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You can go to linode.com/scishow to learn more and get a $100 60-day  credit on a new Linode account. [ INTRO ] There’s nothing quite like the smell of  the trees during a hike through the forest. But the molecules responsible for that  lovely scent do more than just smell good.

They’re used by trees for protection  against pests and pathogens. Researchers have found that  they can even help make clouds. Which means that trees might  not be complete bystanders when it comes to the weather around them.

Their natural emission of cloud-forming molecules may be influencing the climate around them – way more than we used to think possible. And since we need to understand  exactly how our climate is changing, we’re going to need to account  for weather-controlling trees. Let’s quickly go over some organic  chemistry to understand how this works.

Trees produce monoterpenes, using carbon dioxide that they collect from the  atmosphere during photosynthesis. Monoterpenes are made up of ten carbon atoms and sixteen hydrogen atoms, and the exact arrangement produces  slightly different compounds. The most abundant of these is alpha-pinene.

As the name suggests, it’s responsible for the  unique smell of pine trees. Once they’re made, monoterpenes  can be stored in the leaf, locking them up until the tree needs to use them. Otherwise, they’re released by the tree  immediately after they’re produced..

And trees emit a lot of monoterpenes — well over 100 teragrams of carbon per year. That’s a one followed by fourteen zeros grams. Up in the atmosphere,  monoterpenes are quick to combine with ozone and hydroxyl molecules  that are floating around up there.

They form new particles that are  the beginning stages of clouds, known as cloud condensation nuclei.. There are a few ways clouds can get started. And research has shown that  monoterpenes can be one.

And while there’s obviously a lot of variables, the thinking goes that these clouds lead to   increased cooling as well  as the potential for rain! [ehn-aan-tee-oh-murhs] Now, a monoterpene such as alpha-pinene   can actually exist in two forms: plus and  minus alpha-pinene, referred to as enantiomers. These molecules have the same  atoms in the same arrangement,   but they’re mirror images of one another. It’s often compared to left and  right hands.

Same basic arrangement,   but you can’t superimpose them on  each other no matter how hard you try. And there can be differences between  the way enantiomers behave chemically. However, in this case, enantiomers of alpha-pinene have the same reactivity  with ozone and hydroxyl ions.

And because these two forms  react the same in the atmosphere, existing climate models don’t  differentiate between the two. But maybe they should. The authors of a 2022 paper in the journal Nature have found that these different  forms of alpha-pinene are released into the atmosphere at very different times, especially during periods of severe drought.

These researchers looked closely at  the two forms in a closed-system, controlled environment rainforest  designed to simulate the Amazon Basin. This is literally a rainforest  in a really big box, where the chemical and climate conditions within can be controlled and measured by researchers. This means the researchers  were able to do a few things they wouldn’t be able to do  in the natural environment.

First, they could simulate the climate  conditions they were interested in studying, which ranged from normal rainforest  conditions to severe drought. And they could also add carbon-13, a form of carbon with an extra neutron that is used by researchers as a way  to track environmental processes,  because it doesn’t naturally  occur in large quantities. Carbon-13 can be injected into  the atmosphere within the box, where the trees will pick it up and use it just like the more common carbon-12.

That helps researchers determine  which form of alpha-pinene was   being released throughout the experiment. If the alpha-pinene contained the carbon-13 that was added during the experiment, that would mean it was created after they dosed the rainforest-in-a-box with the stuff. And what the team found  during normal conditions was t hat the trees released minus  alpha-pinene in the morning.

And it was freshly made, released almost as soon as it was created by the trees during photosynthesis. In contrast, plus alpha-pinene wasn’t  released until later in the afternoon. And it didn’t contain any carbon-13, which told the researchers  that this form of alpha-pinene was being released from some kind  of tree internal storage system.

So even under normal rainforest conditions, the different forms of alpha-pinene were  being emitted at different times of the day. And as drought conditions  progressed from mild to severe, the trees switched how they released  the different forms of alpha-pinene, and also started releasing a lot  more monoterpenes in general, including other compounds like beta-pinene. The emission of minus alpha-pinene was delayed from morning until later in the day, which the researchers believe was in response to the stressful conditions  created by the drought.

Instead of releasing  freshly-made minus alpha-pinene, the trees conserved energy by  releasing more of it from storage, along with the plus alpha-pinene. And by releasing a lot more  monoterpenes into the atmosphere, it was kind of like an S. O.

S.  signal to the atmosphere, signaling for clouds and rain in  order to lessen the drought stress. Not that the trees know what they’re  doing, but it seems like a strong   clue that trees have evolved ways to help  ease drought conditions on a local scale. They’re not totally at the mercy of the weather – it’s a two-way conversation, and the trees talk with monoterpenes.

All that suggests monoterpenes are more  than just a nice smell to enjoy on a hike. Drought conditions are predicted to increase as the climate crisis continues to rage, so these molecules will become  even more important in the future. This new research shows that even the different forms of monoterpenes are an important piece of  the climate jigsaw puzzle, and it’s important to understand how  their emissions will change over time,   and accurately account for  them in future climate models.

In the end, those trees might be onto something. We all like making it rain. And to do that, we save money where we can.

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