"Lightning kills: it is responsible for dozens of fatalities in the US every year," David Romps of the University of California, Berkeley, US, told environmentalresearchweb. "In addition, half of the wildfires in the US are triggered by lightning. Those wildfires are often the most difficult to fight since they can be triggered far from the nearest fire station. The fear is that a higher rate of lightning strikes would lead to a higher incidence of wildfires."

Lightning is also the dominant source of nitrogen oxides (NOx) in the middle and upper troposphere, Romps said. "Through its control on NOx, lightning indirectly regulates ozone, which is a greenhouse gas and an oxidant, and the hydroxyl radical OH, which plays an important role in setting the lifetime of methane, a powerful greenhouse gas."

Previous studies had found that the lightning rate at individual weather stations tended to increase with precipitation, which can act as a measure of how much convection there is. "This is also common sense," said Romps. Other work had suggested that days with higher convective available potential energy (CAPE) have higher lightning rates. Bigger updraft speeds and larger water contents are generally considered to give higher flash rates.

"We put these two ideas together and tested the hypothesis that the product of precipitation and CAPE – that is, the amount of convection times the vigour of convection – would be a good predictor for lightning," said Romps. "I typically approach hypotheses with a healthy dose of scepticism, and that includes my own. What surprised me, then, was just how well this product of precipitation and CAPE worked."

The team's product explained 77% of the variance in the time series of total cloud-to-ground lightning flashes across the contiguous US. Precipitation, in contrast, explained only 29% of the variation, while the maximum height of convection to the fifth power explained 39%, and CAPE explained 52%.

To look at the effects of climate change, Romps and colleagues from the University of California, Berkeley, the Lawrence Berkeley National Laboratory and the State University of New York at Albany used 11 models from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Analysing values of precipitation and CAPE for 1996–2005 and 2079–2088 showed that each degree of temperature increase is likely to raise the annual mean frequency of lightning strikes by 12%. Previous estimates for the increase in lightning per degree of warming had ranged from 5% to more than 100%. And the models indicated that by the end of the century the rate of lightning strikes across the contiguous states of the US is likely to increase by half.

"We now believe we have a robust method for estimating the future of lightning," said Romps. "We used that method here in a simple way to estimate the future of lightning over the US, but what is perhaps even more exciting is the possibility of using [it] to look at future changes in the spatial distribution of lightning, impacts on wildfire frequency, and changes to atmospheric chemistry."

According to Romps, the next step is to figure out where these changes in lightning will be. "Does the increase in lightning strikes occur where there are already many lightning strikes today?" he said. "Or are regions of the country that get very little lightning in store for much more electrical activity in the future?"

The team reported the results in Science.

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