Lightning may be full of sound and fury, but it also signifies important things on the earth below. Though lightning deaths are falling steadily (there were 23 in the United States in 2013), it continues to ignite some 10,000 wildfires that consume more than 4.1 million acres in the United States each year. Lightning is also, among other things, the leading cause of serious fires in wind turbines. And every flash converts about 7 kilograms of nitrogen into smog-producing nitrogen oxides, to the tune of about 8.6 billion kilograms of NOx per year.
Despite lightning’s importance, we still don’t have a reliable model of how its frequency may change with changing temperatures.
Researchers at the University of California at Berkeley (UCB) and the Lawrence Berkeley National Laboratory, with colleagues from the State University of New York at Albany, report a major step towards projecting how climate change will affect cloud-to-ground lightning production. They detailed their model last week in Science.
UCB’s David Romps and his co-workers stress that their model is a proxy. It describes the correlation between key atmospheric parameters and lightning rates, but it does not—yet—rest on a detailed mechanism. That said, it does broadly predict where and when lightning will strike, and it does a far better job than previous attempts, based on single parameters like precipitation rates, the fifth power of maximum cloud height, or “convective available potential energy” (CAPE, the amount of energy contained in a kilogram of rising air).
Lightning flash rates for the continental U.S. in 2011Image: UC Berkeley
The UCB team combined two of these earlier indicators, precipitation rate and CAPE, to give a value for the mass and energy flow through the ascending air—energy per kilogram per square meter per second. Using actual weather data for 2011, the researchers calculated CAPE-times-precipitation for multiple locations in the continental United States. They then plotted the product against the rate of lightning strikes for the same area (in number of strikes per square kilometer per hour).
The correlation was close, clustering around a linear regression line with an R2 (coefficient of determination) of 0.77. In other words, CAPE-times-precipitation accounts for 77 percent of the observed variation in the tempo of cloud-to-ground lightning. (Precipitation rates alone, by contrast, account for only 29 percent.)
Using a 1-gigajoule ballpark estimate of average lightning-flash energy, the collaborators further calculated that about 1 percent of the available atmospheric energy is regularly converted into lightning.
From these data, Romps and his colleaues project that lightning-flash rates will rise 12 percent (+/- 5 percent) for each 1°C rise in global mean surface temperature—or, given consensus climate predictions, about 50 percent (+/- about 25 percent) by 2100.
Animation of all lightning strikes in the U.S. in 2011. (Note: there is no time scale, so seasonal variations are difficult to distinguish.)Animation: UC Berkeley