Updrafts of warm, moist air set up the conditions for lightning, so thunderstorms are hardly the stuff of polar regions. Or are they? Researchers recently mined a global lightning database and found that the fraction of lightning occurring over the Arctic has grown by more than 300% over the past 11 years. That’s a surprise, the team suggests, and a phenomenon potentially linked to warming temperatures. But other scientists don’t see the same trend in their data.
A team of researchers led by Robert Holzworth, an atmospheric and space physicist at the University of Washington, analyzed data from the World Wide Lightning Location Network (WWLLN). WWLLN detects roughly 700,000 lightning strokes every day on the basis of the radio waves they emit. “Lightning is like a very long antenna in the sky,” said Holzworth, the director of WWLLN. “It’s very efficient at radiating at low frequencies.”
Too Cold, Too Stable
Lightning hotbeds include places like Africa and Southeast Asia. Only a small fraction of the world’s lightning—less than 1%—occurs at high latitudes. That’s because the conditions there are usually too cold and too stable to promote the thermal updrafts necessary for thunderstorm formation, said Holzworth.
But in August 2019 a flurry of lightning strokes occurred within several hundred kilometers of the North Pole. That storm, and others, prompted Holzworth and his collaborators to analyze lightning in the Arctic, which the team defined as the region above latitude 65°N. (The team didn’t study the corresponding area around the South Pole because lightning above the Antarctic is exceedingly rare.)
Eleven Years of Summer
The researchers mined archival WWLLN data from 2010 to 2020. To account for the network’s roughly tenfold increase in detection efficiency since its inception in 2004, they focused on a dimensionless parameter: the number of strokes observed in the Arctic divided by the total number of strokes observed globally. Holzworth and his colleagues furthermore limited their analysis to strokes recorded in June, July, and August since lightning activity is highest in summer months.
Holzworth and his collaborators found that the fraction of lightning occurring in the Arctic increased from roughly 0.2% in 2010 to a little over 0.6% in 2020. That threefold increase is significant, the researchers suggest, and might be tied to warming temperatures in the Arctic.
Global temperatures have been climbing in the past few decades, and the Arctic is warming even faster than other parts of the planet. When Holzworth and his colleagues graphed their dimensionless parameter versus the global temperature anomaly, they found a linear correlation.
The Link Between Warming and Lightning
There’s a causal link between these two quantities, the researchers propose, but admit they don’t have proof. “We know from low-latitude lightning that it is more likely to see lightning in the warm summer months than in the winter,” said Holzworth. “So in the case of the Arctic, it seems a reasonable suggestion that there may be a similar causal link.” Future studies focused on modeling the formation of lightning in different temperature and humidity conditions will be important for demonstrating this link, the scientists suggest.
These results, originally presented at AGU’s Fall Meeting in December, were recently published in Geophysical Research Letters.
An uptick in lightning in the Arctic is potentially bad news, said Yoav Yair, an atmospheric physicist at the Interdisciplinary Center, a private research university near Tel Aviv, Israel, not involved in the research. That’s because lightning can start forest fires, which can, in turn, cause carbon dioxide–containing permafrost to melt. Lightning also produces nitrogen oxides and ozone, said Yair. “You have an additional source of those greenhouse gases.”
Not everyone sees this trend in their data, however. Researchers at Vaisala recently analyzed archival data from the Global Lightning Detection Network (GLD360), which they own and operate. Between 2012 and 2020, they didn’t find a substantial increase in the fraction of lightning occurring over the Arctic. However, they acknowledge that their data set spans a shorter time period than the WWLLN data that Holzworth and his colleagues analyzed.
“From the GLD360 data it is apparent that 9 years of lightning data alone is insufficient to draw long-term climatological conclusions,” said Ryan Said, an atmospheric physicist at Vaisala not involved in the research.
But stay tuned, said Said. “We’re excited to see how these trends develop over the years. We’re really in a golden age of lightning detection.”
—Katherine Kornei (@KatherineKornei), Science Writer