So I was hanging out with my two year old second cousin

in the ocean in Florida.

And we're jumping up and down with the waves and it's fun,

but then the water got all calm and I said, "Oh, no more waves!"

Because, like, that's the top tier of communication I'm going for

with a 2 year old, but then he said, "Don't worry, there will be more."

And I was like "How do you know." And he was like "It's a Cycle."

Yes! It is a cycle! The earth is filled with cycles...

and if my two year old second cousin knows it, you should know it too!

The universe is the great recycler.

All the stuff that we've got on Earth, every last particle

of matter or calorie of energy, has been around since the Big Bang.

It just keeps getting repurposed over and over again.

And when it comes to matter at least,

Earth is essentially a closed system.

All matter gets passed around in continuous biogeochemical cycles,

which are pathways for molecules like water or elements like carbon,

nitrogen and phosphorous to move through all of the earth's

various ecological and geological compartments.

Now, of course we couldn't possibly talk about how all matter gets

cycled around Earth in one video, because the earth is pretty big.

But consider this an introduction to biogeochemical cycles,

starring my two personal favorites...carbon and water.

Now, I'm sure you already know about at least one kind of

planet-wide recycling, because it's the most obvious to us:

the hydrologic cycle, which describes how water moves on, above,

and below the surface of the Earth,

driven by energy supplied by the sun and the wind.

In talking about the hydrologic cycle, it's most useful

to think about all the water on Earth being held

in a series of reservoirs: the ocean, for instance,

or the atmosphere in the form of clouds, or in polar icecaps.

So not only does water cycle through different places,

it also takes different forms at different places in the cycle:

liquid, solid or gas.

And since it's a cycle, there is no beginning and there is no end,

so where we start our discussion is arbitrary...

but we're going to start it off with precipitation:

rain, hail, snow, sleet, graupel... all that stuff is precipitation.

It happens when water that's being held in the atmosphere

condenses, or turns from gas into a liquid,

and then occasionally freezes into a solid, right up in the air.

The opposite of condensation, of course, is evaporation,

the conversion of a liquid into gas.

And when a substance converts straight from

a solid to a gas, that's sublimation.

And when it's from a gas to a solid...that's deposition.

And now you know! But back to condensation.

It's responsible for the formation of clouds,

which happens when air containing water vapor rises and cools,

or is compressed to the point that it can no longer be a gas.

At this point, the vapor forms droplets.

This is the same thing you see happening on your glass

of iced tea on a humid day: the water in the air around

the glass gets cold and turns from gas into liquid.

So, a cloud is just a big pile of condensed water droplets.

In a sense, it's a gigantic, floating reservoir.

Clouds are a handy feature of the hydrologic cycle because

as they drift over the landscape, they move water around

the globe, so water that evaporates over the ocean

can be deposited somewhere else.

Otherwise, if water always got deposited right where it

evaporated, the precipitation would be almost all right

over the ocean, because that's where most of the evaporation

on Earth takes place.

So, wind moves clouds, and as water keeps condensing,

clouds get heavier and heavier until our old friend

gravity takes over and pulls the condensed droplets to the

ground in the form of rain.

Or in the form of snow or hail or sleet or graupel.

So now the water's on the ground, but gravity continues to work on it,

pulling it toward it's resting place, whatever that might be.

It either pulls the water across the surface

of the land toward the lowest point, in a process

called runoff, or it pulls it underground.

Water can be trapped or stored for a little while in places

like lakes, ponds, and wetlands, but most of the water

that falls as precipitation gets pulled lower and lower and lower

as runoff through the creeks,

streams and rivers until it reaches the oceans.

Now, in really cold places, water of course freezes,

and hangs around as ice in certain places for thousands of years

at a time, like at the poles, in glaciers, and on mountaintops.

But when it melts, most of it, too, runs off into the oceans.

So, you see where this is going: Oceans are a big deal.

They're pretty much the biggest deal.

They're the reason that we have the hydrologic cycle

in the first place.

They're also the reason we have awesome stuff like weather

and life on Earth.

The weird thing about oceans though is that they're salty.

And there's a reason for this!

As water runs to the ocean, it erodes minerals like salt

from soil and carries it to the ocean.

Now water heading to the ocean might not taste salty,

but the salt's in there.

But here's the thing, when the water evaporates again,

the salt doesn't evaporate with it. It get's left behind.

You keep this up for a few billion years,

with pure water evaporating from the ocean and then returning

with tiny amounts of salt, and that's your recipe for

a billion cubic kilometers of brine.

And all of this shows that the world's oceans are literally

the last stop for all the liquid water on Earth.

The only way to get out of there is through evaporation,

and that leaves all your minerals behind.

Now, living things also have their role to play

in the hydrologic cycle.

In both plants and animals, the breakdown of carbohydrates

to produce energy produces water as a waste product.

So we lose water through evaporation from our skin,

we also exhale water vapor, and of course we pee it out.

Indeed, most organisms on earth are made mostly of water,

although that water cycles in and out of us pretty quickly.

In plants, water is sucked up through the roots and moves up to the

the leaves, the gas exchange organs, where it evaporates quickly.

This process is called evapotranspiration, and since there

are so many plants here on Earth, it's responsible for a

good amount of the water that enters the atmosphere.

This process is essentially the opposite of condensation,

in that it turns liquid water into gas.

The energy of the sun drives evaporation, whether it's

from the surface of the ocean or from treetops and leaves.

And then once all that water evaporates into the atmosphere,

we're right back where we started!

It's a cycle!

So, now that you know a little bit about the hydrologic cycle,

it's a little easier to understand how the carbon cycle works.

Carbon is one of the most abundant elements in the universe,

and here on Earth, it's always on the move, just like water,

jumping from one reservoir to the next.

And that's a good thing, because

A) all living things require carbon for their structure

and to fuel their bodies,

and B) it's a big component in a bunch of nonliving things

as well: it's in rocks, in the ocean, trapped in ice,

plus it's in the atmosphere, where it helps regulate the temperature.

Without carbon dioxide, Earth would basically be a frozen wasteland.

So lucky for us, there's a whole pantsload

of carbon out there, because we need it.

Let's start out with the carbon in living things.

If you were to take all the water out of your body,

carbon would constitute about half of what remained

in the little pile of dust that used to be you.

And the first biological carbon reservoir is plants.

They absorb a bunch of carbon dioxide out of the atmosphere

because they need it to photosynthesize.

But CO2 is also one of the byproducts of respiration,

a process by which they use that energy.

So, plants take in carbon dioxide from the atmosphere

during photosynthesis, and then release CO2 back out

into the atmosphere during their respiration process

to make ATP for all their cellular functions.

And right now you're like,

"waywaywaywait... isn't the deal that plants get to take in

the carbon dioxide and animals get to breathe it out?"

Well, yes and no.

It's just that plants take in more CO2 from the atmosphere

than they give off through respiration.

The rest is, like, their profit.

It's what becomes the body of the plant.

That's right. That big old massive tree.

All of that mass came from gas. Pretty cool.

So, carbon absorbed by plants has three possible fates:

it can be respired back into the atmosphere,

it can be eaten by an animal,

or it can be present when the plant dies.

And if a tree falls in the right kind of forest

and it's not allowed to decompose normally because a bunch

of other plants all fell right on top of it, and they die,

and get buried, and squish together and form rocks like coal.

We call these carbon-rich geological deposits fossil fuels.

Lately, one of humanity's very favorite pastimes is digging up

all of this old carbon in the form of coal, and oil and natural gas,

and burning it to fuel our our pretty-much-everythings.

But I'll get to that later.

Another extremely important carbon reservoir is the ocean.

Now, carbon dioxide dissolves really easily in water,

and once it's in there, a lot of it's used by phytoplankton,

tiny plant-like organisms that form the base

of the marine food chain.

They use carbon in photosynthesis and they also use it

to make calcium carbonate shells, and when these guys die,

their shells settle to the bottom of the ocean, pile up,

become compressed, and over time, make rocks like limestone.

Now, limestone obviously doesn't burn super well,

so it's not considered a fossil fuel.

But as limestone deposits are eroded by water,

the calcium carbonate is broken down to eventually form,

among other things, carbon dioxide and carbonic acid.

We make lime and cement by heating limestone,

which produces a pretty good amount of carbon dioxide.

And when we do burn fossil fuels such as coal, petroleum products,

and natural gas, it also releases carbon in the form of

carbon dioxide that's been stored for hundreds of millions of years

in the geosphere, which is just a fancy, sciency word for earth rocks.

This process is what started the atmospheric carbon dioxide

levels rising like crazy in the past couple hundred years.

And the excess of carbon dioxide in the atmosphere causes

global climate change, because CO2 in the atmosphere prevents

some of the Sun's energy from re-radiating back out into space.

So, yeah, our planet is getting warmer because we've been

burning through this massive reservoir of carbon that

we had locked underground.

This is causing all kinds of problems that we can see already,

and it's very likely going to keep causing bigger

and bigger problems with time.

And the situation could be helped a lot if we would just stop

unlocking all that carbon and spitting it into the atmosphere,

but in some respects, we don't even have control

of the situation anymore, because of ice.

Remember how I said that carbon is often trapped in ice?

Well, in places like Siberia, Northern Canada and Alaska,

cold places that also have plants, they contain huge carbon

reserves that are trapped in permafrost,

ground that's frozen year-round.

These places are basically frozen wetlands that add another

layer of dead plant matter each year.

But as permafrost melts, these dead plants decompose and

huge amounts of carbon dioxide and methane are released

into the atmosphere, creating a positive feedback loop:

our carbon-burning lifestyles unleashing this other