I’m sure most readers are familiar with thunderstorms from personal experience. Some find the lightning and thunder exhilarating, others terrifying. Others still – usually in places that see them a lot – may rarely even raise an eyebrow.
Regardless, thunderstorms are dangerous. Risks can include cloud-to-ground lightning (and associated fires), flash-flooding (via intense rainfall), damaging wind gusts, or large hail. These can inflict large socioeconomic costs. An average of 144 deaths per year in the USA are attributed to lightning alone, while the annual economic cost from thunderstorms runs into the $ billions.
Left Photo by Unknown Author is licensed under CC BY-NC-ND | Right Photo by Unknown Author is licensed under CC BY-SA
In severe thunderstorms, large hail can cause direct physical damage to property and possessions.
On the other hand, there are large regions on the planet where thunderstorms are a valuable contribution to water budget. The southwestern US depends on them for 70% of its precipitation. Quiet years there often lead to droughts, which can become even more costly than thunderstorms.
Lightning has its own upside, too. Nitrogen dioxide is converted into various compounds which provide essential nutrition to plants. You can often smell this effect in the air after a nearby lightning strike. There’s also a detectable cleansing effect on the air, reducing pollutant content.
The positive-negative balance will vary between regions. For example, a net negative impact is likely in places which regularly see a lot of non-thunderstorm rainfall, such as the UK.
Whichever way you look at them, there’s plenty of reason to take an interest in how thunderstorm frequency is likely to behave with climate change. Here’s a detailed look at the what and the why.
More Moisture Capacity… but Freezing Higher Up
Generally, climate projections accompany rising temperatures with increasing intensity of rainfall. This is because of a fundamental physical connection:
The warmer the air is, the more moisture it can contain as vapour. That means more water available for rainfall.
Predicting how thunderstorm frequency will change is another matter. Reason being, it’s not intense rainfall that generates lightning – instead, colliding ice crystals or other particulates.
The requirement for ice complicates the picture. Air generally cools with height, falling below freezing at some altitude. That level varies with climate – it tends to be higher in warmer or moister conditions.
So, climate warming moves the freezing level to higher altitude, meaning clouds must be taller if they are to contain ice crystals.
Although surface temperatures will also increase, this will be in tandem with warming at higher altitudes too, which will limit changes in the buoyancy of air parcels. In other words, they won’t tend to rise much faster, on average, than they do today. Typical cloud height probably won’t change much.
This aspect of climate change suggests thunderstorm frequency will decrease.
That’s in stark contrast to an increase in typical rainfall intensity.
We’re not done with the question yet, though – there’s particulates to factor in.
Here, things get really complicated. Particulates such as dust specks or pollutants from cars can clash with each other or ice crystals, causing a buildup of charge that increases lightning likelihood.
…but some types also absorb or scatter incoming sunlight, which reduces surface heating. This means near-surface air doesn’t become as buoyant, resulting in shallower clouds that are less likely to reach the freezing level.
This photo shows an example of a ‘pyrocumulonimbus’ cloud: A towering thunderstorm that’s develops in response to hot air rising above a large wildfire. This demonstrates that the inherent dryness of the rising air doesn’t necessarily prevent sufficient condensation of moisture aloft.
This Photo by Unknown Author is licensed under CC BY-ND
A 2010 study into the effects of Amazonian forest fire smoke found a two-phased response. As soot particles are added, there’s initially an increase in lightning frequency. Yet if this addition is continued, a point is reached where reduced surface heating becomes dominant. So, lightning frequency reduces – in fact, so much so that it falls below what would be observed with zero soot input.
The effects are likely to be similar for artificially generated pollutants (e.g. combustion engine fumes). Studying this is complicated by a tendency for higher concentrations to be within more urbanised (‘built-up’) areas, where the urban heat island effect can bring higher surface temperatures.
Dust is a different story, as it’s typically associated with dry air, which is prohibitive to thunderstorms (and tropical cyclones). It’s not clear how atmospheric dust will change with the climate. This is mainly due to the sensitivity of dust concentration to vegetation changes and short-timescale weather events such as droughts and heatwaves. Not only that, but these three factors are all inter-connected as well!
So… we can’t yet draw confident conclusions on how particulates will affect thunderstorms in the decades ahead. This is a key area requiring further research to develop our collective understanding.
Based on studies of weather station data, some countries, such as the USA and Brazil, have seen an overall increase in thunderstorm days in the past 70 years or so. In Europe, the picture is mixed, with a 2019 study finding an increase over the Alps and in central, south-eastern, and eastern Europe but a decrease in the southwest, with no significant trend elsewhere.
That study also scrutinises direct station observations and note substantial limitations to their effectiveness. This is a widespread problem across the globe and one which other methods can only partially and uncertainly fill in for. Satellite observations of lightning are most reliable but have only been operational for a couple of decades – too short for effective trend analysis.
Too Complex for Conclusions…?
Lightning-wise, modelling studies suggest so.
Generally, those that include ice crystal behaviour predict an overall decrease in global-mean lightning activity. However, there’s a lot of uncertainty surrounding how cloud ice content will trend with climate change. Will more atmospheric moisture result in an increase in ice crystal density that compensates or even outweighs a reduction in thunderstorm frequency owing to higher freezing level?
Interestingly, models omitting cloud ice tend to predict an overall increase in lightning activity. This may just be due to insufficient resolution of the problem, however.
For thunderstorm frequency, the prognosis appears to be one of more, but on average less intense, thunderstorms. Globally averaged, flash-flooding risk increases, while lightning risk decreases.
This won’t be a uniform change, though – regional variability is inevitable.
For example, places such as within the Arctic Circle where thunderstorms have historically been extremely rare due to a very dry and cold climate, have seen an increasing frequency in the past decade. Warmth and moisture levels are starting to cross thresholds for thunderstorm initiation more readily, while the freezing level is still at relatively low altitude. In such dry climates otherwise, associated lightning brings with it an increased wildfire risk – as do the higher temperatures.
James Peacock MSc
Head Meteorologist at MetSwift
Cover Photo by Unknown Author is licensed under CC BY-NC-ND