Thanks to the Monterey Bay Aquarium Research Institute for partnering with us on this episode of SciShow.

All of the amazing deep-sea videos you are about to see were taken with their remotely operated vehicles. [♪ INTRO]. If you're taking a trip to the deep ocean, you should know the forecast: it's gonna be cold and dark, with a high chance of snow.

Okay, well, not snow as in “ice crystals” like up here on the surface of the planet. We're talking about marine snow. It looks like snow; it is not.

It's comprised of fluffy bits of organic matter that range in size from about half a millimeter to several centimeters across. And this snow isn't just pretty; it's an essential part of our ocean food webs and our global climate! Now, “organic matter” can refer to pretty much anything that is or was alive.

But in this case, we're generally talking about the remains of plankton. Plankton is a catch-all term for organisms that are largely at the mercy of water currents. And they're often tiny; things like algae, bacteria, protozoans, little crustaceans like krill, and even the early life stages of much larger animals.

When these creatures die, what's left of their bodies starts to sink, becoming part of marine snow. Not all of these snow particles are dead plankton, though. Some are fecal pellets; it's gotta go somewhere!

Marine snow also contains decomposers like bacteria that attach themselves to the falling poo and tiny carcasses. And as the different bits descend, they clump together to form larger and larger “flakes”, which eventually give the appearance of a blizzard far below the waves. And this “snowfall” brings something very important to deeper waters: food.

See, in the shallow ocean where sunlight beams through the water, plankton that can photosynthesize are the base of the food web, just like the plants that are the base of the food web up on land. And in some places in the deep ocean, there are nutrient-rich areas like hydrothermal vents, which provide food for special bacteria that can form the base of their own ecosystems. Instead of using light for energy, these bacteria turn carbon dioxide into sugars by tinkering with chemicals like hydrogen sulfide; a process called chemosynthesis.

But most of the deep sea floor doesn't have these vents, and in the water column below about a thousand meters, there is no sunlight. So locally-produced food is very scarce. And yet, life persists!

Because what they need drifts down from above. Many animals eat the falling particles as they drift down through the water column, like this larvacean. The animal itself is just a small tadpole-like thing in the middle; the rest is the giant mucus net it constructs to catch and concentrate the descending organic matter.

Over time, though, the net clogs, so the larvacean tosses it and makes a new one. Of course, in the deep, nothing goes to waste. MBARI researchers have found that these mucus snowballs are an important source of food for other animals, like this vampire squid!

And at the seafloor, other filter feeders and scavengers scoop up even more of the falling snow. Even with all these hungry mouths, though, some of the snow sticks. The particles that don't get eaten settle on the bottom, and as they decompose, they form a nutrient-rich, topsoil-like ooze that coats much of the vast seafloor.

This is all part of one of the most important biogeochemical processes on the planet: the carbon cycle, which is key to understanding climate change. As far as we know, all life on Earth needs carbon. It's a key component in essential molecules like DNA and RNA and the fats that make up our cell membranes.

So the distribution of carbon in different environments can influence what lives there. Like, without the carbon and other nutrients that sink from the shallows, most deep sea organisms wouldn't exist. But this snow doesn't just impact life in the deep.

By playing a role in the carbon cycle, it affects all life on Earth, including us. That's because the carbon these plankton have in their bodies had to enter the seawater from somewhere. That “somewhere” is generally the atmosphere, because it falls with rain, or it directly diffuses into the water.

Once in seawater, carbon can be used by organisms to build their bodies and shells. Then, when those organisms are digested by predators or decomposers, the carbon can be released as carbon dioxide. If this happens in the shallows, it can diffuse back into the atmosphere.

But marine snow pulls carbon from this water-to-air cycle and tucks it away in oozes on the seafloor. In places that this ooze builds up, it's gradually been pressed into huge deposits of chalk and other forms of limestone. These rocks now cover roughly three billion square kilometers of the seafloor.

And in some spots, they're hundreds of meters thick! In fact, these carbon-loaded rocks are the Earth's biggest carbon storage unit. The carbon can eventually return to the surface.

Tectonic activity can push these ocean rocks beneath continents, where they may melt, and rise upward, and fuel volcanoes that pump CO2 into the atmosphere when they erupt. But that takes many millions of years. Until then, the carbon is essentially locked away.

Over time, marine snow has been storing more and more carbon in the depths. According to a 2019 modeling study, around eighty million years ago during the Cretaceous Period, when dinosaurs like Velociraptor roamed the land, about one million tons of carbon in the form of carbonate were stored each year in deep-sea sediments. But today, scientists estimate over two hundred million are stored annually!

And all the carbon in the deep because of marine snow may have paved the way for our current, comfortable climate. See, over that same time period, our planet cooled dramatically, shifting from the warmer “hothouse” of the Cretaceous Period to our current “icehouse” conditions, where we have permanent ice caps, widespread grasslands, and a cooler climate. That's because carbon dioxide in the atmosphere acts as a greenhouse gas, one that absorbs and traps heat.

So though there are other things involved, in general, the less CO2 we have in our atmosphere, the cooler our planet is. And when more of the planet's carbon is in rocks, there's less in the atmosphere. Of course these days, we've been reversing all of that carbon storage.

We're essentially doing what takes volcanoes millions of years in a geologic instant by burning fossil fuels and cutting down forests. And because of our actions, we're causing the CO2 level in our atmosphere to rise and fundamentally changing our climate. That's why scientists are so eager to understand how our activities affect marine snow.

It will give them a better sense of how the planet will change in the years, decades, and centuries to come. We do not want to go back to the hothouse of the Cretaceous, so in addition to taking some steps to reduce emissions, we want marine snow to keep pulling carbon down into the depths. Right now, our ocean is doing us a very big favor.

Thanks in part to MBARI's crew of robotic floats, we know that it soaks up a quarter of the excess carbon dioxide we pump into the atmosphere and more than 90% of the planet's excess heat. But as atmospheric CO2 levels rise, more carbon dioxide enters the ocean, and that makes the water more acidic, which can disrupt the formation of carbon-storing oozes. That acidity is also messing with plankton communities, which could ultimately affect how much carbon makes it down to the ocean floor.

So scientists are keeping a close eye on marine snow and how it is or isn't changing in response to our actions. Because if current trends continue, we're gonna need all the carbon-storing help we can get. Thank you again to the amazing people at the Monterey Bay Aquarium and their research and technology partner MBARI for collaborating with us on this episode of SciShow.

MBARI's mission is to advance marine science and engineering to understand our changing ocean. You can learn more about their work by following their social media accounts. They have an amazing YouTube channel featuring weird and wonderful deep-sea creatures.

You can also visit their website at mbari.org. [♪ OUTRO].