Pollution in the form of aerosols heightens the risk of lightning strikes, say researchers in Israel, Mexico and the US.
The scientists came to the conclusion by correlating the amount of aerosols globally with the record of cloud-to-ground lightning strikes, while excluding weather as a possible influence. Some 70% of the areas studied exhibited more lightning strikes from polluted skies rather than clean skies.
“Significant increase in aerosol concentration is mostly anthropogenic,” said Ilan Koren of the Weizmann Institute of Science, Israel. “Therefore, these results show a global anthropogenic effect on the climate… If we want to predict future climate change correctly, [the aerosol effect on lightning] should be well represented in global climate models.”
Lightning strikes cost lives and damage infrastructure, particularly in the developing world. In South Africa, for instance, scientists have estimated deaths from lightning at 260 per year, while in India the estimated annual death toll is more than 3,000.
Aerosols have long been suspected to raise the likelihood of lightning strikes. When aerosols are present in high concentrations, clouds are believed to take on a deeper, invigorated structure, containing more and bigger ice particles and allowing stronger updraughts. Such a structure is also believed to promote electrical activity, as ice and soft hail “graupel” particles collide and rebound in the presence of supercooled water.
Studies giving evidence of possible links between high aerosol levels and more frequent lightning strikes are not in short supply. According to Koren, however, it has been hard to claim that the link between aerosol and lightning is causal, as it could always be possible that a common factor – weather being the obvious candidate – is influencing the other two. “The same meteorology that favours deeper clouds could induce convergence and bring more aerosols to the column,” he explained.
The study pitched the times and locations of strong lightning events, taken from the World Wide Lightning Location Network, against cloud and aerosol measurements from NASA’s orbiting MODIS instrument. In the first analysis, the researchers showed the link between areas of polluted clouds and areas of significant lightning activity. In the next step, for a few key regions, they sliced the data into strips of similar meteorological conditions. The trend remained, demonstrating that weather could not be a common driving force.
“It is a data-only study that suggests this global trend,” said Koren. “It is important to show how robust the trend is without using models that rely on many assumptions.”
In general, Koren added, the work broadens knowledge of how aerosols affect cloud formation, and could help scientists on their way to a unified aerosol theory that predicts when high aerosol concentrations will yield larger clouds, more rain and more lightning, and when they will have the opposite effect.
“We are now expanding our research to larger scales, to understand how clouds ‘talk’ with their surroundings and how they affect thermodynamics and radiation properties,” he said. “In parallel, we’re developing new approaches to measuring clouds. We think that in the near future more but smaller satellites could provide valuable data on the internal structure of clouds and cloud fields.”
Koren and colleagues published their findings in Environmental Research Letters (ERL).