Summer lightning strikes across the Arctic could more than double by the end of the century, potentially altering the vegetation and soil in ways that further exacerbate the impacts of climate change.

Researchers used satellite data and climate models to predict how the conditions that spawn lightning might change over the coming decades and found that the resulting increase in lightning could unleash stores of carbon currently locked in the permafrost. The team reported the findings April 5 in Nature Climate Change.

"We expect that lightning will increase much more with global warming and contribute to the ecosystems [and] the climate in the Arctic region as well as globally because it may release huge [amounts] of carbon dioxide from the permafrost," said Yang Chen, an assistant researcher in the department of Earth system science at the University of California, Irvine, and first author of the study. "We have to consider it as a major driving force for global warming in the future."

Vast reservoirs of carbon are currently stored in permafrost soils, comparable to the total amount of carbon in the atmosphere, Chen and his colleagues wrote in the study. However, the Arctic is warming even faster than the rest of the planet. As the permafrost melts, microorganisms can access and decompose the thawing plant and animal remains, releasing carbon dioxide and methane into the atmosphere.

Chen and his colleagues suspect that lightning strikes will contribute to another vicious cycle in the tundra and boreal forests.

Every year, nearly 1.4 billion flashes of lightning are detected around the globe. Regions near the equator generally see more lightning than those at higher latitudes. Lightning is relatively rare in the tundra and northern boreal forests, where the cool air and stable atmosphere are less conducive to thunderstorms.

However, scientists are concerned that climate change is increasing lightning in the frigid north. One recent study found that lightning strikes tripled in the past decade within the Arctic Circle. Lighting strikes can ignite wildfires that burn carbon stored in the soil and vegetation, unleashing greenhouse gases such as carbon dioxide.  

To find out how lightning activity will change over time, Chen and his colleagues first consulted satellite observations spanning May to August in the years 1996-99, taken by NASA's Optical Transient Detector, an instrument designed to measure lightning. The team then compared the lightning flash rates with meteorological data from the same period and saw that summer lightning in this area was strongly tied to two climate variables.  

One, known as convective available potential energy, indicates atmospheric instability. When the atmosphere is less stable, there's more convection — in which warm air rises above cool air — and thunderstorms are more likely to form. 

Also important was the amount of water vapor in the air, which condenses to form ice particles that collide. The friction from these collisions electrifies a cloud, until the electricity is discharged as lightning.

The researchers then analyzed 15 climate models that outlined a scenario in which greenhouse gas emissions continue unabated. They estimated that convective available potential energy would increase by roughly 86% and moisture by 17% over permafrost areas by 2081 to 2100. As a result, summertime lightning was projected to increase by roughly 112% by the end of the century.  

The team also predicted that areas near the northern treeline would see about 0.21 lightning flashes per square kilometer per month in the summertime, which is similar to present-day lightning activity 480 kilometers to the south in boreal forests.

The findings suggest that, under this business-as-usual climate scenario, for every 1 degree Celsius of global warming, the summer lightning flash rate will increase by around 40% in the Arctic tundra, 23% in boreal forests and 25% for the whole circumpolar region. 

"Our estimates for relative increases in lightning across Arctic tundra are about three times as large as the previous estimates for the contiguous United States, highlighting the large changes in atmospheric stability expected at high northern latitudes," Chen and his colleagues wrote in the study.

Lightning is a major source of burning in boreal and Arctic regions, "So when you increase the lightning, the fires are very likely to increase," Chen said.

By burning the surface soils and redistributing nutrients, wildfires alter the habitat in ways that make it more appealing for shrubs and trees than for grasses that are common on the tundra today. 

"The trees will be moving northward to occupy those previously unoccupied regions, so you have basically more fire fuel, which can also additionally promote the burning of wildfires," Chen said. 

And during early spring, he added, "Because of the forest, the snow cover will be much smaller, so albedo will be decreased, and the surface will absorb more energy from the sunlight, so it's going to lead to more surface warming."

Chen and his colleagues estimated that if the vegetation in the Arctic tundra remains largely unchanged, the increase in lightning activity will cause an annual 158% increase in burned area over the tundra by the end of the century. However, if lightning-induced wildfires cause the vegetation to resemble that south of the treeline, the tundra could see a 570% increase in burned area.

By extracting carbon dioxide from the air for photosynthesis, the increased vegetation may somewhat offset the impacts of more frequent lightning strikes. 

"But how much, exactly, this mechanism will contribute to the carbon cycle, I think, is still in an active research area," Chen said.

The team is now investigating whether climate change will lead to a larger proportion of lightning strikes igniting wildfires. 

"We are trying to relate the occurrence of lightning and the fires more directly," Chen said. 

The study, "Future increases in Arctic lightning and fire risk for permafrost carbon," published April 5 in Nature Climate Change, was authored by Yang Chen and James T. Randerson, University of California, Irvine; Jacob T. Seeley, University of California, Berkeley, and Harvard University; David M. Romps, University of California, Berkeley, and Lawrence Berkeley National Laboratory; William J. Riley and Zelalem A. Mekonnen, Lawrence Berkeley National Laboratory; and Sander Veraverbeke , Vrije Universiteit Amsterdam.