Importance of Natural Resources

Climate Science 101: Fundamentals of Climate Science


Thanks again everyone
for coming out here. So I’ll be focusing
on the climate science part of these lectures. I’ll be doing a talk today
and another talk tomorrow. The one tomorrow
will build on what we’re going to learn today. And introduce myself,
I’m Justin Bandoro. I’m a PhD student in the
Program in Atmospheres, Oceans, and Climate. So that’s course 12 here at MIT. And my area of focus is
on atmospheric science and has to do with the layer
of the atmosphere called the stratosphere, which
we’ll learn more about today. So the purpose of
these lectures is to develop a broad understanding
of the Earth climate system. So referring to what
Christoph is showing with his whole
model, we’re going to be learning about the
science for the Earth system part of that– so the Earth
system, so focusing on that. And so today, we’re going
to focus on the first bullet point, which is to develop
a broad understanding of it. And then tomorrow,
we’ll look at how the climate system can
respond to different natural and human caused changes. And like you said, I’m not
focusing on the policy, economics, or governance. These lectures are
on the science. And today’s topics–
we’re going to touch on the history of
climate science, and then go into the structure
and composition of Earth’s atmosphere, and then look
at Earth’s energy budget and how greenhouse gases can
affect Earth’s energy budget and warm the surface,
and then look at different lengths
of variability in the climate system
that range from anywhere from a year to hundreds
of thousands of years. And on the last topic– will be
on the emissions of greenhouse gases and their long-lived
persistence in the atmosphere. So this is a cool
animation developed by scientists at NASA. So this is going back to 1850
and looking at the temperature change since 1850. And as it goes around in
the circle, it will repeat. It’s showing the months
around the circle, and then showing the
temperature change since 1850. So you can see, it’s going out. And the color code is– purple is relatively colder,
and yellow is warmer. So you can see,
as it progresses. There’s two important
things to note. The first is that some years
you can see it contract inwards and comes back
outwards, which shows the variability in the system. So it’s not saying
that every year is getting warmer and warmer– other years where it spreads
out and goes inwards. But you can see the
overall trend where the Earth has warmed globally. So, yes, this is
a global average. It is globally around
0.8 degrees Celsius. So today, we’re going
to try to understand what could be causing this. So to start off with the
definitions of climate– the popular
definitions could be– it’s the average of weather. The other one is
it’s what you expect, and weather is what you get. But here, the climate is
the statistics of weather. So over time, you accumulate
more and more information– you can get the mean of
the weather, but also the variability. Yes, so it’s aggregations
over time scales in more than one year and so
that the seasonal cycle is not considered. And examples of
climate variability are of what most of
you have probably heard of is El Nino, La Nina. So I’ll talk more
about this later, and that just has to
do with the warming or cooling over the Eastern
Pacific Equatorial Ocean. And that affects weather
all across the globe. And then, another length
scale is the Little Ice Age. So that was around in the
1600s, where the Dutch were skating up canals to work. And this is just
an example of where they could be periods that are
colder or warmer than others. And then, along a
longer time scale, you can have these glacial
cycles, which are anywhere from 20 to 100,000 years. And for an example of this, this
is where a glacier is covered, a large part of the
Northern– or the North Hemisphere– or
sorry, North America and reach all the way
down here to Boston. To start off with,
we’ll just dive into the history
of climate science and the Greenhouse Effect. We’ll get into the
science behind it, so it’s just that some
gases in the atmosphere absorb infrared radiation,
and they re-emit it back down to the surface,
which causes a warming effect. But this was actually first
known in the late 1700s by John Fourier. And he first understood
was going on, that these gases would
absorb it and re-emit it. So it would get warmer. But it wasn’t first
quantified until John Tyndall in the mid 1800s, where
he built an apparatus that could actually measure,
quantitatively measure, how much absorption there is
from certain greenhouse gases. And then, continuing on– this was Svante Arrhenius. And in the late 1800s and
early 1900s, he actually– without any of
the global climate models we have these days– was able to figure
out that, or estimate, any doubling of the percentage
of carbon dioxide in the air would raise the temperature
of the Earth by four degrees. So he knew this, or he
was able to estimate this in the late 1800s. And surprisingly,
this number here, which we’ll learn about in
tomorrow’s lecture of climate sensitivity, is pretty
in the middle of what estimates we have today. And lastly, this man– Milutin Milankovitch–
in the early 1900s– he solved the mystery
of the Ice Ages. So ice ages occur because
in the Northern Hemisphere, it receives more or less solar
insulation during the summer season. And the reasons for this have to
do with Earth’s eccentricities. So if you think about how
circular Earth’s orbit is– because it’s not
a perfect circle. But how circular it is,
or the eccentricity, changes with a period
of 100,000 years. And there is also the obliquity. So because we have
seasons, Earth’s axis isn’t exactly
perpendicular to its orbit. So it’s tilted, and
this tilt varies around 2 and 1/2 degrees. It’s around 23 degrees, but
it can vary up to 2 degrees. And that has a period
of around 41,000 years. And lastly, there’s
also precession. So that’s how much it
wobbles around its axis. So you can think of
it as spinning a top. And when the top
is about to die, you notice that it
starts wobbling around. That’s what’s called precession. So all of these together– the combination of all these
41,000, the 20,000, the 100,000 year cycles together– that can explain the Ice Ages,
because it has differences in how much solar
insulation the Earth is receiving in the summer
season in the Northern Hemisphere. What was obliquity again? Oh sorry– so obliquity– so Earth’s axis
isn’t perpendicular, so it’s actually tilted. So when it’s going around the
sun, it’s actually tilted. Well, it’ll stay
that constant tilt. Yeah, so it stays in
that tilt, but then, the amount of that tilt varies. Oh, it varies. So that’s the obliquity factor. Yeah, so it varies by around
two and a half degrees. And that will– if
you think about it– depending on which
season you’re in, you’ll receive more
sunlight than depending on if it was tilted in
the other direction. Great. So we’ll start with the
structure of the atmosphere. So the atmosphere is
divided, commonly, into four different layers. And so the troposphere, which is
also called the weather layer– that goes from the Earth’s
surface to about 10 kilometers. What happens? The cloud tops– they only
get up to 10 kilometers here. And right under here is usually
the level that planes fly at. So this is showing temperature
with respect to height. Kilometer is on the left, mile
is on the right-hand side. And you can see
temperature decreases with height in the troposphere. And then, the
second layer, which contains both the stratosphere
and the mesosphere, is the middle atmosphere. And the stratosphere–
what’s interesting to note is that the temperature
turns around and starts increasing back with height. And this has to do
with the ozone layer, that you all have
probably heard of, which peaks around 30 kilometers
in the stratosphere and ozone absorbs solar UV radiation. And that’s what causes
this layer to warm up. And then, you notice these
things called the tropopause, the stratapause and mesopause. These are just points where
the temperature turns around, so it turns from
cooling with height to– or warming with height. And then, in the mesosphere,
it decreases back again. Then finally, way up
over 90 kilometers, we have a thermosphere, where
temperature turns back around and increases with height. And this has to do
with the interaction with the charged particles from
the sun that heat up this area. But for these lectures,
you don’t really have to consider the mesosphere
and the thermosphere. And we’ll only be focusing
on the troposphere and the stratosphere,
because that’s what’s important for climate. And another thing to
note is the atmosphere is very thin compared to
the radius of the Earth. So if you look up here, going
from 0 to 130 kilometers, the radius of the Earth is
around 6,000 kilometers. So it’s very small compared
to the radius of Earth. Sorry, which region
are the satellites? So the satellites– depending
on which orbit they’re in, they can– some of them are right
at the low end of the– above the mesopause. And some of them can
be way up higher. So this is showing the
temperature climatology of the Earth. So it’s showing the average
temperature for a given month. So this is going through
from January to December. So this is averaged
over 40 years. And the color bar– this is well below freezing. And then, hot and red is
maybe around 20 to 30 degrees Celsius. So it’s showing how much– just the average
temperatures over the globe. And what you notice is that
this belt of warm temperatures around the tropics– it shifts northward or southward
depending on the season. And then, in certain areas– like in Siberia– you can see
changes of around 50 degrees Celsius from summer to winter. While in the tropics,
some of these regions only have a change of around
3 degrees Celsius. So you get more extreme
variations in high latitudes in the Northern Hemisphere. And the other thing
you might notice is that in the
Southern Hemisphere, the changes aren’t as drastic. As you can see over here,
they’re very drastic. It’s not as drastic over
the Southern Hemisphere, and that’s because a
large portion is ocean. And in the Northern
Hemisphere, is where you have more of the land. So the ocean damps out
the seasonal variability in the Southern Hemisphere. And this is showing
the same thing but for sea surface
temperatures. And you’ll notice
again, you have the warm belt over the tropics,
but it’s not zonally symmetric. So what that means is there’s
regions where the ocean is much warmer than others. And other features to note– you can see the Gulf
Stream coming up here for North America, that brings
warmer water up our East Coast, and over to Europe– and the Kuroshio
Current in Japan. So this is just to show
broad view of the average, what we think contains average
temperatures on land and also in the ocean. So next part we’re going to look
at is atmospheric composition. So this is a pie chart,
and blue is nitrogen, N2. And red is oxygen, O2. And the first thing
that stands out to you is that most of the atmosphere
is nitrogen and oxygen. So 78% nitrogen, 20%
oxygen. And you only have this tiny sliver here. 1% is argon, and you
have this tiny sliver is here with water
vapor, which is 0.4%, and these trace
gases, which include carbon dioxide, methane,
and other inert gases– and they also have
ozone and nitrous oxide. Sorry, this is everything
below stratosphere? Yes, there’s not much
above the stratosphere. 80% of the masses
in the troposphere– so there’s not much above there. But is the composition the
same going through the– I’m going to touch on that next. You’re right. So concentrations of
nitrogen and oxygen are relatively
constant with height, but you can get very
drastic variations. So the top one here shows water
vapor concentrations, which are also called mixing ratios. And it’s a log scale down here. And it has temperature, which
is the solid black curve. And then water vapor
is the dotted curve. And you can see, water vapor– because this is a log scale– the concentration of water
vapor drops exponentially with height. And this has to do with
temperatures cooling. So the amount of water vapor
you can hold in the air depends on the temperature. So as it gets
colder, water vapor precipitates out
of the atmosphere. And so that’s why these
concentrations drop. And another one that changes
with height is ozone. And so like I was
referring to before, we have the low
ozone layer here, and it peaks in
the stratosphere. And this is what’s called good
ozone, because it protects us. You also shouldn’t confuse it
with the ozone from pollution in the troposphere, which
is bad for health effects. So that’s just
another example that– nitrogen and oxygen are
relatively constant with height concentrations. Some of these gases
vary in the vertical. And it also varies temporally
as well– so not just in height but over time. So this is showing from
CO2 from the Mauna Loa Observatory in Hawaii– concentrations of
carbon dioxide. That’s also measured in parts
per million or mixing ratios. That’s how much you
get given volume. And you can see, this is
going back from late 1950s, when it started,
up to present day. And you can see the
increase in CO2 over time. But what you also notice– the black curve is the average. So it’s like a low-pass filter
or just averaging over years and moving through it. But we can also see,
there’s these variations that are occurring every year. And this has to do with– in the Northern Hemisphere,
as I was pointing out before, there was more land in
the Northern Hemisphere. So in wintertime
there, CO2 levels peaked, because you have
less photosynthesis. So that’s why you can see that
it’s also changing seasonally, as well as overall
increasing trend. And it’s getting up over
400 parts per million. So the next thing we’re
going to talk about is Earth’s energy balance. So we have the sun, and
it emits a luminosity. The power or energy
that it emits is 3.9 times 10 to
the power of 26 watts. To put that into
perspective, you’re household light bulb puts
out around 100 watts. So just showing the order
of magnitude of what’s coming out of the sun. You can also think of this
in terms of radians, which is the flux of energy for area. So if you just looked
at a certain area, right on the outside of the
sun, which is the photosphere, you’ll measure 6.4
times 10 to the seven watts per meter squared. So that’s how much energy is
passing through the surface in surface area. That’s based on the
surface of the sun– Surface of the sun. So this is the radiance– just the outside edge
of the sun, which is called the photosphere. And then, because the
inverse square law, which says that intensity
drops as a factor of 1 over the radius squared– so as you get further away,
the intensity or the radiance diminishes, as you
get farther away. So the separation between
the Earth and the sun is around 1.5 times ten
to the power of 11 meters. And you can see, once it gets
to the top of the atmosphere, the radiance or intensity
reaching the Earth is 1370 watts per meter squared. And this value is referred
to as the solar constant. And this, as we’ll see later– the output from the sun also
has an 11-year cycle with it. Yes? Is that what actually reaches
the surface of the Earth? No, sorry, this is what’s at
the top of the atmosphere. I’m going to touch on that next. So the total absorb radiation– so if you take that
solar constant and– if you think about it, the
surface that the sun sees is just a circle,
at any given time. It just sees a circle. And the area of a
circle is pi r-squared. And you can use the
radius of the Earth, and you can multiply
the radius of the Earth by the solar constant. And that will give you the
total absorb solar radiation. But you also have to take
into account the albedo. So the albedo is a
fraction of incident or incoming solar radiation
that is reflected back to space. And this can be from
clouds, can be from ice. So the average
value is around 0.3, so 0.3 of the incoming solar
radiation is reflected. So it never touches
the Earth’s surface. So you have to take
this into account. So you do 1 minus ap, so
that’s going to give you 0.7. So this gives you the total
absorb solar radiation. And then, you have to think
about that total absorb solar radiation as distributed
all over the Earth. So if you think
about Earth model it as a sphere, which is pretty
good, the area of a sphere is 4 pi r-squared. So if you take this value and
divide it by four pi r-squared, this gives you the absorption
per unit area– so the energy per unit area that’s averaged
over the whole planet. So we can think about
what temperature the Earth would be if we
didn’t have an atmosphere. So if we didn’t
have an atmosphere, we can estimate Earth’s
surface temperature by using Stefan
Boltzmann’s law, which has to do with the black bodies. So black body is
a theoretical body that it’s a perfect emitter
and a perfect absorber. So all incident radiation
upon it, it absorbs it, and then, it re-emits it. So that’s why it’s
called a black body, because it has no color. And so if you think about it– what Stefan Botlzmann’s law
tells you for a black body is that f is the
radiation that’s emitting is proportional to
this constant times the temperature of the body
to the power of a four. So this is a total
absorption per unit area. And so Earth has
to be in balance, so that what’s
absorbed is emitted. If you take [INAUDIBLE]
and you plug in sigma t to the power of four, and
you solve for the surface temperature, which is
the effective temperature of the Earth, this
value comes out to around 255 Kelvin or
minus 18 degrees Celsius. So if we didn’t
have an atmosphere, this would be the temperature. And as we all know,
that’s way too cold. And the actual observed
surface temperature is 15 degrees Celsius. So this tells you
the atmosphere has to be important,
because else it would be much colder than it is now. So what can be
contributing to this? So coming back to
John Tyndall, he first measured the infrared
absorption of atmosphere gases. So I keep talking about
infrared, or IR, so why do you keep focusing on
this spectrum of radiation? And this has to do with– again, coming back
to the black bodies. Depending on the temperature
of the black body– so [INAUDIBLE] showing the
sun at around 6,000 Kelvin. So Earth, which is
around 303 Kelvin. So the peak in the wavelength
of emission on the black body is inversely proportional
to its temperature. So the sun, which is very hot,
emits at a very small peak wavelength. So this is showing you the
visible spectrum of light. And its peak is
right in the middle, right at the edge of the
spectrum of visible light. While Earth–
since we just saw– if you take the Earth as 303
Kelvin, around 15 degrees Celsius, its peak is
around in this region here, which is the infrared–
so just under 10 microns, it peaks at. So that’s why we’re interested
in infrared absorption or absorption of
atmospheric gases. That peak at 303
degrees Kelvin– is that [INAUDIBLE]? Sorry, so this is showing
the black body spectrums for two different objects– one that’s at 6,000
Kelvin, which is– it’s hard to tell here
that’s the red one. And then, this black
one or blue one– Oh, it’s two different curves. It’s two different curves, yes. So this one, which is
blacker blue is at 303. And this one, red one,
is at 6,000 Kelvin. So his main conclusions were
that nitrogen and oxygen are transparent in both
infrared and solar radiation. So these spectrums we see here– both nitrogen and oxygen– they don’t absorb from any
of the solar radiation, and they don’t absorb any of
the outgoing infrared radiation from the planet. However, there are
certain molecules, like the trace gases I
was telling you before, that only make up a fraction
or a small fraction of Earth’s composition that are incredibly
important, because they absorb in infrared. So this is water vapor,
carbon dioxide, ozone, and some other gases. And he speculated
how fluctuations in water vapor and CO2 could
affect Earth’s climate. So this is a complicated figure,
but I’ll walk you through it. So at the top again, this is
similar to what I just showed. The solid red line is showing
the black body spectrum of the sun that you would see
at the top of the atmosphere. So this is if you’re right at
the edge of the atmosphere, and you’re looking at the sun,
what spectrum you would see. So don’t focus on this part yet. The red curve is– you’d see at the top of the
atmosphere from the sun. And again, you can see
it peaks in the visible. And there’s a
little [INAUDIBLE].. And it goes from the UV. It goes down. And then, these
three curves here are the black body
spectrum that you’d expect if you were
standing at Earth’s surface and looking down. So if you’re just looking
down at Earth’s surface, what would you see coming up? And these are the blue curve,
which is around 303 Kelvin. And then, the other
one is showing 210 versus the black one,
which is at 310 Kelvin. And then, we come back to
these solid filled parts. So for the one on
the left-hand side, this is showing what you would
see at the surface of the Earth if you’re looking up. So this is telling you how
much of the black body spectrum is absorbed by
atmospheres and the gas as the solar radiation
comes down to the surface. And you can see
here, this is just showing the total percent
that’s absorbed and scattered. And then, it shows that for
each gas or scattering process. So a large portion,
as you can see, is absorbed by oxygen and ozone. And so this is in the UV, so
the very small wavelengths. So that’s what’s absorbed. So it doesn’t get
down to the surface. And then, there’s also what’s
called Rayleigh scattering. So this is scattering,
because these wavelengths are very similar to the size
of molecules in the air– so nitrogen, oxygen. So they
scatter the incoming radiation, so it doesn’t even get to
the surface and scatters it back outwards. And so you can see, if
you look at the sun, you can think of it as these
gases taking chunks out of the black bodies. So they are taking
these chunks out of it. So what we see at the
surface ends up being this. That Rayleigh scattering? Yes, it scatters at the
very small wavelengths. Now you have those
charts listing oxygen and ozone and methane. Yes, first I’m just
trying to point out what absorbs the solar radiation. So you can see, some
of these molecules– water vapor absorbs some of
the incoming solar radiation, as well as– carbon dioxide has
a little band here. But a large portion is
absorbed from oxygen and ozone and scattered back out
from the atmosphere. I’m trying to understand the
difference between Rayleigh scattering, because you
mentioned that’s from– Sorry, yeah, so
Rayleigh scattering– –the size of the molecules. How is that different from– It’s not absorption. Oh. So here, yeah,
I’m showing total, so it’s taking the total
absorption and scattering. So absorption would be
oxygen and ozone absorbing the incoming solar radiation. But scattering is just when– you can think of it as bouncing
off and just reflecting. So it’s sent away
from the Earth. Yes, so it’s sent away, so
it never reaches the surface. And this is because
it’s preferential, it’s smaller wavelengths. And coming back, on the
right-hand side, you can see– so this blue solid
curve is what’s seen at the top–
so say if you’re at the top of the atmosphere,
and you’re looking downwards, what spectrum would you see? And can see, it’s very
different from what you’d see at the surface. And this has to do
with water vapor. So you can see, water vapor
absorbs very strongly. It’s actually the strongest
greenhouse gas in the infrared. So it absorbs a lot. But you also have
carbon dioxide, and then other constituents like
methane and nitrous oxide, which absorb, and IR. So this part here is what’s
called the atmospheric window, so some of the infrared
radiation is allowed to escape. But that’s only 15% to 30%
of it, while almost all of it is being absorbed
by the atmosphere. And like I said, water vapor
is the strongest absorber. And this is the same that
John Tyndall concluded. And another important
feature is that the carbon dioxide and the water vapor
bands don’t completely overlap. So any increase in
here, in carbon dioxide, will absorb much more of this
outgoing infrared radiation. So now, with this information
on greenhouse gases, we can go back to that model,
or that simple model we had with no atmosphere, and
then add in an atmosphere and see what happens to the
Earth’s surface temperature. So we have to make some
assumptions to do this. The first one is that the
atmosphere is completely transparent to solar radiation. So all incoming solar radiation
gets down to the surface. And then, the atmosphere is also
a opaque to infrared radiation, so that all outgoing
infrared radiation is absorbed by the atmosphere. So the infrared emission
is from the surface and from the atmospheric layer. And here in this model,
it’s a simple model, we’re only considering one
layer, so a single slab of the atmosphere. So you can think
about it like this. So coming back, this
is the same value we had as the radiation
that gets down to the surface of the Earth. And then Earth absorbs
that, and then, it radiates at some temperature outwards. Then, you have the
atmosphere here, which absorbs all the
outgoing infrared radiation and radiates it back,
both downward and upward. And then, you have to
take that into account when we’re going to
calculate the surface temperature of Earth. But just to figure out what
temperature the atmosphere should be at– so this value, as we found
out before, was 255 Kelvin. And so in balance,
the radiation entering must equal to the
outgoing radiation. So this means, just by that, the
temperature of the atmosphere is now the effective or emission
temperature of the planet. So we know the temperature
of the atmosphere. And then at the surface,
we have this balance, so that the radiation
the surface is emitting is equal to both
that of the downward from the atmosphere and also
the incoming solar radiation. And if we rearrange
this equation, we can solve for the
surface temperature, and we get a value of
around 30 degrees Celsius. And this is better than
before, but it’s a bit too hot. And this is because of
the assumptions we made. So assumptions
again– like I said, there’s a window where
some of the IR radiation is allowed to escape. And not all the
solar radiation that comes down to the surface is
absorbed by the atmosphere. And you also have
other processes that can transport
heat from the surface that I’ll touch on later, that
are convection and conduction. You mentioned there the
temperature of the atmosphere, and then, you talked about
the temperature of the Earth. But which is controlling that? Is it the temperature of
the Earth or the atmosphere, or is it a combination of both? So you can think of it as– these all have to
be in equilibrium. So the incoming
radiation has to equal to the outgoing radiation. And through this
simple assumption, this means the temperature
of the atmosphere now has to be equal to the
same value we had before, in the case with no atmosphere. And then, why–
because the atmosphere radiates both upward and
downward and longwave or infrared radiation. The surface now has– in addition to the solar
radiation coming in– the atmosphere radiating
infrared radiation back downwards as well. So you have to take
this into account. And that’s what causes the
temperature of the surface to go up to what we had before. So we can think about
this in terms of energy budget of the atmosphere. So this is more complicated than
that simple one we had before, but I’ll walk you through it. So this value at the top is
the incoming solar radiation. So this at 370 watts per meter
squared, divided by four. 340 watts per meter
squared is coming in. 79 of the watts
per meter squared is absorbed by the
atmosphere– the oxygen, ozone, and some other gases. And then, you know, I was
talking about the albedo before, so this is
what’s represented here. So around 100 watts total is
reflected back out to space. So this has to do
from both clouds. So there is a contribution
coming from both clouds, then, there’s a contribution
from the surface, from ice, snow, and
other areas that reflect the incoming
radiation back to space. So out of the 340 total, 100
is lost or reflected back out to space, which means that only
161 is absorbed by the surface. And then, you can see– so it’s coming down. And then, you can also
see that at the surface, it’s radiating 398
watts per meter squared, back up into the atmosphere. And then, what’s
happening is that it’s absorbed by the atmosphere. It radiates it back
downwards, and then balance. These are the values
of both of them. And then, you have
this portion here, which is the atmospheric window,
where some of the IR radiation is allowed to escape back up. And then, the total
outgoing thermal radiation is around 240. And if you add the
240 up with the 100, you get back up, so that
they’re roughly equal. So the incoming is
equal to the outgoing. And then, you also have
these processes that– radiation isn’t the only way
you can have energy coming up from the surface. You can also have
evaporation and, also, sensible heat, which is
a conduction from just having these layers in close
contact with each other. Energy is transferred. So this is just the overview
of the Earth’s energy budget. And these are all
measurements that were made, so either from satellites
or ground-based instruments that can measure the
fluxes of total radiation. And so we can see here that
if we add greenhouse gases to the atmosphere,
what happens is that it’s going to be
radiating more energy back down to the surface. And then, in turn,
the surface has to adjust by warming up
to [INAUDIBLE] background. All right, we’ll move on to
the next section, which has to do with climate variability. So there are two types of
natural climate variability. The first one is external
forcing of the climate system. So this has to do with changes
in the orbit of the Earth, which affects the amount of
solar radiation impinging on the hemispheres
during the summer. And like I said before, the
Milankovitch cycles can range from 10,000 to 100,000 years. And what’s also
considered external, even though it’s actually
in the climate system– it’s inert but it’s also these
large volcanic eruptions. So eruptions from volcanoes
will send sulfur dioxide up into the atmosphere that
will condense onto particles. And you can think of
these as little mirrors in the atmosphere that will
reflect radiation back. And I’ll touch on these
two in a little bit. Another one is the
solar variability. So the output from the sun– that 1370 value that
I was talking about before isn’t constant. It has approximately
an 11-year cycle. And then, the second source
of climate variability is internal. So all of these– you can think of
these as external. So if these didn’t happen,
the climate system– it wouldn’t change. But because of internal
climate variability, even without these, you get
these non-linear interactions in a complex system. And I’ll talk more about The
ENSO, or El Nino Southern Oscillation. Sorry, no, this is El Nino. Then, there’s also
another mode called the NAO, which is the
North Atlantic Oscillation. So these are just intrinsic,
internal variability in the climate system. So, yes, as I was
talking about before– how volcanic eruptions
can affect the climate– this is a photo taken from
Mount Pinatubo in 1991. You can just see all
of the emissions. And what happens
is that when you have these large
volcanic events, it inputs sulfur
dioxide up, up right at the top of the troposphere
and into the lower stratosphere. And then, these
condense onto particles and form sulfuric acid. And the, these
sulphuric acid particles are like– like I said
before– tiny mirrors. And then, they reflect the
incoming solar radiation back out to space. And this is showing an
example of how far you can see into the atmosphere. So it’s called optical
depth from a satellite, before and after the
eruption of Pinatubo. So you can see, just
a month after, you have the sulfuric acid
particles all over focused on the tropics. But then, as you get
later and further out, it covers the entire globe. And this causes cooling
of Earth’s surface. So what this is showing is– the solid black line is
observed Earth’s temperatures. And then also with
it is this red line, which is a model, and then lots
of simulations from this model. I’ll be talking more
about global climate models in tomorrow’s lecture. But what you can see
here is these lines are volcanic eruptions. So Mount Agung from
Indonesia in the 1960s. You have El Chichon. And then you also have Pinatubo. If you look here,
you can see a drop in global surface temperatures
following these eruptions, because these effect the
Earth’s radiation budget, where less radiation is coming
down to the surface because of these eruptions. And then, the other
climate force or mode of external variability
is from the sun. So it was discovered that you
have these sunspots that you can see on the sun– and that the number
of sunspots you see are related to– so this is
showing the sunspot number from 1600 to present day. And you can see the sunspots–
there’s some sort of cycle on it– these ups and downs going up. And these are 11-year cycles. So this is showing the sunspot
numbers along with the incident solar radiation. You can see it varies, because
it’s going up and down. And this is from the
1970s to present day. So you can see the
incoming solar radiation is related to the
sunspot number. So this is also another mode
of external variability, where either that or
changing the energy budget by how much radiation
is coming into the planet. And although it’s hard to see– so this is again showing
what I had before, but then, this is the
global surface temperature. And if you take an
11-year moving average, you can see that you
have these ups and downs in Earth’s temperature
that are related to incoming solar radiation. And then lastly, on
intrinsic climate variability or internal climate
variability, you have what’s called the El
Nino or La Nina oscillations. So this is showing the
average sea surface temperature anomalies for
December 1982 to February 1983. And you have a large warming of
the Equatorial Eastern Pacific. And then, you
compare that to this down here, where you have large
cooling of Equatorial Pacific. And these warming
and cooling phases can affect the weather
in North America but, also, all around the world. And because you have easterly– from the East– winds
blowing from East to West– and then, during the El Nino
phase, you weaken these winds. And this is related with
enhanced precipitation across the Equatorial Pacific. And in the US, because
that’s where we are, you have more rain in the South
and cooler winter temperatures in the Southeast. And then, in the La Nina
phase, which is the cool phase, you have stronger winds
along the equator. And that’s related to
a reduced precipitation across this region. But in the US, you have
less rain in the South, and winter temperatures are
warmer in the Southeast. And this is because
the atmosphere– you can think of it like if
you hit a bell– so if you ring a bell in one part of the
atmosphere very strongly, you’re going to hear it
elsewhere in the atmosphere. So that’s basically
what this is. Because these are such large
changes in ocean temperatures that it effects the
atmosphere and circulation in North America and, also,
all the way as far as Europe. Wait, so are those
the same thing as the high end of the cycle? Is one of those the
low end of the cycles? Yes, so this is
the El Nino phase, where you have warm temperatures
over the sea surface. And this is La Nina phase, where
you have cooler temperatures over the surface. And here is showing the related,
all over the world, changes. So with the El Nino phase,
which we just had in 2016, you have much warmer– it shows you where it’s
warmer in the world. And then, La Nina
phase– it shows you where it’s cool and, also,
precipitation– where it’s more wet and more dry. And this is just to
show you that you can have these fluctuations in
temperatures, precipitation, and other conditions that
are intrinsic to Earth and not related to the external
changes and external forces. So these will happen without any
changes in external radiation coming into Earth. And what’s the primary drive? What causes those– That’s a very good
question, and it has to do with just these modes. So you have these changes
and upwelling and downwelling in the ocean. And so if you get enough stress
on the ocean surface over time, it’s going to
accumulate and cause changes in
circulation, which will cause both of these events. yes, up there? Yeah, also, it looks like
when the middle part is cooling or heating, the rest
of the ocean is still opposite. Yes, that’s true. So doesn’t that
balance it out somehow? Well, you have to think in
terms of what’s causing– they are called teleconnections,
or teleconnection patterns, and circulation. So you have to think
about how it changes the mean state of the climate. And in here, the mean state
climate are easterly winds. And so if you change the winds,
that will change precipitation. And so these guys are
also changing with it, but it’s predominantly what’s
going on in the tropics that’s affecting it, because even
though this looks kind of large, it’s right in here
where you get the most warming and cooling of the sea
surface temperatures. How long does it [INAUDIBLE] Unlike the sun that I
was showing you before, where it’s relatively
11 years, this changes from two to four years. So you can have a
year or two where it’s in the El Nino phase. Then, you can have
another year or two where it’s in the
neutral, so it’s normal. But then, you can have the
year, where it’s La Nina, and then, the next
year can be El Nino. So it’s not predictable
like the sun, where you have the 11-year cycle. It’s still has two to
four year cycle in it. So that’s why in
2016, where we had the record-breaking
temperatures all over the world, that was the El Nino phase
and where it’s predominantly associated with
warmer temperatures. So that was helping push it
way up above the records. And so the last part
I’ll quickly go through is changes in greenhouse gas
concentrations over time. So what this is showing–
the green is carbon dioxide. The orange is methane. And then, the red is
showing nitrous oxide. And it’s showing changes
in these three gases’ concentrations. And you can see some lines. These lines are when we
had direct measurements, so we can directly
measure these species. But these circles are what
we have to rely on beforehand from ice core. So these gases were
present in the 1850s. And then, you have these
bubbles and ice core data. And if you drill further
down into the ice, you can go further
back in history, and you can get observations
from back here, that are estimated from
the ice core data. So you can see, will all three
of these greenhouse gases, they’ve been
increasing with time– with the CO2 from
fossil fuels, methane. You also get it from combustion
but, also, from agriculture– you can get methane. And then, N2Os, also from
the agriculture, [INAUDIBLE] fertilizer where it’s
increased as well. So this is just showing
that all three of these guys have increased with time. And if we look at
the focus on CO2– so this is showing
gigatons of CO2 per years. So the emissions of CO2, using
the same thing that I had, over the same time
period I had before– and you can see, it’s showing
the changes due to forestry. So that’s deforestation. If you burn trees
and cut down trees, you’ll increase the amount
of CO2 in the atmosphere. But since the 1900s, the
large fraction of this is from fossil fuel combustion. Cement use, which also releases
CO2 and glaring or fracking from natural gas. So you can see, this is showing
the global anthropogenic CO2 emissions. And compared to 1750, and just
showing you the cumulative CO2 emissions. So if you integrate these
values over time up to 1970 and to 2011, you can
see how much of it is from fossil fuel emissions
and from land use changes. And one question
you might ask is how do we know that the increase
in CO2 in the atmosphere is related to fossil
fuel emissions. So you can use this
method of isotopes. So if you look at
the atom of carbon, there’s different
isotopes of it, where you have
different neutrons. So you can have different
neutrons and in the atom. And so you have C12,
which has 12 neutrons– C13, which has 13 neutrons– And C14, which has 14 neutrons. And carbon is present
in all living things. And life has a preference
for lighter C12 carbon. And this is because when plants
breathe CO2 and photosynthesis, they prefer C12. So plants absorb C12. And when they die,
they sediment. And then, eventually, and
same with all other living organisms, this
is what eventually goes to the fossil
fuels that we’re pumping out of the ground
or these past living things. If you look at the C13 to C12
ratio, what we would expect is that it should be
decreasing as we’re pumping more C12
into the atmosphere if we’re burning fossil fuels. If you look at observations–
so this again showing global emissions of carbon. And then, right here, it’s
flipped, so that going up is decreasing. So if you look at
the C13 to C12 ratio, it’s decreasing with time. So it shows that we’re changing
the ratio in the atmosphere. And this is from the
emissions of fossil fuels, or the combustion
of fossil fuels. So this is, again,
showing the carbon cycle and how much perturbation or
changes in the global carbon cycle are caused by
human activities. And you can see,
we’re putting out 34.1 petagrams,
which is one times 10 to the power of 12 kilograms
of carbon into the atmosphere. But the atmosphere has only
taken 16.4 of this number. And this is because the ocean
takes up the carbon dioxide from forming bicarbonate. And the ocean is becoming more
acidic by taking up the CO2. But there is a level, of
how much you can take up. But it’s continuing
to take it up. And you can also see
that the land is taking– the plants are respiring
some of the extra CO2 we’re putting up
into the atmosphere. Around 16.4 of it is
left in the atmosphere. And this is showing that
figure but over time. So if you look at
focus on the top first, this is what I had before
with gigatons of carbon or CO2 going up the that
we’re emitting. And if you look
at how much of it is going where– so
the dark blue is ocean, so it’s showing how much
the oceans are taking up. And then, the light
blue is the atmosphere, so you can look at the
atmosphere burden of CO2. And then, also, the
land sink– so much CO2 the land is taking up. And you’re going to see, over
time, a larger portion of it is going into the atmosphere. And the ocean is taking
up some of it as well. And the importance of this–
so this is showing for CO2– if we keep ramping up CO2– so from 1800s to– so this done with
model simulations, where CO2 is increased up to– so right now, we’re just at this
400 parts per million level. And then, if you keep increasing
it to 550, 650, 750, 850– and then, once you
reach this peak, you cut off all CO2 emissions. So you are not emitting
anymore into the atmosphere. What happens? How long does it take for
the CO2 to go back down? Even here, if you cut it off
at this level, it decreases. But then, it still stays
high, and much higher than it was in the
pre-industrial time. And you can see
that the more CO2 we keep putting into
the atmosphere, the longer it’s going to take
to get rid of all of this. And this is because there
are different processes that can take out the carbon
from the atmosphere. Like I was saying, you can
have the photosynthesis. And this is a short time
scale, from one to 100 years, but the oceans can take it up. But the sedimentation
of the carbon that first created fossil fuels
takes thousands and thousands of years. So this is calcium carbonate
sedimentation and, also, silicate weathering. But this is just to show
that all the CO2 we’re putting into the atmosphere is
going to take a very long time before it can even get back down
to this level we had before. Could you go to
the previous slide? Yeah. Why is there a big variability
between the three [INAUDIBLE] over time? So what I didn’t mention in this
is that the ocean part in here is– we measured the land
in the atmosphere. And the ocean is inferred
from the rest of it, from the variability. But the variability
has to do with– like I was talking
about before– this has to do with the
land use changes, where you have these large
deforestation events, coupled with the atmosphere,
where you can have– like I was saying– the El
Nino and other variations that affect plant growth. But, yes, that’s what
the interannual variation is due to. And, yes, so this
is just to show that it will take a very long
time to remove all this carbon dioxide, which is the
important greenhouse gas from the atmosphere. And every gigaton more
carbon we’re putting up, it’s going to take a
long time to remove it. So in summary, these trace gases
of water vapor, carbon dioxide, methane, and nitrous oxide– even though they are
a trace and make up a very small portion
of the atmosphere, they’re opaque to outgoing
to infrared radiation, and they’re responsible
for the greenhouse effect. And then, because of
the Greenhouse Effect, the surface must warm
to be in balance. So if we put up more of these
concentrations are changing– are increasing, decreasing–
the surface of the Earth has to respond by
cooling or warming– and that the variability of
the climate system can span anywhere from one to 100,000
years and increase in CO2 since the pre-industrial levels– is from fossil fuel
emissions and the removal of CO2 from the atmosphere
is a very slow process. And that’s it for today. And I’m leaving up here some
resources and good books, if you want to look into this
more, because I don’t have time to touch on everything
in detail today. But here are a few
books you can look at. And you can also check out
the Global Change website and look at our educational
resources we have online if you want to learn more. And tomorrow, I’m
going to be talking about how both human and
natural caused forcings and how the climate
system responds to it and how we can identify
the temperature changes that we’re seeing are
related to human activity. And I’ll take any questions
for a short amount of time, because I went over, but
I’ll take a few questions. Getting back to that El
Nino and its reverse, so what is the
fundamental cause of that? So that has to do with
changes in ocean circulation, where you have– because it was first measured–
because you had one station here and another station here. And they measured
changes in temperature. And there is a connection
between both of them. And that has to do with the
upwelling and downwelling changes in the
ocean circulation. And this is because the
changes in those circulations are from accumulated
stresses of wind. I was talking about
the easterly winds– if you keep stressing
the ocean over time, it’ll all accumulate
until it’ll just flip, and it reaches these
different equilibria– or these were stable. But to get from one stable
equilibria to another, it’s like a sudden shift. So you just get a shift
into these perturbations. If you look at some of
the resources I put up, it’ll give a more in depth
view of [INAUDIBLE] climate variability in here as well. Or see me after, and I can point
you towards some links on it. It’s not an easy process. You can do the whole
course on just trying to understand that modeling. Yes. You remember that you can
trace the increase in the CO2 in the atmosphere and
anthropogenic sources, because the C13-C12
ratios going down. What is it going down in? Just in the atmosphere itself–
like probes in the atmosphere? Right, you can take
a sample of air. So this is only since the
1980s, when you had technology to look at the isotope ratios. So if you take a
sample of air, and you can measure how many have
C12, how many are C13, how many are C14, you can see
how this ratio of C13 to C12 is changing over time. And because, like I said,
life has a preference for lighter carbon
because of photosynthesis, and the planets
preferentially absorb C12– and that’s what all the carbon
and the fossil fuels is. So for burning
fossil fuels, we’re emitting more C12
into the atmosphere. And so if you keep increasing
the amount of fossil fuel combustion, we’re going to get
a higher ratio of C13 to C12. What you’re not saying is fossil
fuels have lower C13 ratio, is that right? You don’t actually say what
the ratio is for fossil fuels. So, yes– if you look
at the C13 to C12 ratio, it has higher C12. So this ratio, if you’re
measuring C13 to C12 in the atmosphere, it should
be falling as we’re increasing fossil fuel combustion, because
we’re putting in more C12 into the atmosphere, because
that’s more in fossil fuels– the C12– because like I said,
the fossil fuels got there from an
existing life beforehand. And you say there is
a preference for C12. I mean, is that like mostly
C12 or is it like 2% more? I’m not an expert in plant
biology or this area, but there are people who
are experts in this area. And photosynthesis,
preferentially, takes up CO2 that is lighter– in the lighter isotope of CO2. But just in a gross
way, is this talking about a 50% increase in that,
or is it just a few percent? I do not know this. I’m not going to say that,
but I can look it up. If you want to talk to me
later, you can look it up. Does the Earth magnetic
force have anything– Oh, so changes in
the magnetic field? So that slide I had way
before of the atmosphere– so I didn’t point
it out, but there’s something called the
magneto sphere, which is above the thermosphere. So it’s the magnetosphere, and
that’s where the magnetic– like, where the flips, magnetic dipoles,
and magneticism is. Yes, so that’s taking a
lot of these ions that are coming from the sun. And I’m not sure
how much changes in the magnetic dipole of the
Earth affects the climate. I’m not sure how
much it, but I don’t think it’s negligible
compared to what we’re talking about here
within the troposphere and stratosphere. Yes, come tomorrow, and we’ll
talk about greenhouse gas, or how we can– forcing mechanisms and
feedbacks in the climate system. [APPLAUSE]


Reader Comments

  1. Excellent introductory lecture, from our lay perspective, we appreciate your inclusion of relevant equations in addition to weaving a terrific overview of the basic science. If you had time to record other lectures please post, or direct us to the links.

    Thanks for your effort!

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