It was the clever headline that caught my attention: “April flowers bring May showers?”
But it was the latest research about pollen from the University of Michigan and Texas A&M that got me digging a little deeper and eventually arriving at the subject of clouds and climate change.
The bottom line is a possibility that pollen from trees and flowers can break apart during a rainstorm. The broken pieces can then float up into the air and seed the clouds for the next rainstorm.
Allison Steiner, associate professor of atmospheric, oceanic and space sciences at U-M, began exploring how pollen might seed the clouds after sweeping a layer of pollen off her front porch one morning and wondering what happens after the pollen drifts into the air.
Atmospheric scientists have never paid much attention to pollen. It is generally believed that pollen grains are too large to seed the clouds. Instead, most attention has been focused on man-made aerosols, such as particles from a coal-fired power plant. High in the atmosphere, the particles can encourage moisture in the air to condense, the initial step in the formation of rain.
But people with allergies may recognize that their symptoms grow worse after a rainstorm when the air begins to dry out. As Steiner explains in an M-I news release:
“When we were looking in the allergy literature we discovered that it’s pretty well known that pollen can break up into these tiny pieces and trigger an allergic response. What we found is when pollen gets wet, it can rupture very easily in seconds or minutes and make lots of smaller particles that can act as cloud condensation nuclei, or collectors for water.”
In a laboratory at Texas A&M, Sarah Brooks, a professor in atmospheric sciences, soaked six different kinds of pollen in water, then sprayed the moist fragments into a cloud-making chamber. Brooks and her colleagues found that three fragment sizes — 50, 100 and 200 nanometers — quickly collected water vapor to form cloud droplets, which are 10 times bigger than the particles. (It takes about 6 million nanometers to equal a quarter of an inch, so we’re talking about very small particles.) Brooks noted in a Texas A&M news release:
“Scientists are just beginning to identify the types of biological aerosols which are important for cloud formation. Our results identify pollen as a major contributor to cloud formation. Specifically, our results suggest that increased pollen could lead to the formation of thicker clouds and longer cloud lifetimes.”
The effect of cloud formation on global warming may be the most important mystery in climate science today, according to Jasper Kirby, a particle physicist who is leading a team of atmospheric scientists from 15 European and U.S. institutions. Consequently, the effect of aerosols on cloud formation must be equally important.
Clouds are known to cool the planet by reflecting sunlight back out to space, but they can also contain heat at night, so cloud formation plays a critical role in determining the rate of global warming. To better predict global warming, one has to better understand when and how clouds are formed at a “very fundamental level,” Kirby told reporter Rae Ellen Bichell in “Yale Environment 360.” Kirby added:
“By fundamental, I mean we have to understand what the gases are, the vapors, that are responsible for forming these little particles. And secondly, we have to understand exactly how quickly they react with each other and how they form the aerosol particles which … constitute the seeds for cloud droplets. And this process is responsible for half the cloud droplets in the atmosphere. It’s a very, very important process, but it’s very poorly understood.”
In the upper atmosphere, aerosols can directly reflect sunlight back into space. These include man-made aerosols from industrial pollution as well as natural aerosols, such as volcanic eruptions and desert dust and now possibly pollen. Check out NASA’s webpage on “Atmospheric Aerosols.”
Steiner, who is doing the pollen experiments, said understanding natural aerosols is critical to understanding climate change:
“What happens in clouds is one of the big uncertainties in climate models right now. One of the things we’re trying to understand is how do natural aerosols influence cloud cover and precipitation under present day and future climate.
“It’s possible that when trees emit pollen, that makes clouds, which in turn makes rain and that feeds back into the trees and can influence the whole growth cycle of the plant.”
For people more interested in the allergy aspects of this story, I found a website called pollen.com, which identifies a variety of ways that weather can affect pollen and thus allergies:
- A mild winter can lead to early plant growth and an early allergy season,
- A late freeze can delay pollen production in trees, reducing the risk of an allergic reaction,
- Dry, windy weather increases the spread of pollen and worsens allergy symptoms,
- Rain can wash pollen out of the air, reducing the risk of exposure to pollen, but
- Rain can also increase the growth of plants, especially grasses, increasing the pollen levels.
For a research report about how rain can break up pollen into smaller particles to trigger allergies, check out “Thunderstorm-associated asthma in Atlanta, Georgia” by Andrew Grundstein et al.