real situation of huge fires in the west, a situation where these fires did not exist and a series of experiments that included and excluded the effect of some smaller, local fires.
The differences were dramatic. The impact of the distant fires in the west and the local fires together increased the occurrence of heavier rainfall (more than 20 millimeters of rain in one hour) by 38 percent. Under these warm conditions, hailstorms with hail as large as two inches (almost the size of a baseball) were 34 percent more frequent. However, the distant fires had a much greater effect.
“The impact is very big,” says Fan. “It was surprising.”
How do fires affect storms?
By examining the results of the models, they were able to identify two main reasons why the downwind fires had such dramatic consequences.
They saw a very high pressure area around the western wildfires. This was probably the result of the enormous release of heat, gases and particles. The air flows from high to low pressure. The additional high pressure in the west enhanced the winds that flowed eastward into the storm area. While it was bone dry near the fires, humidity upwind was normal or even slightly higher. The extra strong winds carried that moisture directly to the storm areas.
Because wet air and strong winds are key ingredients for severe thunderstorms. As wet air rises, the water changes from gas to droplets. This process releases extra heat into the surrounding air. Because warm air rises, the extra heat causes the air to rise faster and more explosively. This releases even more heat and therefore rain. Strong winds often magnify the strong, meandering currents in the atmosphere, making the storms more intense.
The fire had another effect. The thick blanket of ash and other particles drifted downwind to the storm area. Water condenses more easily if it can cling to something. The more particles, the more opportunities to attach. If these particles are sucked into a strong updraft that moves into increasingly colder air above, the chance of hail increases. Previous research has shown that smoke particles often caused larger hail in the central United States.
“It’s not like the event was caused by smoke caused, but it is strengthened by it.’ So says Pablo Saide, an atmospheric scientist at UCLA who was not involved in the study. It is a good example of how climate change affects the weather, he says. Not by adding more events, but by changing the nature of the events, which usually makes them more intense.
He says the research also shows how climate change connects remote communities. Events such as wildfires in the West now affect a much larger population.
Still a lot to figure out
Forest fires are expected to increase proportionally due to climate change. California fire officials have already warned that there is no longer a “season” for wildfires. Fires are possible all year round and are becoming more frequent. Storm seasons are also getting longer. With more overlap, the chance of more violent storms resulting from fires also increases.
Exactly how these processes work and how big the consequences can be is still unclear. Mike Fromm, a meteorologist at the Naval Research Laboratory, wants a much more comprehensive study of both continental-scale dynamics and the details of rain and hail production. “There are still many questions about the relationship between environmental pollution and the crackling thunderstorms,” he warns.
However, it is becoming increasingly clear that what happens during the fires is not limited to the fire area. Rebecca Buchholz is an atmospheric scientist at the National Center for Atmospheric Research. She studies the health effects of wildfire smoke, which drifts far from the original fires. For her, this research emphasizes that fires in the western United States not only have a local, but an extra large downwind effect on the continent.
This article was originally published in English on nationalgeographic.nl