Now let's talk about snow and non-linear feedbacks.
want to go back to the example we talked about earlier, we looked
at the balance between energy in and energy out in two cases.
If you have a planet that's mostly covered with
forest, it absorbs most of the light coming in.
The albedo is low. The planet will be hot.
You have a planet that's covered
mostly with snow. It's highly reflective.
Less energy is absorbed.
The planet will be cold.
We consider what that means for the surface.
There's actually a feedback loop.
Where if you have a planet that's cold, it has a lot of snow.
And it stays cold.
You've a planet that's warm.
It's mostly dry land that absorbs lots of energy.
That's
warm.
So you can see this is a situation that has two equilibrium.
The flux out scales this way.
The flux in has two branches, a cold branch and
a warm branch. The curves cross at two points.
Actually three points.
These two points here are stable
equilibrium, this one's an unstable equilibrium.
And there's two possible places where the planet could be.
What makes this point stable is that if I
increase the temperature a bit the surface stays dry.
The flux in doesn't change, but the flux out increases.
So if the planet temperature fluctuates up, it will radiate
more that will drive it back towards this equilibrium point.
Same thing is true here. The planet gets a little warmer, it
radiates more draws it back here, gets a little colder the planet radiates less.
As long as the planet remains completely snow covered or completely dry.
The planet properties don't depend on the surface temperature in
this regime and this regime that makes these two regime stable.
On the other hand this is an unstable solution.
If I have a planet that's partially dry and partially snow covered and I
increase the amount of snow by a little bit that decreases
the amount of energy absorbed, that makes it colder.
That decreases it more, that makes it colder.
This is an unstable situation.
What's a bit worrying is we live in that situation now.
And we can see this happening right now in the arctic.
If you look at the arctic, the arctic was once almost completely covered with snow.
Snow is highly reflective.
Most of the sunlight that hits the surface reflects back up.
The ocean water, on the other hand, absorbs, this
case about 93% of the energy that strikes it.
So that, as you decrease the amount of snow,
increase the amount of water, the polar regions absorb more energy from the sun.
They get hotter.
They got hotter, more snow melts, there's more water to absorb
the energy, its gets hotter and there's a runaway process that many people
fear is leading to the melting, the complete melting, of the polar ice caps.
We can see this happening in Greenland.
This is a map in 2000, of Greenland's albedo.
You'll notice that most of Greenland in 2000 was completely covered with snow.
Snow has a very high albedo so it was reflecting a lot of light.
Near the edges here where we have water some bare land it absorbs a lot of light.
Unfortunately, Greenland's snow seems to be melting.
You can see Greenland's albedo is decreasing.
It's absorbing more energy and many of us fear that Greenland is suffering from a
runaway effect, where it's albedo will decrease, it
will absorb more energy, it will get warmer.
More and more of it's snow
will melt, and we may see a very rapid heating of the whole polar
cap region, due to this instability and the increase in greenhouse gases.
Could this runaway have happened in the past and could it go the other way?
Could we have had a period of time where the amount of snow really increased?
The geological record seems to say, yes.
There seems to have been several periods of global glaciation, with the most recent
one occurring about 650 million years ago. Remember that the Earth
is about 4.5 billion years old. We live here in the present.
So in the history of the Earth 650 million years ago wasn't that long ago.
It was about oh, 85% of the history of the Earth had already passed by that point.
And these periods that were called Snowball Earth are a really
fascinating bit of Earth's history that
we've only begun to understand relatively recently.
To understand why Snowball Earth happened we have
to talk about something called the carbon cycle.
And understand what sets the amount of carbon dioxide in the Earth's atmosphere.
As we saw when we looked at what sets the planet's
temperature, an important term is the optical depth in the planet's atmosphere.
How much energy is retained and re-absorbed by the atmosphere?
In the Earth's case, a very important part of that story is carbon dioxide.
We look at the Earth's carbon dioxide content, we have to look at what
is the source of carbon dioxide, and what are the sinks of carbon dioxide.
An important source of carbon dioxide is volcanoes.
As volcanoes vent to the atmosphere
a significant fraction of the carbon dioxide on the planet comes from
volcanic venting. This carbon dioxide is returned
to the Earth through what's called silicate weathering.
Rain brings down water, combined with carbon dioxide,
weathers calcium silicates, produces calcium
ions that flow down as the rivers bring the weathered material to the ocean.
The calcium, the carbon dioxide, the calcium carbonates get absorbed in the
water, settle to the bottom of the ocean, gets subducted back into the plates.
Get returned to the Earth's interior and
the carbon dioxide is then returned through volcanoes.
If the system is equilibrium, the amount of carbon dioxide
going in through weathering balances the amount of carbon
dioxide going up through volcanoes. Now, this, this system
is not always balanced perfectly. The process
is over here of silicate weathering, and the rate of vulcanization.
A volcano with total carbon dioxide there somewhat decoupled.
And there may have been periods, we think there were periods in the Earth's history.
Where these two got out of balance.
So, we look back about
600, say 630, 650 million years ago,
the continents had arranged themselves, so most
of the continents, most of the land on the planet, was located near the equator.
Why is that significant?
Well, near the equator it's hot and it's wet.
There's going to be a lot of rain, a lot of silicate weathering.
So at this point in the Earth's history,
we had lots of weathering, lots of CO2
going into the oceans and into the Earth's surface.
But less, no particular increase in CO2 coming up.
What we think happened, is that this increase in silicate
weathering, decreased the abundance of carbon dioxide in the Earth's atmosphere.
Let's go back
to our picture here, where we balance the flux in from the star to the flux out.
What happens if we lower the amount of CO2 in the atmosphere?
Once we do that, the optical depth goes down.
That shifts the curve this way.
If this is where we are today, as we lower the amount
of CO2, we move in this direction. And you can see that we move
from a balance here, say, and as we
lower the optical depth, less CO2, the planet retains
less heat, and it could stand equilibrium at a lower planet temperature.
So that the temperature drops. The two curves
cross here, rather than here. Well, what if this process continues?
It can continue until the temperature drops so low that there is
no more equilibrium point on this part of the curve where you have mostly dry land.
You can be driven to the situation where the only stable solution
is down here.
Where the planet is mostly covered or, perhaps entirely covered with snow.
So if you have, have too little carbon dioxide you're driven to this situation.
Where the planet very quickly could become entirely covered with snow and ice.
And we find ourselves stuck down here in a world
that has a very high albedo because of all the snow
cover, and is very cold.
Many scientist believe that during the snow ball period of the Earth's history.
The mean temperature of the Earth was comparable to that of Antarctica.
The whole planet was like Antarctica. Snow covered and frigid.
Perhaps with tiny thin zones of, of slushy water
near the equator where life may have managed to persist.
This sounds like a pretty dreary situation and one that, if you just look at
this picture here, we seem to be, it looks like we might get stuck it.
For down here,
it's the smell.
There is not enough energy to warm things up
and to be driven back to the solution here.
Fortunately,we are saved by the fact that when the earth becomes snow
covered, the weight of red-, weathering drops tremendously.
We don't have rain falling on land.
We don't have this the calcium and carbonates flowing into the water.
This process of CO2 going from the atmosphere,
into the earth's surface, turns off. On the other hand, the volcanoes continue.
We can see this happening in Iceland, where we can see volcanoes go off beneath
thick layers of glacier. So we end up in a situation where CO2 goes
into the atmosphere
from volcanoes.
But is not returned because there's no weathering.
So the CO2 slowly starts to rise.
And our Snowball Earth planet starts to have
an increasing amount of CO2 in its atmosphere.
Now we start out here at low temperatures, but
as we increase CO2, the equilibrium starts moving to
a warmer temperature.
Now initially it doesn't make much difference, because
we find ourselves in this branch of the solution.
And the planet's still snow covered in cold,
but as the CO2 rises, the equilibrium point.
The point where this curve, from this term balances this term.
Shifts to higher,
and higher temperatures. And as we increase CO2, our optical depth
goes up, and our curve shifts this way at the given temperature.
With a higher optical depth, the flux out is smaller, so the equilibrium shifts and
eventually we reach this point where there is no stable solution on this branch.
Things get so warm that the snow starts to melt.
And in fact, the planet shifts way over here and becomes very warm.
What this suggests is a very violent melting, that we undergo
a very rapid period where the Earth's temperature shoots way up.
And there's some evidence that the Earth's temperature
may have been, of a mean temperature, perhaps, 20
degrees, maybe even more, warmer than it is today.
So we went from this very cold state, where we built up the CO2.
The CO2 got so high that the snow started melting.
Once it started melting, you start exposing a lot of dry land.
And there was a runaway melt that led to, perhaps, a very violent Earth.
There seems
to be evidence for giant waves associated with an enormous release of, of energy,
as all this snow melted, and the Earth's temperature zoomed, to a very high value.
We find things like what are called cap carbonates.
We seem to see thick layers of this carbonate rock associated with
this rapid sea level rise, as all the snow very quickly melted.
So, we've had pretty violent events in
the Earth's past. And you can see these violent events are
often driven by the fact that you can have these positive feedback's, that once the
snow starts to melt, that exposes a lot of water ocean water that can absorb energy.
Exposes more land that could absorb energy.
Makes the planet warmer, and
there's a runaway process.
These kinds of feedback's play a very important
role in determining the Earth's history and will
likely play an important role in determining the
history of other planets in our solar system.
And when we start to think about planets outside our solar system,
we have to imagine that processes like these are going on there.
When we look at planets that are 10 or
20 light years away, we right now do not have the detailed information
we need to construct this kind of detail history of how the planet evolves.
But we have to suspect that these simp, kind of processes
happen elsewhere, and that planets can actually have quite complex histories.
So that if you were a astronomer, sitting on a distant
star, 650 million years ago, observing the Earth, you would
see our planet, not as it is now, but as a snow covered planet.
Rather cold, and one that you might suspect never
held and would never hold life and you'd be wrong.
Lemme end there.
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