Cloud seeding by trees could alter precipitation, climate

Certain pollens shed potent ice-nucleating compounds

Bees and their nectar-loving friends aren’t the only means of transporting pollen long distances. Water molecules grab some wind-strewn pollens and whisk them up into the clouds, where they can become the seeds on which ice crystals form. These tiny bits of plant tissue can raise the temperature at which cloud water will freeze by up to 18 degrees Celsius, a new study finds — increasing the reflectivity of clouds and likelihood that droplets become heavy enough to rain out.

Pollen grains from birch, pine and juniper — those with the best ice fostering prowess — show no similarity in surface shape, a feature scientists suspected might play a role in pollen’s ability to seed clouds. Instead, new data indicate water-soluble surface chemicals are responsible. B.G. Pummer/Vienna Univ. of Tech.

This ice crystal formation, called nucleation, is made possible not by the pollen itself but by water-soluble chemicals on some pollens’ surface, suggesting that biological material may not be required to seed clouds, scientists from Austria and Germany report online October 5 in Atmospheric Chemistry and Physics Discussions.

Researchers have long known pollen grains can reach some clouds. But these carriers of male reproductive cells — the plant equivalent to sperm — are relatively large and heavy, notes microbiologist Brent Christner of Louisiana State University in Baton Rouge. So few scientists had suspected that enough pollen could reach clouds to play a major role in cloud chemistry.

But the picture changes if the active agent on pollens is a nano-size molecule that can wash off and be lifted to high altitudes, Christner says. “Then the implications are there for pollens to potentially play a big role” — especially if the nucleating molecules have nearly the same potency as certain bacteria, the gold standard for natural ice-seeding agents.

The new study was prompted by an interest in how the biological promotion of ice formation might affect the light filtering capacity of clouds — and ultimately affect climate, explains coauthor Hinrich Grothe, a physical chemist at the Vienna University of Technology. As ice forms, clouds become more reflective, leading to atmospheric and surface cooling, he explains. As their moisture falls out, clouds will thin, allowing more warming sunlight to reach Earth’s surface.

A German group had reported almost a decade ago that pollens in general were weak ice nucleators. Grothe decided to test the ice-promoting ability of 15 different pollen types — including pines and other trees along with shrubs, grasses and herbs. Under their experimental conditions, distilled water alone froze at roughly –36°. Adding pollens from corn or ragweed had no effect, but those from stinging nettle, willow or hazel allowed water to freeze at –24° to –25°. And pollens from birch, pine and one type of juniper permitted freezing at –21° to –18°.

The temperature changes were largely unaffected by removing the pollen itself, suggesting that some surface molecules left behind are responsible for the change in freezing point. Grothe and his team have confirmed that the mystery molecules are not proteins but are still investigating their true identity.

The new study contributes to the growing body of data showing that biological entities “can have important, and often unexpected, influences on the weather,” says microbiologist Noah Fierer of the University of Colorado in Boulder.

“This is a fascinating paper that I believe will solicit broad interest,” adds atmospheric scientist Paul DeMott of Colorado State University in Fort Collins. While the new report did not confirm pollen’s impact in high-altitude clouds, he says, “it does suggest a heretofore unrecognized and possibly important pathway for biological influences on the atmosphere.”

Janet Raloff is the Editor, Digital of Science News Explores, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer, specializing in toxicology. To her never-ending surprise, her daughter became a toxicologist.

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