Global warming could have an electrifying effect. Climate calculations suggest that U.S. lightning frequency will increase about 12 percent for every 1 degree Celsius in warming, researchers report in the Nov. 14 Science.
Leading climate simulations forecast a roughly 4 degree rise in global average temperature over the next 100 years, notes lead author David Romps, an atmospheric scientist at the University of California, Berkeley. “So for every two strikes you get today, you’ll get three strikes at the end of the century.”
This lightning surge will have negative repercussions, Romps warns. Lightning strikes ignite most natural wildfires, spark ozone-producing reactions in the lower atmosphere and kill on average around 50 Americans each year and injure hundreds more. “Climate change is going to alter weather in ways that will become quiet noticeable,” he says.
Currently about 20 million lightning bolts touch down each year within the continental United States. Lightning storms form when rising water vapor cools in the atmosphere creating a cloud. As the water condenses, the cloud heats up and climbs like a hot-air balloon. At high enough altitudes, small ice crystals form and collide, transferring electric charge like rubber shoes shuffled on carpet. Over time negative charges collect on the bottom of the cloud. If the difference in electric charge between the ground and the thundercloud becomes large enough, electric current will arc between them as lightning.
Because hotter air temperatures produce more water vapor, climate scientists agree that lightning activity will probably increase as the planet warms. Predictions of the extent of this increase have ranged from 5 percent to well over 100 percent per each degree in warming. This large discrepancy arose, Romps says, because previous estimates used indirect techniques that didn’t consider the physical conditions that influence thunderstorms.
In their new approach, Romps and colleagues considered two key factors related to thunderstorm creation: precipitation rate and the amount of energy available to make air rise. The team worked out a simple formula that relates the product of these two climate conditions to the lightning flash rate. Comparing their calculations with U.S. climate data collected in 2011, the researchers’ formula successfully predicted 77 percent of the variability in the U.S. lightning strike rate over the course of the year.
The researchers applied their formula to 11 global climate simulations. On average the simulations churned out about 12 percent more lightning strikes in the United States per each degree rise in global temperatures.
The additional lightning strikes could make climate change worse, says atmospheric physicist Colin Price of Tel Aviv University. Wildfires bellow carbon dioxide into the atmosphere and the superheated air left behind by lightning strikes can produce nitrogen oxides, which react to form low-altitude ozone that can cause local warming. Even renewable energies are at risk, Price says, because lightning bolts can strike and disable wind turbines.
The new study should help scientists better prepare for increased lightning activity, Price says. But he points out that the results are currently limited to only the United States as a whole. “With faster computers it should be possible to apply it at smaller scales,” he says, “but their calculations might not work for the tropics or over the oceans where thunderstorms behave differently.”