29. Global Warming (continued)

RONALD SMITH: The first
question was one of the most difficult ones
on the exam, I believe. And the point is that you were
given, first the Gulf Stream coming up along the east coast
of the United States. At a particular latitude, 35
degrees North, you are given information that this is 50
kilometers wide, has a depth of 1 kilometer, and the pressure
difference across it Delta P, is given
by 4000 Pascals. And you are asked to compute how
much water is flowing, the volumetric flow rate, in
the in the Gulf Stream. So the way I did it was first
to use the formula for geostrophic speed, which is
the pressure difference divided by the length over which
the pressure changes.

That's the so called
pressure gradient. And then down below goes 2, the
density of water, times the rotation rate of
the earth, times the sine of the latitude. That formula is given to you at
the top of the second page. You just need to cancel out the
volume and solve for U, and you'll get that formula
for geostrophic flow. So then what goes into this is
a Delta P of 4000 Pascals, a length of 50 kilometers, 50,000
meters, 2 the density of seawater is about a 1025. The rotation rate of the earth
is 7.27 x 10 to the minus 5. And the sine of the latitude,
it's going to be the sine of 35 degrees. When you put in all those
numbers, I got 0.9 meters per second for the speed of flow
in the Gulf Stream. To get the volumetric flow rate,
that's going to be the rate of the product of the
area and the flow speed. The area is given by
the product of the depth and the width. So it's going to be 50,000
meters times 1000 meters. So that's going to be 5 times
10 to, what the seventh? So when you put this value into
there and this value into there, I got about 47
x 10 to the sixth cubic meters per second.

That's the volume, how many
cubic meters of water per second are passing through some
line you draw across the Gulf Stream. I asked you to express
that I asked you to express that in sverdrups. A sverdrup is 1,000,000 cubic
meters per second. So this is 47 sverdrups. Any questions on that? Yes. Student: What is the–why is
that the same as the lenght? PROFESSOR: This is the,
well sorry. So this is a pressure
gradient. So it's how pressure changes
with distance.

So I may have messed up my
symbols here, but that would be that distance. I gave you the pressure
difference across the Gulf Stream. I gave you the width
of the Gulf Stream. And so the pressure difference
divided by that length over which the pressure changes
is the pressure gradient. That is units of Pascals
per meter. How much does this pressure
change per meter as you walk across or swim across
the Gulf Stream? Questions on that then? Student: So is the pressure
gradient force not pushing the air in the direction it's
moving, it's actually pushing it across– PROFESSOR: That's right. So presumably there'd be a high
pressure over here, low pressure over there. So if you were to do a little
force balance on a piece of water in the Gulf Stream, the
pressure gradient force would be off in that direction
from high to low. Because there's higher pressure
on that side of the parcel that on that side. And that would have to be
balanced by a Coriolis force equal and opposite to that.

And then in order to get that
Coriolis force, the fluid would have to be moving in that
direction, that's you, so we have that right angle
relationship between the velocity of the material
and the Coriolis force. So I didn't ask you about the
direction, but it would be off to the Northeast, the
way I've drawn everything in this diagram. Other questions on that? OK, question two. Now for that you needed this
diagram on the next page. And what I recommended
Are you all set? I'm going to withdraw it because
I'm not ready to use the board yet. But that's terrific. Thanks so much. I appreciate that. AV Guy fixing projector: I would
just blank it at the moment if it's PROFESSOR: That's fine.

How do I blank it? Blank it. Yes. Yes, terrific. So you were given the
temperature and salinity for two water masses and you could
plot them on this S-T diagram. And A plots up here
somewhere and B plots down there somewhere. And from the lines of constant
density, which I'm not going to be able to draw very
accurately, you can find that B is more–so I've got
the numbers there. So the density for B was 1027.5
and the density for A was 1026.5 units kilograms
per cubic meters.

Now I mentioned in class that
quite a number of you have forgotten that you need to put
a 10 in front of the numbers that are given here. For example these things are
written something like 27.5. You need to make that into
1027.5 in order to have proper units of kilograms
per cubic meter. To remember that, you could look
back at the properties of water on the front page, where
I give a typical density for seawater as 1025 kilograms
per cubic meter. A number like 27.5 would be
ridiculous for water. That would be a factor
of 100 too light. The other thing is then A turns
out to be less than B. And therefore because density
must increase going down, water mass A must be
the higher one in the water column. And that was the second
part to that question. Question three, I used the
formula, change in salinity is given by the initial salinity
minus d over capital D plus little d.

And the problem said the heavy
rain adds a half meter. So d is plus 0.5 meters. And that fresh water is mixed
down to a depth of 50 meters. So the effect of that added
fresh water is felt all the way down to 50 meters. That means I put in 35 parts per
thousand for S0 and then it's minus 0.5 over 50 plus 0.5
and that comes out to be minus 0.346 parts
per thousand. And because the original
salinity 35, the new salinity is then 34.65 parts
per thousand. I just subtracted
that from the 35 to get the new salinity. Questions on that? Question four was about El
Nino and in the eastern tropical Pacific, so in the east
there, the SST would be higher than usual, the air
pressure would be lower than usual, precipitation higher
than usual, biological productivity lower than usual.

And explaining the relationship
between A and D, that would be like this. So if you have warm water near
the surface and cold water beneath, it's going to be very
difficult for nutrient rich waters to reach the surface
because of that stability in the water column. If the nutrients can't reach the
euphotic zone, then you're going to have low biological
productivity. Question five was about the
last glacial maximum. CO2 in the atmosphere was low,
the isotopes in fresh snow on Greenland would be lighter than
normal, because in that cold condition there would be
more precipitation between source and Greenland. More water would have been
rained out, the heavier isotopes rain out,
you end up with lighter ones on Greenland. The isotopes in deep sea
sediments would be heavy because with ice stored
on land, it would be isotopically light. The remaining water in the ocean
would be isotopically heavier and the sediments
would have picked up and retained that signal. Sea level would be low because
water is stored on land. So then the relationship
between C and D, oxygen isotopes in deep sea sediments
in sea level, well I've just said that, so sea level low
means that water is being stored on land in
the ice sheets.

And because that is light
isotopes, because of the evaporation process,
that's going to make the oceans heavy. Questions on that? In recent centuries we have
a perihelion in January. Explain how the climate would
be different if due to procession– All right, so let me put on the
board this side review of the plane of the ecliptic. The sun will be off center and
earth will be here and here. And this will be today. But this will be say 10,000
years from now. And today the perihelion is in
January, which means the tilt of the earth is like that. If it was going to be in June,
then we know that the tilt of the earth would be like this,
because this is northern hemisphere summer,
which is June. So the tilt is like that. So then what I wanted you to
explain was basically how these two climates would
be different. And one thing is that in this
season the northern– in this situation, the northern
hemisphere summers, being perihelion,
would be warmer.

The northern hemisphere winters
would be colder because of that distance. So the northern hemisphere
seasons would be stronger. The southern hemisphere seasons
would be weaker, because in the winter tilted
away, you're closer to the sun, tilted towards, you're
further from the sun. So the proper answer would be
that the intensity of the seasons would be changed, but
oppositely early in the northern and southern
hemispheres. Questions on that? Sea ice is frozen seawater. Thickness, let's say
1 to 4 meters. Salinity, it starts
out as seawater. It loses quite a bit
of its salt when it freezes, but not all. So a typical salinity for sea
ice is between 5 and 20, somewhere in that range,
whereas seawater is 35. Icebergs on the other hand are
compacted snow, an entirely different origin than sea ice. And their salinity is
essentially zero, since it's fresh water snow has fallen
on the glacier. Whatever formed the iceberg.

Questions there? Student: You said that the
salinity of sea ice is what? PROFESSOR: 0. Now it may be that if it's been
floating in seawater for a while a little bit of seawater
has kind of worked its way into some
of the cracks. But if you find a chunk of pure
ice in the iceberg, it'll have 0 salinity because it came
from fallen snow some years or centuries before. Student: Sea ice. PROFESSOR: I'm sorry
you asked me about sea ice. Sorry. I answered the wrong question.

Sea ice is fresher than
ocean water, but has some salt in it. Yes? Student: Can I talk you
after class about PROFESSOR: Of course. OK. Now question eight. What was the question? OK. Recent trends in sea ice. So you may recall that it's
conventional to judge both of these in September. Now in the northern hemisphere,
September is the minimum in sea ice and in the
southern hemisphere that's the maximum in sea ice. But that makes sense because in
the northern hemisphere the maximum in sea ice, which occurs
say in March or April, fills the entire basin. So it's not a question of cold
conditions giving us more sea ice in winter in the Arctic
Ocean, it's already coast to coast. Now a little more may
spill out into the Pacific and the Atlantic, but in terms of
the Arctic Basin, it's full. So that would not be a sensible
way to measure changes in the arctic sea ice. And for the southern ocean, at
the end of the warming season, there's very little
sea ice left.

It retreats mostly right back
to the coast. And so that wouldn't be a sensible
way to measure. Instead we measure it at its
maximum in the southern hemisphere, which
is in September. Anyway in the arctic, sea ice
is rapidly decreasing. In the southern ocean it's
approximately constant by the measure I've just described. Question nine is computing
the mass of salt in the world ocean.

What I thought you would do
there was to estimate the depth of the ocean at
about 5 kilometers. Estimate the surface area
of the ocean as about 2/3 of the global. Multiply the two together to get
a volume of ocean water. Multiply that times density to
get a mass of ocean water, and then use the salinity of 35
parts per thousand to get how much salt.

And I ended up and your number
may be slightly different than mine but I ended up with about
60 x 10 to the 18th kilograms of salt. Was the answer I got. Yes? Student: When you're converting
from volume to mass are you using density of
water or seawater? PROFESSOR: I think
I used sea water, but the difference is very slight. It's only 1025 versus 1000. So your answer would be off by
if you chose one versus your other, the answer would be 2%
different, which I don't think for a rough calculation like
this is very significant. Finally question ten. The Little Ice Age is the cool
period I'm sorry what was question 10a? Antarctic bottom water.

Antarctic bottom water
is I changed the exams. It's an old version. The Antarctic bottom water is
that cool, cold water mass in fact, formed at the bottom of
the ocean, formed near the shores of Antarctica. A terminal moraine is a pile of
rock and soil deposited at the tip of a moving glacier. Equatorial upwelling is rising
water from the diverging Ekman Layer, flow at the equator,
due to a reversal in the Coriolis force.

Mid-ocean ridge is the shallow
region of water, of shallower depth in the ocean connected
with where ocean crust is being created by solidifying
material from the mantle and then it's a spreading center for
ocean crust. And the Ekman Layer is the ocean flow driven
by wind stress at right angles to the wind. You could also have talked
about, well the fact–what kind of a force balance it
has, but some kind of a definition of the Ekman Layer
there was needed.

OK. So that's exam three. So we're going to finish
up the discussion– Question. Julia. Student: What was the
average of the exam? PROFESSOR: I'm not sure. Do you guys know what the
average is on this? 82. Better than last time. Was that the other
question too? Yes. So we're going to finish up the
global warming discussion today by talking about
emission scenarios. Now again, the primary
reference here is the IPCC reports. So everything in the diagrams
I'm showing are almost entirely from the IPCC reports
which you have, or you can get very easily. But there's another report
as part of it, SRES. It also is on the IPCC website
which stands for What does it stand for? Special Report on Emission
Scenarios. And I'll be talking
about that today. It should have been easy
to remember, given the title of the slide.

So the idea is here that some
economists and some industrial engineers got together to
imagine how the emissions of carbon dioxide in the atmosphere
might proceed over the next 100 years, based on
certain population and economic assumptions. And they tried quite a
variety of different things, as you will see. And then for each of those, they
handed those off to the climate modelers. And the climate modelers ran
their climate models with these different carbon dioxide
concentrations. And the result is a set of
projections into the future of how both CO2 and Earth's
climate will change. And that's what we're going
to talk about today. So here are most of the IPCC
emission scenarios. Time is on the x-axis
from 1990 to 2100. Emission rate is on the y-axis
in units of gigatons of carbon per year. Gigatons of carbon per year. Remember that's not the mass of
CO2, that's the mass of the carbon in the CO2. So if you want to compute the
math of carbon dioxide, just correct for the ratio of the
molecular weight of a carbon dioxide molecule to the
carbon by itself.

44/12 would be that
ratio, right? Carbon dioxide is
44, carbon 12. So yes. Just multiply these
times 44/12. And the A1 is broken up into
some subcategories. Generally the As have quite a
bit of increasing emissions over the next 100 years. The two B scenarios are a
little more optimistic. they climb and then declined
for B1, or climb and then increase at a very much
slower rate for B2. These documents were published
using data from about 2000 and projecting it from about 2002
and we have a few years now to look and see which of these
lines we've been on the last five years. It's a little bit hard to
tell, because they don't diverge so strongly in
the first few years. They're all pretty similar. But from the articles I've read
recently, it looks like we're a bit closer to the higher
projections than we are to the lower ones, if you look
what's happened over the last five years.

Now, this may have changed a bit
since 2008 when we began to have these economic
difficulties. So you're going to want to read
the literature carefully, but as of about 2008, 2009, it
looks like we were on some of the more discouraging
trajectories, in terms of CO2 emissions. Now from these, with a little
bit of understanding about how carbon is put back into the
biosphere, you can come up with total cumulative Well
sorry, this is just summing them up.

Total carbon dioxide cumulative
emissions, so just adding those together to get
the different scenarios expressed in a different way. From that, with a bit of
understanding of how some of that carbon dioxide will be
recycled back into the biosphere, you can come
up with carbon dioxide concentration projections
over the next 100 years. I don't know why this
artist has put the two up on top there. That's not a conventional
way to write CO2. So don't be misled by that. I think this diagram is still
accurate in spite of that loss of credibility given
by putting the two in the wrong place.

But again, you see that the B1
scenarios are leveling off, whereas the A scenarios are
climbing very rapidly, especially the A2 scenario,
which has us reaching over 800 parts per million by volume
by the year 2100. Now are you familiar with this
organization called 350.org? So this is what's his name,
McKibben's organization. With some scientific basis, I'm
not sure everyone would agree, but with some scientific
thought, they've decided that 350 should
be the limit we should strive for on CO2.

But remember, we've already
passed that. We're at 397 already. So but just for record, you
could put that on this diagram, 350.org would have you
put the limit right there. It helps you to understand how
far in exceedance of that number we are and will
be in the future. Any questions on this diagram? Yes. Student: What are the different
criteria they used to create different scenarios? PROFESSOR: I don't have those on the tip of my tongue. They have to do with the way
certain economic sectors will develop and that way–which
countries will dominate production of certain items.
It's rather detailed, and that SRES report goes into that. It makes rather interesting
reading. I apologize for the fact that
I don't have those different economic definitions prepared. Student: But it's based on
the economic growth? PROFESSOR: Economic
growth, economic– where production occurs. Not only a total gross, but in
what country production shifts to and things like that. Student: So it takes into
account shifts to different forms of energy? PROFESSOR: Exactly. Some of that's in there too.

That's right. That's right. So now the climate modelers
perform their magic. And as you know, there's about a
dozen or so of these climate models run by different groups
around the world that do these projections. So you get a lot of different
projections. The number of things gets
multiplied because we now have all these different scenarios,
and we have all the different models running on all the
different scenarios. So you get a lot of
different output. It's a little hard to
manage sometimes. But I want to show
you this diagram. Again, this is from
IPCC report. It shows the surface warming
a based on a pre-industrial reference and versus
time, 1985. This just only goes to 2025. This is a short time
scale here. It shows something called
the commitment curve. That is code for constant
composition. In other words, that essentially
says no further CO2 emissions starting in the
year 2000, essentially. Now the temperature does
continue to climb on that, because even with constant CO2
emissions, you still have to warm up the oceans. Remember the oceans are putting
a lag on all of this, because of their enormous
heat capacity.

So this continued rise is due
to mostly trying to warm up the oceans, even though
the greenhouse effect is kind of fixed. Question, yes. Student: Is that only
anthropogenic on the outside, or is it all across
the outside? PROFESSOR: Well it's
constant atmospheric composition, is the
way it's defined. [Correction: The assumed carbon
dioxide concentration is about 370ppmv).] And then these different
scenarios follow each other pretty closely over
this time frame.

But if you remember back to the
emissions scenarios, or to the composition one, your past
2025 before they really diverge very much. So it's not surprising that even
though these scenarios are wildly different, you don't
see much of that up to the year 2025. But you do see a lot of rise. I mean now you're up to a degree
or so of warming and the rate is rather impressive. Now when you go out to a much
longer time scale, that's when you see the big differences. So here is up to the present
day, this is actual data, and here's the constant composition
commitment curve that you saw beginning to peel
off in the previous one. While the other ones continued
together, but by the time you get to 2050 now, they're
beginning to diverge quite strongly. And by 2100 they really
are quite different. I don't want to project how long
each of you will live, but I expect that a lot of you
in the classroom will be around maybe in 2080.

And so that is the world that
you will probably live to see, with some variability. But I think if the last five
years is any guide, probably you'll be up in this upper range
if things continue as they are going. So that's a warming of about
again this is based on this reference, not pre-industrial
but that's a warming of about 3 degrees Celsius from
the current day. Then they begin to level out. All of these scenarios begin
to level out, except for A2 perhaps, because of
the assumptions that have been made. And in fact by the time we are
removing fossil fuels at a high rate for the next 100
years, we will have depleted a fairly significant fraction
of the fossil fuel. So this turning over is
not all our choice. Some of this will turn over
simply because the remaining amount of fossil fuels
to be burned is getting to be so small. Questions on this? Again, you'll find this
in the IPCC report. Now when you plot the same data
on a larger time frame, going back 1000 years,
so here we are today.

So what they've done is taken
that historic proxy data that I've shown you before, with a
little bit of a hint of a medieval warming, and a little
bit of a hint of a little ice age in here, and then our
current kind of two phased warming in the 20th century,
and then put these IPCC scenarios tacked on to that,
it helps you to put in perspective as to how the
changes relate to what we've seen over the last part of
the Holocene period. It's quite a steep and dramatic
rise compared to the flat climate we've
had recently. Any questions on that? Yes. Student: How about relative to
periods significantly prior to PROFESSOR: Yes.

So that's important. Now. I don't have the diagram here,
but they're loaded in the previous presentations. You can go back and see that. And of course what will happen
is when you get 10,000 years back so here's 1000 years back
when you go 10 times more, then you're back into the
Pleistocene, the LGM, the Last Glacial Maximum, and then
this temperature drops about 5 degrees. So take that distance and put it
down here and that'll give you a different sense. Right? That'll give you a sense of
well, OK, this is higher than any of that, but as an
absolute change it is comparable to what we
had going in and out of the ice ages.

It was all down here however,
so this is unique in its warmth, but not unique in its
magnitude of the fluctuating. That's a good point
to keep in mind. So you can be fooled by just
what period of geologic history you've used here to form
a basis for comparison. Other questions on this? And of course it won't be
uniform, the warming. Here's the warmth they
anticipate under three of the scenarios, B1, A1, B and A2 up
from 20 to 29, most of the warmth is in the northern
hemisphere. Up to the end of the century
then, much more warming, but again concentrated
in the northern hemisphere, high latitudes. Values as high as 7
degrees Celsius. My god, that's a
lot of warming. That's an amazing amount
of warming. Certainly there would
be no arctic ice. Certainly there'd be no glaciers
in the northern hemisphere, mountain glaciers,
under that climate. OK, any questions on these
IPCC projections? Yes. Student: What about in
comparison to the Pliocene, the period that we said
was comparable– PROFESSOR: The Pliocene, right.

So that would then be comparable
the Pliocene also seemed to have much higher
temperatures at the high latitudes than we have today. So this kind of scenario is one
of the reasons why there is a lot of research on the
Pliocene, because they need too had this kind of warmth in
the northern high latitudes. And we don't understand why that
is exactly, but it may well be something
similar to this. Except that it didn't seem that
the CO2 values were as high back then during
the Pliocene. But it'd be worth reading a
couple papers on the Pliocene to see to what extent they if
you just Google Pliocene climate, you can quickly
just use a Google Scholar for example.

If you want to get the peer
reviewed literature, go into Google Scholar and search for
Pliocene climate and you'll find a lot of recent papers that
are trying to deal with just this issue. So a lot of problems then we
perceive could be connected with this global warming. And these are all
pretty obvious. I just list them here. And it may not be complete. We expect increasing drought,
which will– and some human populations as
well as animal populations will be forced to migrate. They'll be some extinctions
probably. The one that's most talked about
will be the polar bears.

My strategy for global warming
is that if I just buy a house 300 miles north of New Haven,
that'll pretty much account for the global warming that'll
take place in my lifetime. Pretty clever, right? A lot of deep thought
went into that. But imagine the polar
bears, right? They live in this arctic
environment. It's going to warm up. They're going to lose the
sea ice very quickly from which they hunt. They aren't going to mind
the warm so much. They can probably handle that,
but they normally do their hunting off the sea ice. Without sea ice, they won't
to have a way to eat.

And therefore we're probably
going to lose the polar bear pretty quickly. That ecosystem will be gone. There will be frequenct
heat waves. For example, a few years ago we
had a heat wave in Europe. I believe that was in 2003
that killed some tens of thousands of people. And we had one just a year and
a half ago in eastern Europe. And the projections are that
these will occur very frequently as we get towards the
middle and the end of the current century. And of course, if you're living
up north that's not too bad, or if you have
air conditioning.

But air conditioning is a
problem because that uses energy which may require fossil
fuel burning, which would put more CO2 in
the atmosphere. So that's kind of a
downward spiral. The ice on land will melt. The mountain glaciers we spoke
about, and the ice sheets of Greenland and Antarctica
we talked about. And because that, ice on land is
supported by the land, when it melts, that lifts
sea level. Remember if ice is already
floating in the ocean and you melt it, that doesn't
change sea level. But if ice is supported by the
land, as a mountain glacier would be, or a large ice sheet
would be, that will cause sea level to rise. And it could be the order of
several meters, which would have a big impact on coastal
development.

Today many rivers flow all
summer because they get for example, in California the
rivers that come down out of the Sierra Nevada range, they
do decrease their flow in summertime because there's
not much rain. But they keep it going because
there's enough glacial ice melting through the summer to
provide those rivers with water even in July and
August. Well, that will certainly change. And there's all these
rivers coming down out of the mountains. If there's not rain in that
season, those rivers will certainly go dry. Because without the ice and snow
to store water at high altitudes and this will be a big
difference between today. Many, many rivers around the
world are flowing in summer only because of snow pack
melting, that stores that water until late summer. Tropical diseases will
move forward. And I hesitated to put this in
there but I did, because it's not likely that would happen,
but it's constantly discussed in the literature, in the
scientific literature, as a possibility.

That is, if you look at the
planet Venus with its unusually warm climate surface
temperature for Venus is 460 in Celsius, 735 in Kelvin it
has the solar system's strongest greenhouse effect, has
a rather high albedo, it reflects a lot of sunlight. But nevertheless it as a very
high surface temperature because of its high
greenhouse effect. And the idea is that it probably
wasn't always like this, but some kind of process
amplified itself. Probably it started to warm
up, that for some of this carbon that was in the surface
of the planet to come off the planet and form carbon
dioxide which warmed the planet further. Water may have played a role
too, but now most of the water is gone. Water may have played a role
in getting Venus to its hot state, but most of
that's gone now. Anyway, either a water vapor
feedback or a CO2 feedback probably took Venus from an
earth-like state to its current state. And so there is some
worry that this could happen to earth.

We could get to some point
where suddenly these two feedbacks, carbon dioxide and
water vapor feedbacks, might then take control of the climate
and run away and give us something that's much, much
higher than any of these IPCC projections. It's not likely, but you can
read about it in the literature. There are advantages however. There are vast regions in
the northern hemisphere, especially Canada and northern
Asia, where agriculture is mostly limited by lack
of summer warmth.

And so you would find greatly
increased agricultural productivity in Canada and Asia
under these IPCC global warming scenarios. Also at the present time, many
more people die from cold every year than from warmth. And of course, my heating
bill will be less, so I'd enjoy that. And it's now known and well
documented in the literature that when CO2 concentrations
rise, plants grow more quickly because it of what's called
CO2 fertilization. And so crops will grow generally
more quickly. Some would argue that the
crop–the nature of the plant structure however changes and
makes that plant material less nutritious. So be careful. It's not only the mass of the
plant that you grow, but weather–if you're going to eat
it, whether or not it's nutritious for humans. So be a little bit careful
on that one. But there's no doubt that CO2
fertilization is already being seen in forests and
in agriculture. So that's a real factor,
a real positive factor.

So these again are
pretty obvious. If we wanted to reduce global
warming what would we do? Well none of these are easy. Many of these are impossible. But I list them anyway, being
the eternal optimist. Reduce human populations, reduce per
capita use of energy. One way to do that would be to
increase energy costs so that each of us would work harder
to reduce our per capita use of energy. Reforest the continents, because
when you grow a tree, you sequester a certain amount
of carbon dioxide. There are a couple of
problems with that. I've mentioned one
of them already. A tree typically only lives 60
or 100 years and then it will die and that carbon dioxide
will be returned. Within 20 years, it'll be
back in the atmosphere. So it's not a permanent way
to store carbon dioxide. And also recently in the
literature, it's been pointed out, and it's really quite
obvious when you think about it, forests are very dark
in their coloration. Their albedo is very low. And so if you add more forest,
you decrease the average albedo of the planet, which
would warm the planet.

So be careful about
that trade off. Fertilize the oceans. For a while we were talking
about putting iron into the oceans, because that turned out
to be a limiting nutrient for phytoplankton growth. And phytoplankton draw in CO2
just like plants on land do. The question once again though,
how long would it stay in the oceans? Would it fall to the bottom to
be covered over, or would it just return back into
the atmosphere. Stop third world economic
growth. Well that's kind of a joke,
because how in the world would you do that? Of course in the first world
we use much more carbon dioxide, we emit much more
carbon dioxide per capita than the third world does.

And that's because we have a
higher standard of living and the third world aspire to have
the same standard of living that we do. And so that's going to be where
a lot of the increasing CO2 emissions will come from. Shift to nuclear energy. Nuclear energy does not emit
any carbon dioxide. Shift to renewable energies of
various types, wind, solar, geothermal. I'm going to be talking about
these, by the way, in the last week of the course. We're going to talk a bit
about renewable energy. The big thing that's talked
about these days is CCS, Carbon Capture and Storage. It's removing carbon dioxide
from the atmosphere and burying it down deep in the
crust of the earth. A lot of research is being
funded, including a big grant here in the geology department
at Yale to work on some aspects of this. The question is– of the
questions is would it stay down there? I mean it's light material. You'd like to combine it or
condense it in some way that it's stable and would stay
down where you put it. But after all, it is a material
that would like to gasify and come back out.

And so there again I would worry
about how long it would stay buried down there. And then various geoengineering
hypotheses have been made, such as constructing
some kind of a shade over the earth to prevent
some of the sunlight from reaching the earth. We're out of time. I've got a few more comments
about this for next Monday, but enjoy your weekend..

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