WEAVING through the sweaty tangles of a Panamanian forest, Steve Yanoviak is hunting a killer. Its prey isn’t the monkeys, bats or multicoloured birds that cram the branches, but the foundations of the forest itself – its trees. Each day, this killer strikes thousands of times around the world, but leaves no evidence behind. “Tropical trees die standing. They bear no scars,” says Yanoviak.
Catching it in the act takes monumental effort. That’s because the likely culprit isn’t a living organism, but instead a familiar force of nature: lightning.
Advertisement
Yanoviak, an ecologist at the University of Louisville, Kentucky, is just one of many researchers around the globe confronting the profound, underappreciated effects that lightning has on the natural world. It ignites wildfires that reset ecosystems. It can boost greenhouse gases, and unleashes other pollutants in an instant. And in the tropics, it is the grim reaper that singles out the most magnificent of ancient forest trees for destruction.
What’s more, lightning is probably on the increase, and that’s because of us. Climate change seems to be driving up the frequency of strikes, while population growth and changes in land use are exacerbating their effects. The toll on both the human and natural spheres has sparked a new urgency in getting to grips with this everyday phenomenon.
It all begins harmlessly enough, with moisture-laden hot air that rises from the warm surface of Earth. As it cools, water vapour condenses around microscopic particles – things like dust, pollen, sea salt and smoke – to form droplets and clouds. As they rise and cool further, these droplets turn to ice. Lighter ice particles rise to the top of the cloud and tend to lose electrons, becoming positively charged, while heavier falling particles tend to gain electrons and become negatively charged. The electric potential between the two mounts until giant sparks form within the cloud to restore balance.
When sparks really fly, however, is when the negatively charged base of the cloud starts inducing a positive charge on nearby bits of ground, concentrating it in protruding objects such as trees, spires or people. When those two oppositely charged areas are close enough, a lightning bolt can make contact with the ground.
For most of us, the destructive potential of a lightning strike is best marvelled at through a window. Though the data is patchy, strikes are estimated to kill up to 24,000 people a year globally and injure 10 times that number. The effects are disproportionately concentrated in lower income countries, where often “there is absolutely nowhere for anybody to go”, says Ron Holle, co-author of the book Reducing Lightning Injuries Worldwide. In Bangladesh, for example, lightning killed 64 people within just four days in 2016. In a recent study, Holle highlighted how lightning strikes in the country peak in April and May, exactly the time of year when farmers have to harvest their rice.
But the effects of lightning on human health and well-being can be more insidious. In a few microseconds, it can heat a sliver of air to 30,000 °C, leading to rapid chemical reactions that shatter the normally sturdy bonds within oxygen and nitrogen molecules to create nitrogen oxides, which are pollutants. While lightning-caused wildfires have always been an intrinsic part of the forest cycle, making space for new growth and triggering the germination of some seeds, they are an increasingly unwelcome phenomenon in drought-beleaguered zones that are becoming more vulnerable to fire. When tinder-dry Australia experienced record wildfires in early 2020, for example, lightning was a principal trigger.
Unlucky strikes
One of lightning’s most profound impacts on natural ecosystems, however, might have been overlooked had ecologist Lucy Rowland at the University of Exeter, UK, not had a spooky encounter in the Amazon rainforest. She had poked needles into a few trees to test the flow of sap up and down their trunks, before retiring to the safety of her camp. Then, lightning struck. When Rowland returned, her instruments were smouldering and ruined, but the tree showed no signs of damage itself. A few months later, however, it and its neighbours were dead.
This is odd. When lightning strikes a tree in temperate climes, the damage is generally obvious. Electrical current is thought to run down the moist layer inside the bark, which expands explosively under the high temperatures, sending chunks of bark flying. No one knows why tropical trees escape this fate, though it is possible that they somehow distribute the current differently.
This silent destruction got Rowland’s colleague Tim Hill, also at the University of Exeter, thinking. With a back-of-the-envelope calculation using satellite lightning data and tree censuses, he concluded that, if every tree had an equal chance of being struck, lightning would be insignificant for the fate of the forest because most trees are small and unimportant. But if the biggest trees were to be struck more often, says Hill, that would change everything. Larger, older trees are keystone structures of the forest: they are biodiversity hotspots, bug hotels and generally play a major part in its survival and well-being. They also disproportionately contribute to the storage of water and carbon dioxide. “The top few per cent can store 50 to 60 per cent of the carbon,” says Hill.
Figuring out whether they are preferentially struck by lightning could therefore be of huge consequence. “It’s a potentially important effect that no one really thinks about,” says Hill. That is hardly surprising. Determining lightning’s impact on tropical trees turns out to be incredibly difficult when the killer strikes randomly and unpredictably, when there are hundreds of thousands of potential victims and when there is no sign of the kill for weeks.
Hill is now equipping 20,000 tropical forest trees in Nigeria and Ghana with coils of wire. When lightning strikes, its current induces a magnetic field in the coil, which is instantly logged to identify exactly which tree was struck.
Yanoviak is taking a different approach. Since 1996, when lightning struck a tree terrifyingly close to him during a storm over a forest on Barro Colorado island in Panama, he has been working to catch it in the act. Working together with his colleague Evan Gora, also at the University of Louisville, he has now installed a system of surveillance cameras above the Panamanian forest canopy, as well as meters on the ground below that can detect an electrical surge. Algorithms triangulate this data so Gora knows where to search, with the help of a drone, for signs of tree death by lightning.
They are still collecting data, but so far their results indicate that, to their astonishment, half of the deaths of large tropical trees are down to lightning – not least because it seems a single strike can bounce off one tree to affect an average of five others. Assuming that other tropical forests respond in a similar way, lightning might kill about 190 million tropical trees each year globally, and damage half a billion others.
“If you had asked me, of all the large trees that die in the forest, what fraction are dying from lightning, I might have said maybe 5 per cent,” says Yanoviak. “We’re now beginning to understand that lightning is far more important than anyone expected.”
Our basic ignorance about lightning runs deep. We don’t even know how much of it is happening globally, because it is mostly monitored by local ground-based systems. “Tying them together in some kind of a patchwork is very difficult,” says Jochen Grandell at the European Organisation for the Exploitation of Meteorological Satellites.
Our best estimate is that there are about 1.4 billion flashes worldwide each year. But as the planet heats up and the atmospheric convection necessary to create lightning increases, there are good indications that these numbers are on the rise.
“Our best estimate is that 1.4 billion lightning flashes occur each year”
In 2014, David Romps at the University of California, Berkeley caused a stir with as the planet warmed. It was tricky work: large-scale global climate models operate at such low resolution that they can’t portray the behaviour of small and fickle phenomena such as clouds and thunderstorms.
The assumptions he had to make led to a model that even Romps calls naive, but it correlates with the lightning incidence in modern records. It predicts that, for every 1°C rise in global temperature, there will be a 12 per cent hike in lightning incidence over the US. Projecting forward to the end of the century, that means business-as-usual greenhouse gas emissions would lead to a 50 per cent increase in lightning strikes in the country.
These numbers may sound dramatic, but they tally with the data we have – and this could indicate potentially severe consequences, not least for forest ecosystems. Sander Veraverbeke at Vrije University in Amsterdam, the Netherlands, has been studying the incidence of lightning across the spruce and pine forests of northern Canada and Alaska, and has found that by 2 to 4 per cent a year for the past 40 years.
In unpublished work, Veraverbeke has collaborated with Romps and others and found a similar trend for the entire ring of boreal forests – in Canada, Alaska, Scandinavia and Siberia – as well as for the Arctic. “We expect quite significant increases in lightning – not only more, but it moves further north,” he says.
In these forests, as in warmer climes, lightning strikes and attendant wildfires clear patches to allow new growth. They are essential for the reproduction of some species, such as black spruce. But the more wood that burns, the more carbon is released – and more clear patches means more sunshine can penetrate the permafrost. That is troubling because of the huge amount of trapped carbon that could be released into the atmosphere as the permafrost melts. It could even be a damaging feedback loop, says Veraverbeke. “Because of warming, you have more lightning. Because of lightning, you have more fire. Because of fire, you have more emissions. Because of emissions, you have higher temperatures.”
Not everyone agrees that climate change is driving up the incidence of lightning. Last year, Declan Finney at the University of Leeds, UK, argued that Romps’s model failed to consider how much evaporated water actually turns to ice in clouds – an essential ingredient for lightning. , which took this effect into account, predicted a fall in lightning incidence. Late last year, Romps published a second paper that played with his and Finney’s approaches. It concluded that they both pointed to an increase in lightning in the US, while disagreeing with each other on the incidence over tropical oceans, and were both inconclusive for anywhere else.
While the likes of Romps and Finney debate the models, veteran lightning scientist Colin Price is trawling what past data he can find for hard evidence of change. Price got hooked on lightning as a child, watching summer afternoon thunderstorms race over Johannesburg, South Africa, where he grew up. Now at Tel Aviv University in Israel, he has been studying “thunder day” data. This has been recorded for centuries at weather stations all over the world by observers who simply made a daily note of whether they heard or saw thunder and lightning that day. The data is crude and buried in basements and drawers around the globe, “but basically, it’s the best we have going back in time related to thunderstorms”, says Price.
Stormy future
These records do show increases in thunderstorms over the past century in places ranging from south-east Brazil to the UK and from Japan to Alaska. Since 1970, “we see a dramatic rise”, says Price, adding that the trend goes on for too long to be the result of natural climate cycles, such as El Niño. He then between a warming climate across the African continent and the number and size of thunderstorms – a thought-provoking 40 per cent increase for every 1°C rise. “It’s difficult to say that that’s causing the increase, but they are fairly well-correlated,” says Price.
If the correlation holds, the consequences could be severe for tropical forests, with the death rate of the biggest trees increasing by a fifth by the end of the century. That is likely to have repercussions. Most obviously, the death and decay of these forest giants will release carbon that has been locked away in them over decades into the atmosphere, changing our climate models in ways that could take years to comprehend.
In more recently planted forests, an increase in lightning needs to be factored in to decisions about where, how and what to plant, says Guy Midgley at Stellenbosch University in South Africa. He is still upset about fatal fires that broke out in pine and eucalyptus plantations after lightning strikes in the country’s Western Cape in 2017. These non-native plantations and escaped trees growing in surrounding grasslands have increased the fuel load. The combination of increasing drought, fire-prone species and more frequent lightning raises increasingly important questions about the sense of such tree-planting schemes, he says. He isn’t alone. Researchers recently concluded that such climate-related changes were risky enough for plantations in California that the land would absorb more carbon if it was left as grassland.
If lightning is really set to strike more, one key to staving off the worst effects on the natural world, as well as protecting human life and property, will be an improved ability to spot lightning coming. Big storms can be forecast days ahead, but smaller ones can whip up, unnoticed, in a few hours anywhere when the conditions are right.
Fortunately, the tech is now arriving. In the US, the “nowcasting” of imminent lightning storms on the east and west coasts was transformed by the launch of the Geostationary Lightning Mapper satellite in 2016. Europe aims to launch an even more ambitious version, the Lightning Imager, in 2022. Grandell predicts that in a decade, such information will trickle through to all. People fishing on Lake Victoria in East Africa, for example, will be warned by SMS message that lightning is sweeping in and that they should return to the shore.
Meanwhile, the work to establish the consequences for the ecosystems we rely on, in Panama and elsewhere, continues. The true complexity and power of an awesome natural phenomenon is only now being revealed.
