If We’re Altering Rain, Hail, Any Doubts Left on Climate?

By Andrew Freedman

One of the biggest hurdles to overcome when communicating climate science is the resistance many have to accepting the notion that human activities are capable of altering the earth’s climate system. After all, the planet is a pretty big place, and the climate was doing its thing long before humans arrived. To some, the abundant scientific evidence showing that manmade emissions of global warming gases, such as carbon dioxide, are likely the key driver behind recent global warming seems, well, kind of arrogant.

To these folks, I say check out a recent study that had nothing to do with global warming. By showing that human activities can have measurable impacts on small-scale weather phenomena – in this case, thunderstorms that spawn tornadoes and hail – the report highlighted that we’re already able to influence weather on a daily basis. 

And if we are capable of modifying thunderstorm behavior, it’s not a large leap to understand that we’re also altering the atmosphere on much broader scales.

The scientists behind the research – Thomas Bell of NASA’s Goddard Space Flight Center in Greenbelt, Md., and Daniel Rosenfeld of Hebrew University in Jerusalem, did not set out to prove a point about global warming. But the weekly storm cycles they identified and the physical mechanisms they proposed are responsible for these cycles contain important lessons for how people think about human interference with weather and climate.

According to the study, which was published last fall in the Journal of Geophysical Research, human activities – specifically aerosol pollution from cars, trucks, and power plants – can give thunderstorms a boost by altering the way that rain and hail forms, as well as the release of heat energy within a thunderstorm. This is mainly the case, they found, during the summer thunderstorm season in the eastern U.S.

The report noted storms that form in more polluted air masses tend to grow taller, form larger hailstones, and may be more ideally suited to spawning tornadoes than storms that form in cleaner air. Bell and Rosenfeld identified a pattern in severe thunderstorm phenomena such as hail and tornadoes, which shows that, in the eastern U.S., they are more common during the week than on the weekends. Previous research has also shown that a similar weekly cycle exists in lightning activity as well.

The researchers zeroed in on the pollution angle when they realized that it, too, has a weekly cycle, and it’s one that matches closely with the thunderstorm-related data they examined.

The hypothesis has a solid grounding in both the physical theories of how thunderstorms work, as well as observational data; although NASA’s Bell said there is a very small chance they stumbled on a random match between pollution cycles and weather variability.

“We know two things go up and down together and we can come up with a reasonable physical theory for why it might happen, but the nail in the coffin would be for someone to actually measure the amount of pollution at the base of a storm that was forming, and you’d have to do this over and over again to get some statistical averages,” Bell said.

The mechanisms behind the weekly cycle are rather complicated, but they’re worth a brief discussion, because they show how we’re changing weather at small geographic scales – raising the question of what we’re doing to larger-scale weather and climate patterns.

Thunderstorms that form in dirty, humid air tend to behave differently than ones formed in more pristine environments. Scientists have found that water droplets don’t just randomly condense from water vapor like you may have learned in your high school earth science class; instead they need a little nucleus of dirt, salt crystals, or sometimes even insects to grow around. Pollution from cars, trucks, and power plants add such particles to the air, and by providing more material for water vapor to condense around, the pollution causes dirtier clouds to form more numerous, but smaller, water droplets.

The smaller water droplets are less likely to fall quickly as rain. Instead, they are more easily vaulted high in the cloud, where temperatures are below freezing, where they may be reshaped into hailstones. Perhaps more importantly, the freezing process releases heat energy, which helps supercharge the storm, as if it just gulped downed an energy drink.

“The additional ice that’s formed aloft is actually releasing heat energy into the atmosphere, and that gives the rising bubble of air an extra boost,” Bell said.

The critical factor is how much pollution is available to a nascent thunderstorm, which means this is a localized phenomenon. Since pollution peaks midweek, it seems the storms do, too, according to the study.

As Bell said, if a weekly cycle in pollution, which pushes aerosol levels up or down by 10 to 20 percent, is having a detectable influence on the atmosphere, then the high level of background pollution must be altering weather in significant ways also.

“When you think about it, we’re changing the climate every week back and forth, it’s pretty incredible,” Bell said.

The study demonstrates that human activities can influence small-scale, high-impact severe weather events. In revealing the possible links between weekly fluctuations in severe weather events and manmade pollution, it offers a fresh way of looking at the much broader issue of global climate change.