Wildfires can swell destructive storms half a continent away | National Geographic

Wildfires in the western United States are becoming more widespread, intense and destructive due to drought and unprecedented heat caused by climate change.

However, its influence is not limited to the West. New research shows that fires can affect the weather far beyond its borders. Heat, ash, gases and other small particles emitted into the air affect weather patterns on a large scale. Ultimately, they can increase rainfall and hailstorms hundreds of miles downwind by 38 percent.

It starkly illustrates how closely interconnected the different regions affected by climate change are. said Jewen Fan, a scientist at the Pacific Northwest National Laboratory and one of the study’s lead authors. That was posted on Monday Proceedings of the National Academy of Sciences. “It’s really exciting how much damage can be done with this increase,” she says.

Fires change weather near and far

Forest fires have increased in intensity in recent years. In response, scientists have begun mapping the ways mega-events affect weather, but so far only on a massive or relatively small scale.

Plumes of smoke reaching the high atmosphere can affect temperatures in one hemisphere or even globally. For example, the massive bushfires of 2019 and 2020 in Australia spewed so much smoke into the stratosphere that it looked like a volcanic eruption. Sunlight was blocked out and the southern hemisphere eventually cooled in the years that followed.

Other scientists have investigated how wildfires cause dramatic and sometimes impressive weather phenomena in the environment. With truckloads of tools, research teams headed to the whirlpools of intense fires to understand why they formed. They also flew planes through pyrocumulonimbus clouds high above. Burning embers and lightning bolts are often fired from this.

However, during California’s devastating 2018 wildfire season, the worst on record at the time, Jiwen Fan began to wonder. Could increasingly frequent and severe fires in the western United States affect not only the weather nearby, but 1,500 miles downwind?

Major weather patterns in the United States tend to move from west to east as the winds blow. In mid-July, early in the expected wildfire season, the Carr Fire broke out in California. Fan noticed that a few days after that, a huge storm that lasted days swept through the High Plains. States like Wyoming and Colorado experienced torrential rain, hail the size of a baseball, and winds of up to 90 miles per hour. The storm caused over $100 million in damages. Could the two have been related?

Her team had the right tools to explore this question. First, they examine a decade of meteorological and wildfire data, looking for other major fires that occurred just before major storms. This combination was actually very rare. This is because storm season in the central United States is at its peak at the beginning of summer. In the past, this season ended by the time the wildfire season began in August and September. But forest fires started earlier and earlier under the influence of drought and heat as a result of climate change. As of 2010, the team has found large storms in the central United States coinciding with large fires in the West.

They focused on a storm from 2018. Using a weather model to which heat and smoke effects from the fires were added, they simulated a storm lasting several days. The real situation with huge fires in the West, a situation where those fires did not exist and a series of experiments where the impact of some small local fires was and was not included.

The differences were dramatic. The effect of the far west fires and the local fires combined increased the rate of heavy rainfall (more than twenty millimeters of rain in an hour) by 38 percent. In these hot conditions, hailstorms with hailstones up to two inches in size (about the size of a baseball) were 34% more common. However, distant fires had a much greater impact.

“The effect is very significant,” says Fan. “It was surprising.”

How do fires affect storms?

By examining model results, they were able to identify two main reasons behind the fires’ dramatic downwind effect.

They saw a very high pressure area around the western wildfires. This was probably due to the massive emission of heat, gases and particles. Air flows from high pressure to low pressure. Additional westward high pressure boosted winds that flowed east in the storm area. While it was dry near the fires, downwind the humidity was normal or slightly higher. Very strong winds carried that moisture directly into the storm regions.

Excessively moist air and strong winds are major components of severe thunderstorms. As the moist air rises, the water changes from a gas to droplets. This process releases excess heat into the surrounding air. Because warm air rises, this extra heat makes the air rise faster and more explosively. This releases more heat and thus rain. Strong winds often increase strong, meandering currents in the atmosphere, making storms more violent.

The fire had a second effect. The thick blanket of ash and other particles drifted downwind toward the area of ​​the storm. Water condenses more easily if it can cling to something. The higher the number of particles, the higher the chances of bonding. If these particles are sucked into a strong updraft that moves into the increasingly cold air above, the chance of hail increases. Previous research has shown that smoke particles often cause larger hailstones in the central United States.

“It’s not as if the event created smoke it causes, but it strengthens it. So said Pablo Side, an atmospheric scientist at the University of California who was not involved in the study. It’s a good example of how climate change affects the weather, he says. Not by adding more events, but by changing the character of the events, usually making them more intense.

He says the research also shows how climate change connects remote communities. Events like wildfires in the West are now affecting more people.

There is still a lot to unpack

Forest fires are expected to increase proportionally due to climate change. The California Fire Department has already warned that there is no longer a “season” for wildfires. Fires are possible all year round and are becoming more common. Thunderstorm seasons are also lengthening. As overlap increases, so does the chance of more violent storms as a result of fires.

Exactly how these processes work and how significant the consequences can be, remains to be investigated. Mike Fromm, a meteorologist at the Naval Research Laboratory, wants to do a more comprehensive study of both the dynamics of continental scale and the details of rain and hail production. “There are still many questions about the relationship between environmental pollution and sparkling thunderstorms,” ​​he warns.

However, it is becoming increasingly clear that what happens during 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 steers away from the original fires. For her, this research confirms that fires in the western United States not only have a local impact, but a very significant downwind effect on the continent.

This article was originally published in English on nationalgeographic.nl