Hank: Thanks to Linode for supporting this SciShow compilation video. You can get a $100 dollar 60-day credit on a new Linode account at linode.com/scishow.
[intro]

Some things just put us in a trance when we smell them: like old books can smell [smells book] - like, that smell! What is it? [wheezy voice] I love it so much!

When it comes down to it, they are just a bunch of paper. So. We're going to explore the chemicals that make up our favorite and least favorite smells in this SciShow compilation to understand why, for example, books smell way more satisfying than a stack of paper. There are all sorts of chemicals involved in binding, waterproofing, and other processes that put books on the top shelf of smells. And Michael can tell us a few of them now.

[slide: "Why Do Old Books Smell So Good?"]

Michael: Walk into an old library or a secondhand bookstore, you'll be surrounded by that comforting old-book smell. You know the one some people describe as "a little musty with hints of vanilla or coffee." Or even "newly cut grass." Or maybe you prefer the smell of new books, which can seem crisp and fresh. But what causes books to have such distinctive scents? Well, it comes down to a handful of chemical compounds found in the paper and bindings of the book.

See, paper is made up of wood pulp, so it has a lot of organic compounds, which are just chemicals that contain carbon. Specifically, paper has a lot of the polymer cellulose, which is a long chain of the molecule glucose, and that's bound together with the help of lignin, another complex organic polymer found in plant cells. And over time, these chemical compounds react to light, heat, and moisture in their surroundings and start breaking down. In the process, they release volatile organic compounds - or VOCs - which easily vaporize and enter the surrounding air.

There are different kinds of these VOCs, and which ones are released depends on how the manufacturer made the paper and bound the book. If you detect a hint of almond, you're probably smelling benzaldehyde, a ring of carbons connected to another carbon that's double bonded to an oxygen. It's naturally found in almonds, which explains the scent. A vanilla-like fragrance is thanks to vanillin, the main compound that gives vanilla its smell and flavor. If you smell something sweet, it's likely because of ethylbenzene, a ring of carbons connected to a short carbon chain -

- short carbon chain that's often used to manufacture plastic. It's also in things like inks and paints. If your'e detecting a light floral aroma, you're probably smelling 2-ethyl hexanol, a kind of alcohol that's often used in solvents, but also in flavors and scents. New books release different kinds of VOCs because modern manufacturing processes use different kinds of chemicals, like hydrogen peroxide to bleach the paper, and alkyl ketene dimers to make paper a little water resistant.

Scientists and historians can use these VOCs to learn more about the age and condition of older books, or to reveal parts of their history, like whether they've been exposed to smoke or had water damage. And learning more about old book smells can help historians determine which ones are degrading and need to be better preserved and protected. So it turns out we can learn a lot from the smells in books, not just the words in them.

[slide]

Hank: There's nothing like curling up with a good book on a rainy day; combine the perfect book smell with the calming smell of the rain, and you are in for a good time. But water, very clearly, doesn't smell like anything, so you might be enjoying the soothing smell of... rocks? No. Plants? Or ozone? Well, let's let Michael explain that one, too.

[slide: "Why Does Rain Smell?"]

Michael: Lots of people love the smell of rain; it's an aroma we associate with freshness and cleanness and... wetness. But water doesn't really smell like anything, so what exactly is that wet aroma? Well, it turns out that even though rain often smells clean to us, much of its scent actually comes from dirt and rocks. The smell of rain has its own scientific name, "petrichor," from the Greek word for rock - "petra" and "ichor,' the blood-like substance that was said to flow through the veins of the gods. 

But the source of the smell isn't the rocks themselves; it mostly comes from plants. Especially during long, dry periods, some plants release oils that are rich in fatty acids, some of which you might recognize as food ingredients, like palmitic and stearic acids. Plants release these acids when water is scarce, because they bock other seeds in the ground from sprouting, which reduces competition for water. And over time, these oils build up on soil and rocks, and when rain falls, it kicks them up into the air and causes them to release fragrant, volatile compounds that, to us, smell fresh, vegetal, and altogether pleasant. 

But that's only one component of rain smell. Petrichor also contains a -

- also contains a chemical that's released by soil bacteria called "geosmin," or "Earth smell," and as the compound that gives soil the smell that we think of as earthy. Geosmin is actually a pungent kind of alcohol; it's what gives some vegetables and wines their loamy, dirty flavor. Certain soil bacteria release geosmin when they die or when they go dormant, especially in hot, dry conditions. And when rain finally comes along, it disperses the chemical into the air in the form of a fine mist. So, a lot of what you detect when you smell rain are actually compounds that living things have produced to get through dry spells. This would explain why the smell is particularly strong if it hasn't rained in a while. 

But there's one more source of rain scent that you can smell even before it rains - ozone. It's just a molecule of oxygen, but it has such a distinctive aroma that its name also comes from the Greek word "ozein" - "to smell." Ozone, or O3, is produced when a lightning ionizes a molecule of oxygen - or O2 - in the air, separating the atoms. When they recombine, some of them form groups of three, creating ozone, which has a sharp, bracing scent that gives petrichor its invigorating quality. 

So the next time it rains where you are, take a moment to stop [inhales] and smell the chemistry.

[slide]

Hank: So after a nice rain, you can really smell that earthy, plant smell that viscerally puts you in your happy place. But if there is no rain forecast, you can still get some of those smells from mowing the lawn. Or I guess the smells are different plant smells, because we're really smelling the grass communicating with bugs. Here's a peek in to that world.

[slide: "What Makes Fresh Cut Grass Smell?"]

Michael: You know that grassy smell right after you mow the lawn? It's kind of fresh or maybe reminds you of spring and warm weather. Grass doesn't normally smell like that though, so what's the deal when it gets cut? Well, when their leaves are injured, grass and other leafy plants make organic compounds for protection. Some of these chemicals called "green leaf volatiles," or GLVs, evaporate into the air and produce that signature scent. And we think these GLVs are important signals that attract the predators of grass-munching insects.

When a leafy plant is damaged, it makes lots of chemicals to protect itself; some of these chemicals are signaling molecules inside the plant. Jasmonic acid and salicylic acid, for example, help the plant synthesize compounds that make it less appetizing, or defend it against -

- against fungal and bacterial infections. Other signalling molecules, like traumatic acid, tell the plant to make more cells to close up the wound. Green leaf volatiles act a little differently though; they're volatile organic compounds, which means they easily become gases and are released into the air, basically acting as a call for help.

See, before lawnmowers existed, leafy plants were hurt by insects, like caterpillars eating them. The wounded plant releases a bunch of GLVs, which include chemicals like aldehydes, alcohols, and esters. Some of these chemicals are the culprits behind that fresh, grassy smell. But more importantly for the grass, these chemicals act as a dinner signal to other insects, like parasitic wasps, which can lay their eggs in the caterpillars and eventually kill them. 

To figure out how important these smelly chemicals are to plants, a researcher from Texas A&M University was studying a mutant strain of corn that couldn't make GLVs, and corn is, botanically, a grass. Over time, this mutant corn had more insect damage than it's GLV-producing counterparts in a lab setting and in the field. Predatory insects just don't show up as often to eat their herbivorous prey without a chemical signal to tip them off. 

So, next time you smell that fresh cut grass, remember that it's just adding insect to injury.

[slide]

Hank: So you might not be the primary target for these plant smells, but you can still enjoy them. And with a gas-powered lawn mower, you might associate that fresh-cut-grass smell with another favorite, gasoline. Here's Savannah to explain why gas is on the list.

[slide: "Why Do People Love the Smell of Gasoline?"]

Savannah: Gasoline is made mostly of hydrocarbon molecules, as well as additives that help the car run smoothly. One of these molecules is benzene, which gives gas its distinctive smell. But we don't really know an evolutionary reason to like this random molecule in gasoline, so it might come down to odor hedonics, which is a sciency way of saying we like what we like. We tend to like familiar smells and those that we formed a positive association with in the past. 

So we might have caught a whiff of benzene while doing something fun, like going on a road trip as kids or getting candy from the convenience store. Then, we linked those smells and emotions in our minds. And it's easier to link emotions to smells and other senses, because the brain cells that let us -

- that let us smell things send information to a specialized smell center in the brain before linking directly to the emotion part of the brain, called the amygdala. This pathway is different from our other senses that have to go through a less specialized intermediate part of the brain before they're sent to the amygdala. So that connection between emotion and smell is stronger than, say, taste. 

Plus, our brain chemistry encourages the association. Bezene causes our brains to release more of the feel-good chemical, dopamine, which makes us feel a bit euphoric. But, like many things that trigger a dopamine release, too much of a good thing can be bad. Inhaling too much benzene can break our DNA, which causes cells to die. And depending on which cells die, that can mean a weakened immune system, reproductive problems, or even cancer if you whiff enough of it. So that's a bit terrifying, but the good news is if your'e not a professional pumper or around gasoline a lot, the amount of benzene you're exposed to shouldn't affect you. 

Still, it may be best to keep the time at the pump to a minimum. And if you don't like the smell of gas, maybe that's not entirely benzene's fault, because, well, there are a bunch of other components of gasoline that don't smell so great: Things like long-chain amines have a fishy or rotten smell, and some people say that long-chain carboxylic acids smell like goats, which might explain why gasoline isn't appealing to all of us.

So there are legitimate reasons why you might love or hate the smell of gas. But whatever camp you fall in, I'm sure we can all agree that the smell of freshly baked chocolate chip cookies is much, much better.

[slide]

Hank: Now, the smell of gasoline can be a little bit polarizing, so let's take a breather to talk about Linode. You're almost definitely watching all of this thanks to cloud-computing technology, like the stuff they do at Linode, a cloud-computing company from Akamai. Linode provides access to some of your favorite internet services, from streaming videos to storing files. It's the stuff that developers and enterprises use to build, secure, and deliver their products all over the world. And if you don't believe that "all over the world" part, you can run a speed test in any remote area by connecting with one of Linode's international facilities all before any commitment.

Then, you can start Linode by clicking the link in the description down below or heading to linode.com/scishow for a $100 60-day credit on a new Linode account. And thanks to Linode for supporting this SciShow video because it really stinks when you can't get the internet stuff you love.

Now, the internet or the smell of gasoline might scratch an itch for you, but if you just really love the smell of a new car, well, you might be getting a little more than you bargained for at the dealership. 

[slide: "Is That New Car Smell Dangerous?"]

Savannah: The molecules that make "eau de new car" belong to a category called "volatile organic compounds," or VOCs. In general, VOCs are everywhere; they're in nail polish remover, they're in grandma's antique wood furniture, they're in leaves - you name it. And in terms of health effects, they're a mixed bag. Some have little to no known effects on human health, and some should really stay out of your lungs. When something releases VOCs, it's called "off-gassing," and how big of a deal this is comes down to what compounds are doing it and in what quantities.

In vehicles, off-gassing comes from things like glue, paint, upholstery, and all the plastic keeping you on the road. Since new vehicles haven't had a chance to air out, they tend to off-gas more. And the list of VOCs that have been detected in them is about as long as a drugstore receipt. Studies have found anywhere from 30 to more than 250 VOCs in vehicles, and some common ones include styrene, acetaldehyde, benzene, and formaldehyde. Again, different VOCs have different potential health effects, but the four I just mentioned are all carcinogens. 

Now, this doesn't mean they definitely cause cancer. Labels like these come from the International Agency for Research on Cancer, and focus on how strong the evidence is that something could cause cancer in the right conditions. For instance, the formaldehyde and benzene in new-car smell are "class I carcinogens," or known carcinogens; this means there's strong evidence that they can cause cancer. Meanshile, styrene and acetaldehyde are "class II carcinogens." The evidence for these compounds isn't as conclusive as it is for the stuff in class I, but neither of these class labels say anything about how these carcinogens affect your personal risk of -

- risk of getting cancer. There's a lot more that goes into that. 

But remember, there are two parts to figuring out if the VOCs in new-car smell are a health risk: the compounds, and how much of them there are. I mean, anything can be dangerous in large enough amounts, including oxygen and water and even my cat, Huckleberry; he is so fluffy, but at what cost.

So, car manufacturers are expected to figure out how many of these molecules are usually floating around in their vehicles. Different countries and companies have different standards for emissions and how they should be tested. But to give an example, one international organization requires new cars to sit in a chamber at about room temperature while researchers monitor off-gassing; if the VOC levels are below safety thresholds, the car is good to go. 

Except, if you've ever gotten into a car on a sunny day and scorched your legs on the fake leather seat, you know cars don't live in room-temperature bubbles. And unsurprisingly, field studies have found that VOC emissions can change with the weather. Most studies have focused on the air temperature inside vehicles, and they found that when air temperatures were higher, so were VOC levels. For instance, in a 2020 study done in China, levels of some VOCs could be more than 100 times higher in the summer than the winter.

But you know how sometimes the air inside a car is just kind of warm while the dashboard is hot enough to cook a breakfast burrito? Researchers have noticed that, too. In one 2023 paper, scientists focused not on air temperature, but surface temperatures inside a vehicle. Researchers in China parked a new car outside for 12 days in the summer and took measurements as the weather changed. They found that at their highest, the formaldehyde and acetaldehyde levels were about 35% and 60% higher than the national standard, respectively. Even on a cloudy day, acetaldehyde levels exceeded the limit by 20%. Now this is just one car, so it could be an outlier. But at least in this case, the researchers calculated that at this levels, the total amount of VOCs imply a high cancer risk for drivers. And this isn't the only study to come to that conclusion. 

A 2021 paper for the US didn't focus on the vehicles' ages, -

- ages, but after analyzing more than 2 dozen studies about vehicle VOCs, they still concluded that formaldehyde and benzene levels were too high for California standards. Because of this, they found that over 60% of local commuters had a 10% chance of being at a higher risk for developing cancer. Again, that doesn't say anything about anyone's personal risk; just that the overall risk is higher.

So, how long does it take for VOC levels to drop? If you're currently Googling "new car trade-ins" in a different tab, maybe hold on, because it depends. A study published in 2020 monitored VOC levels in a new vehicle for almost two years. They used the detection method that didn't pick up formaldehyde, so no data there. But for the compounds they did measure, they found that the peak level of total VOCs had dropped almost 25% by the vehicle's second summer. Unfortunately, they also found that for certain molecules, the off-gassing rates didn't really decrease with time, and that includes aldehydes, the family acetaldehyde is part of. So, buying a used car doesn't solve the whole problem, but it seems to help as would leaving the windows open whenever possible. 

And on a larger scale, it might be encouraging to know that studies like this aren't just for consumers; they also inform car manufacturers and regulatory agencies who have the power to research and use different materials in their vehicles. Ultimately, there can be a lot of good that comes with a new car, especially as they become more efficient. So if you have the ability to buy any vehicle sitting there looking pretty on the lot, there's a lot to consider, including if it's worth picking an older model that's had a chance to air out.

[slide]

Hank: Some things that smell appealing to us are really a sign of potential danger, like this carnivore's pee; it smells like popcorn. Olivia's gonna need to unpack that.

[slide: "Why These Weird Carnivores Smell Like Popcorn"]

Olivia: wander through the forests of Southeast Asia, and you might suddenly wonder if there's a movie theatre nearby. That's because part of Thailand, Malaysia, Vietnam, and Indonesia are home to the binturong, a cat-like carnivore with pee that smells like buttered popcorn. Yum?

Binturongs - or "bearcats" as they're sometimes called, although they aren't closely related to bears or cats - like to hang out high in the trees of dense forests where they can avoid ground-dwelling predators while they snack on fruit, insects, small rodents, or leaves. They're covered in shaggy, black fur with long, goofy tufts behind their ears, and they have grasping or prehensile tails, which they use to clamber along the branches. 

But probably their most distinctive feature is their popcorn-y smell, which mostly comes from the urine they use to mark their territories. They leave scent marks by squatting and spraying urine onto their legs and bushy tail and then rubbing those onto nearby branches. They can also use a specialized gland near their anus - called the "perineal gland" - for this. But researchers think the scents released from there are different and less appetizing.

The tasty smell of their pee is thanks to a chemical called "2-acetyl-1-pyrroline" or just "2-AP" for short. It's an organic heterocyclic aroma compound and an imine, which is basically just a fancy chemistry way of saying it's a chemical that has a smell, contains carbon atoms, has a circular structure made of multiple elements, and there's a double bond with a nitrogen atom thrown in there somewhere.

In popcorn, 2-AP forms when high heat causes a reaction between the kernal's sugars and amino acids - a chemical process known as the "Maillard reaction." But binturongs aren't actually caramelizing their pee, so biologists aren't sure how they make it, though they think bacteria in the gut or near the urethral opening might be involved. It's also possible other compounds contribute to the buttery aroma. But when researchers identified a bunch of different components of binturong urine in 2016, 2-AP was the only one found in all the samples. And it hung around at room temperature, meaning it would probably also hang around for a while on a tree.

What's really interesting though is that male binturong urine had a lot more 2-AP than female urine, and researchers were able to link the level of 2-AP in an animal's urine -

- urine to the amount of the sex hormone androstenedione swishing around in their blood. Males have more of this hormone in their blood than females over all, but levels of it increase in females when they're in heat - right when they're most fertile. That means other binturongs might be able to tell whether there's a territorial male or a female ready for mating nearby - all from how popcorn-y the branch they're scurrying on smells.

And while knowing what's behind the binturong's unique buttery smell is pretty cool all by itself, it might also be kind of important, because binturong populations have declined more than 30% over the last couple of decades. So if 2-AP puts these vulnerable animals in the mood or signals when a female is fertile, it could help conservationists breed them in captivity. And that could help ensure these wonderfully weird animals keep forests smelling like popcorn for centuries to come.

[slide]

Hank: It's hard to say why popcorn pee would be an advantage to these animals, but it's pretty clear why other animals, like stink bugs, smell so pungent. But maybe you're wondering how they do it. Well, let's let Michael tell you.

[slide: "Why Do Stink Bugs Stink?"]

Michael: If you've ever made the mistake of squishing a stink bug, you know exactly how they earned that name. If you haven't, might I suggest keeping it that way. Stink bugs give off an awful smell when they're attacked or squashed; it's a defense mechanism and a pretty powerful one. There are several species of stink bugs, which together make up the family "Pentatomidae," but the most common type is the brown marmorated stink bugs, an otherwise banal, penny-sized insect that's native to Asia but has invaded all over the world. Some call them "shield bugs" because of their distinct trapezoidal shape. The hardened shell of their broad bodies acts as a defensive armor, protecting them from predators and the elements.

But their most recognizable defense is their unmistakable odor, something like a cross between cilantro and a skunk. The smell comes from a waxy liquid that contains aldehydes, compounds that have a central carbon atom double-bonded to an oxygen and single-bonded to a hydrogen. They also tend to be very smelly. A common one is formaldehyde, which is used for preserving tissues. If you ever -

If you've ever hung out in the back rooms of a museum or a biology lab, you might be familiar with that particular stench. But aldehyde aromas can also be found in your kitchen since they give some foods their smells, including - you guessed it - cilantro.

The aldehydes found in stink bugs are contained in specialized scent glands on the bottom side of the animal's thorax, or the middle body part, and they're highly concentrated, so even a little bit causes a big stink very quickly. Some research suggests these aldehydes also have antifungal and antibacterial properties, so they may be helping stinkbugs fight diseases at the same time.

But as anyone who's been sprayed by a skunk can tell you, making a stink is a pretty good defense mechanism, so it's not surprising that these bugs don't seem to have very many natural predators. That's what's allowed them to go global and become a huge pest. They're known to eat more than 100 types of plants, including valuable crops like apples, corn, and soybeans. In 2010, it was estimated that the Mid-Atlantic states in the US lost about $37 million from damages to apples alone, so finding ways to control their population is important for agriculture. But they're not just bugging farmers.

When colder weather rolls in, stink bugs look for a comfy place to overwinter, like your home. When one finds a cozy spot, it releases pheromones that encourage others to join it. Other than offending your nose, stink bugs are pretty harmless to humans. But if you don't want them to snuggle into your house for the winter, it's best to keep your doors and windows sealed tight. If some get in anyway, whatever you do, just don't squish them unless your nose is prepared.

[slide]

Hank: Little things like bugs or even rubber bands can pack a big stinky punch. Olivia's gonna need to get down to the molecular level to explain their distinct smell.

[slide: "The Strange Physics Behind the Smell of Rubber Bands"]

Olivia: If you've ever spent a few hours wrapping things up with rubber bands, like if you're trying to explode a watermelon or a pumpkin or something, you know that distinctive smell that rubber bands let off when you stretch them. That scent is the result of the weird physics of rubber, because stretching a rubber band actually heats it up. This is something you can actually feel for yourself at home:

Take a rubber band, stretch it quickly, then gently touch it to your upper lip. I say "your lip" because lips actually have a higher density of -

- density of temperature-sensing nerve fibers than your fingertips. Anyhow, it's slightly warm, right? Now, let the rubber band relax quickly but carefully - don't fling it at someone's eye or something - and then touch it on your lip again. It's much cooler, and that's because of something called "entropy." Although entropy can be a bit of a scary term, for our purposes, it simply refers to the amount of disorder in a system. 

If you zoom into the molecular level, rubber bands are made of long chains called "polymers." When the rubber band is at rest, these polymers are disorganized, kind of like a big pile of spaghetti. But when you stretch the rubber band, the spaghetti strands become straighter and more organized, more like a box of straight, uncooked spaghetti. And because the rubber band is more organized it has less entropy. 

But the second law of thermodynamics states that the entropy in a system can't decrease over time, so to maintain the same amount of entropy when the polymers are stretched out, the rubber band has to become more disordered in some way. And that way is some of the molecules in the rubber band moving faster. When molecules move faster, their temperature is higher, hence the warmer feeling of the stretched out rubber band. Now, this also happens to make the rubber band smell more, and that's because higher temperatures cause substances to have more volatility - basically, they evaporate more easily. For example, water at room temperature doesn't evaporate very fast - it's volatility is low. But if you heat that water up enough, it starts to turn into steam - it's volatility becomes high. Water doesn't have a strong smell, but the same principle applies to odorants: chemicals that do have a smell to us. 

Natural rubber contains dozens of odorants, including things like limonene - a lemony scent found in citrus fruits - and trimethylamine - a compound with a strong fishy smell. You might not think rubber bands smell lemony or fishy, but mixed with all the other smelly molecules freed by the heat of stretching, you get that distinct rubbery smell.

[slide]

Hank: So strong smells can come from the tiny molecules in a little rubber band or from the vast ocean. Even the biggest things on Earth get their smells from tiny stuff hiding inside, like plankton. And when I say -

- I say that plankton give the oceans their smell, do not worry, I'm only referring to the smell they give off when they explode. All right, Rose is gonna give a little more context.

[slide: "Why Does the Ocean Smell Like That?"]

Rose: Once you've smelled the ocean, you can't un-smell it; that sort of sharp, sometimes eggy odor is just that distinctive. And it also plays a big role in ecology and maybe even in the climate. Because, sure, there's salt in that whiff of dead fish, but the key ingredient in this smell is something you probably wouldn't expect - exploding plankton.

Collectively, the tiny plant-like organisms floating in the sunlit layer of the ocean are called "phytoplankton," and they - as well as certain bacteria - make a chemical compound called "DMSP" (dimethylsultoniopropionate). Among other reasons, phytoplankton make DMSP to help protect themselves from too much UV radiation, balance the water content in their cells, and even deter predators. But sometimes, no amount of this compound will prevent them from being a tasty snack, and that's where the fun begins.

When something like a a hungry predator comes along and breaks open a phytoplankton cell, the DMSP is released from the cell into the water. And there, it becomes an important food source to microbes. As the microbes graze on the DMSP, they ultimately break it down into smaller molecules such as dimethyl sulfide, or DMS. The "sulfide" part means it contains the element sulfur, and that's what gives the ocean its unmistakable odor - it's a bunch of microbes breaking down their food.

Now in some places, this sea smell can be really pungent, leaning towards the rotten-eggs end of the spectrum, while in other locations you might barely catch a whiff. The difference depends on how many phytoplankton live in the area. Specifically, it takes a group of actively growing phytoplankton, called a "bloom," to ultimately make a strong odor. The larger the bloom, the more DMSP there is for microbes to break down, and the stronger the sulfur smell.

And it's not just humans that notice it; animals from seabirds to seals to whale sharks will follow the scent to track down their next meal. That's because their prey, such as fish and zooplankton, -

- and zooplankton can often be found chowing down on the phytoplankton that make up a bloom. But there's actually another story here, too, because DMS in particular also plays another more surprising role on Earth - helping to form clouds.

DMS is the largest source of biologically produced sulfur on our planet, and since it's a gas, it quickly make its way into the atmosphere. On the way there, it goes through chemical reactions to become a variety of sulfur compounds. Then, these compounds act as condensation nuclei in the atmosphere, which means there's something water vapor can collect on to form a cloud. And clouds can have a surprisingly large effect on the climate, like fluffy white clouds reflect sunlight back to space, which helps keep the planet cool.

At the same time, through the process of photosynthesis, phytoplankton are also removing carbon dioxide from the atmosphere. So, in a way, these tiny creatures are some of the Earth's most important climate regulators. And the process is self-reinforcing; absorbing more CO2 means more energy for bigger phytoplankton blooms, which then produce more DMS and more clouds as a result.

Unfortunately, like other important climate processes, things are starting to get a little out of balance. As the oceans absorb more of the carbon dioxide we're emitting, they're becoming increasingly acidic, and that's not good for phytoplankton growth or the production of DMS. So ultimately, scientists have a lot left to learn about the complex interplay between the ocean, life, and our atmosphere, and how it will change in the future.

One thing is for certain though: The smell of the sea is unforgettable.

[slide]

Hank: While some of us can't imagine life without the refreshing smell of the ocean, it's definitely an acquired smell. So, if you don't like any of these smells, you can always just wait for winter when they are not as strong. Here's why.

[slide: "Why Does Everything Stink Less In Winter?"]

Olivia: Seasonal changes bring all sorts of new sights, sounds, and smells. But sometimes, even the same things can smell totally different to us in the cooler months than the warmer ones. It has to do with both what's there -

- what's there to smell and how our noses work.

If you've ever been in a big city on a hot summer day, you may have noticed that there's a... certain bouquet. Part of this comes from the fact that bacteria becomes more active on hot days, so grosser things get grosser, like a dumpster in the hot sun. Meanwhile, the cold might stop decomposition in its tracks. But there's another factor; it comes down to how smells actually reach our noses.

Smells are a type of volatile organic compound, meaning they evaporate easily and they need to evaporate before they can reach our noses. The thing is, some of these compounds only evaporate at higher temperatures. As a result, there can simply be more scent molecules floating around as the temperature goes up. It's the same reason a bowl of cold soup doesn't smell much compared to a piping hot one.

But it's not just that there's more to smell in the summer; the air is also spreading smells around more efficiently. That's because gas molecules move around more quickly in hot air, so they pass odors along faster. So in the summer, there are more smells and they're travelling faster.

Then, there's humidity. It's probably not news to you that in the northern hemisphere, summer air is more humid than winter air. And research suggests that molecules of water vapor likely act as buoys for odor particles and carry them around when under drier condition, they'd probably stay put. All of these effects add up to give garbage on a hot day that certain... je ne sais quoi.

As temperatures drop, things start to stink less. But you may also notice that crisp edge that gives winter air its distinct winter smell, like the smell of the first frost. Turns out, the harsh sensation of cold air triggers another sensory nerve called the "trigeminal nerve." This nerve detects touch, temperature, and pain in your face. It also interacts with your olfactory system, which is responsible for smell. The trigeminal nerve isn't actually part of the olfactory system, but the two are closely related in ways that we don't fully yet understand. Because it sits cosy with a lot of other nerves, -

- a lot of other nerves, the trigeminal nerve can cause some crossed wires among our senses, which is why, for instance, some people get the urge to sneeze when they look at a light. 

But the triggering of this nerve can also affect how we perceive smells. It effectively combines the sensation of temperature with our perception of scent, and that combination between our olfactory system and our trigeminal nerve is why things like mint smell "cold" and pepper smells "hot." And it's part of the reason why winter smells like winter. 

There are many environmental differences between the seasons, and those can have a big impact on what smells are even around in the first place. But the way smells travel and interact with our noses can change our experience of smells for even cooler reasons.

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Hank: Our noses help us navigate tons of stuff, from books to bugs. And they tend to go underappreciated because it's easy to focus on the smells we don't like more than the ones that make us happy. But a lot of this, of course, comes down to personal preference. So sound off in the comments if you dislike any of these commonly-enjoyed smells, which I'm sure some of you do, or if you can't get enough of the ones that everyone else seems to hate. We'll see you down there.

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