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The distinct, earthy smell that fills the air after rain following a long dry spell can trigger some serious nostalgia. This scent, petrichor, goes right to our brain’s memory and emotional centers for reasons that may be to do with our very evolution. But as climate change alters Earth’s weather and environment, petrichor is going to transform, too.
First, it helps to know why petrichor even happens. During dry periods, soil-dwelling bacteria produce a compound called geosmin that deters insects such as fruit flies. The chemical signals to the flies that the plant material the bacteria grow on isn’t appetizing and that it might be toxic. Geosmin also attracts spore-spreading animals, such as springtails, which help disperse the compound even more widely.
“Humans are extremely sensitive to geosmin,” says Johan Lundström, a psychologist specializing in olfactory perception at the Karolinska Institute in Sweden. “If there was one drop of geosmin stirred up into an Olympic-sized swimming pool, we’d still smell it.”
While bacteria in soil produce geosmin, plants discharge aromatic oils during a drought to create a shield to protect against harsh heat and minimize water loss. When raindrops hit dry ground and plants, the impact creates microscopic bubbles filled with geosmin and oils that rise into the air, pop and scatter into tiny particles that can then fill human nostrils.
Exactly how petrichor changes will depend on where you are as the effects of rising global temperatures on weather and climate patterns will vary from place to place and year to year. Overall, parts of the world that get more rainfall will experience less petrichor, whereas areas that have droughts will experience more, says Jeppe Lund Nielsen, a professor of microbial physiology at Aalborg University in Denmark.
Droughts cause geosmin to build up, and when they occur with hotter temperatures, bacteria produce even more geosmin as heat speeds up chemical reactions inside the microbes. “If you get a higher temperature, you also get a higher production of the compounds,” Nielsen says. So when it finally does rain, more geosmin is released. But if soils get too dry, “the geosmin will dry up and not be released,” Lundström says.
Where rain falls more frequently, there will be less petrichor because the aromatic compounds won’t have enough time to accumulate. And heavy downpours—which are increasing as the planet warms—are too forceful to trap the tiny air bubbles required to launch the scent into the air. Instead the compounds are diluted, pushed underground or mixed in surface floodwaters.
Soil degradation—which has also increased as a result of higher temperatures and heavier downpours—plays a role, too, because it means fewer geosmin-producing bacteria can grow.
Any changes to geosmin won’t stop at what humans smell. Insects that react to petrichor will also have to adjust their behavior, Nielsen says. For example, areas with large amounts of petrichor will likely see an increase in mosquitoes, because the smell indicates to these insects that the dry ground is now a moist fertile environment in which to lay their eggs.
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