RONALD SMITH: Yeah, so this is a long list of things
that one might do to reduce global warming.
But they're all very difficult, if not impossible.
So are there any points that you want to make about these?
Any questions about what I mean by them?
I think we did go over this last
time, if I'm not mistaken.
There are lots of other recommendations or suggestions
that have been made under the general title of
geoengineering.
But one I indicated there is to install an umbrella-like
object out in space, somewhere between the earth and the sun,
so that it casts a shadow on the earth.
And, therefore, reduces the total amount of sunlight
hitting the earth.
A related suggestion is that we put a layer of small
particles, kind of like volcanoes have done naturally
from time to time, put a layer of small particles in the
stratosphere that will increase the albedo of the
earth and reflect the Sun's radiation back into space, and
thereby cool the earth.
The question for something like that, though, is what
material do we put in there?
They would have to be very fine particles or they would
just fall out.
If they have some kind of weird chemistry, like sulfate
particles, what would they do to the chemistry of the
atmosphere?
So whenever you think of something, some little clever
idea pops into your mind, unfortunately there's probably
100 unintended consequences on these geoengineering ideas.
So they have to be thought through extremely carefully
before any of them could be implemented.
While climate research is quite a mature area now, there
are some areas within it that require further thought.
For example, we're still missing a complete theory of
the ice ages.
We think that maybe this Milankovitch forcing has
played a role.
But that Milankovitch forcing is very weak actually.
The obliquity change, the tilt of the earth's axis, is only a
couple of degrees one way or the other.
The procession theory, which changes the orientation of the
tilt of the earth's axis, changes the way the seasons
work, but oppositely in the two hemispheres.
So if you're looking globally, that influence cancels right
out completely.
So you have to make up some argument about why one
hemisphere is more important than the other.
So we're missing a complete theory there.
We're also missing a theory of Venus' climate history.
How it got from being an earth-like planet to something
that's much, much hotter.
And that's important, as I mentioned, because there's
some worry that the earth's atmosphere might follow a
similar path.
Theory of solar variability, now we know that the sun has
gotten hotter over the millennia, over
the age of the earth.
And we know that there's a sunspot cycle, an 11 year and
a 22 year cycle, on the number of sun spots on the
surface of the Sun.
And we think that that may have had a very minor
influence on the amount of radiation reaching the earth.
But that theory is not well fleshed out.
There are some statistical relationships
that have been proposed.
But the physical mechanisms for this make almost no sense.
And so we need to further understand why the sun might
vary in its strength and what influence that
would have on the earth.
The theory of positive feedback I'm going to be
talking more about that in just a minute--
the idea that there are these processes on earth that will
tend to amplify climate change.
The ones that we've talked about already have been ice
albedo feedback.
Right?
More snow in a colder climate makes the albedo larger,
reflects more radiation, cools the earth more, and that might
spiral on itself.
There's the water vapor feedback.
A warmer climate would put more water vapor in the
atmosphere certainly.
Water vapor is an important greenhouse gas that would warm
the climate further and so on.
And then one that's been talked about more recently,
seems to be going on with the last 100 years or so, is
carbon dioxide feedback, where a warmer climate is beginning
to allow carbon trapped in permafrost to decay and be
added to the atmosphere.
Also ocean water, when you heat it, it can hold less
carbon dioxide dissolved in water.
So that CO2 is going to be added.
So this acts on a slower time scale perhaps, but there's
some worry that there's a carbon dioxide feedback that
may be starting to happen.
That one has to do--that last one has to do with this next
one, how greenhouse concentrations are controlled.
We don't have a complete understanding
of the carbon cycle.
Or how some of the other greenhouse gases, like
methane, ozone, oxides of nitrogen there's not a
complete understanding of how they are controlled in the
earth's atmosphere.
The last IPPC report, the one that you've been looking at,
the AR4, is full of apologies for how little they can say
about regional climates.
The problem is that these global climate models roughly
agree in their predictions of how the global temperature
will change.
But they disagree, to a large extent, about how the climates
of different continents and different sub regions within
continents will change.
So this is a big area of research.
In fact, my group is interested a little bit in how
global changes will influence local and regional climates.
We don't understand very well how clouds would change,
aerosols, how those two things would impact the earth's
albedo in a different climate.
And this question of whether we can re-bury carbon dioxide
into the earth is receiving a lot of funding and a lot of
research at the moment.
Those are some of the active areas.
If any of you were to get involved in climate research,
there's a good chance you'd probably be working on one of
those problems. Although, maybe I've left off a couple
of newly emerging ideas.
So the last thing I want to talk about in this set of
lectures is something called climate sensitivity.
And it's basically an attempt to simplify and quantify how
earth will respond, how the earth's climate will respond,
to changes in radiative forcing.
I'll define radiative forcing in just a minute.
There are two different common definitions for climate
sensitivity.
So I need you to be aware of both of these.
So when you pick up a paper on climate sensitivity you can
spot which definition the author is using.
One definition is, how much would the earth's climate warm
if there was a doubling of CO2?
That's a commonly used definition for climate
sensitivity.
The other one is, how much would the earth warm if you
increased the radiation hitting the surface of the
earth by one watt per square meter?
Now these two things can be related, roughly, because a
CO2 doubling when you add CO2 to the atmosphere you increase
the absorption and the
re-emission of longwave radiation.
And a doubling of the CO2 is approximately adding 3.7 watts
per square meter of radiative forcing back to the
surface of the earth.
So you trap the outgoing longwave radiation, you warm
the atmosphere, and then that readmits back to
earth about that much.
So you can go back and forth between those two definitions
using that kind of relationship, that quantity.
Now the reason I like it, the reason I want to stick it in
here, is that it's being used more and more as a compact way
to compare different models.
For example, the National Center for Atmospheric
Research has a global climate model.
The Department of Energy has a global climate model.
NASA has a global climate model.
Several in Europe, the Russians have one.
How do you compare them?
One way is to use this measure of their sensitivity.
It's been found in that way that there are some
significant differences between these models.
And it's been a help to find out why these
models are so different.
Also, it's a nice way to summarize some paleoclimate
data of the type that you and I have been looking at a
couple of weeks ago.
And I like it because it illustrates the role of these
climate feedbacks.
So it turns out that these climate sensitivity values,
almost no matter how you compute them, are larger than
you would expect if you left off the feedbacks.
And so by that increase in sensitivity you can show what
role these climate feedbacks are having.
So for all these reasons, we're going to
take a look at this.
Now, so the definitions.
Radiative forcing, extra radiation at the earth's
surface relative to preindustrial time.
That's the way radiative forcing is defined in the
global warming debate.
I wouldn't call that a universal definition.
But in the global warming debate
that's how it's defined.
These are the primary feedbacks we're talking about.
And occasionally you'll find a discussion of a runaway
climate where the feedbacks, the positive feedbacks, are so
strong that they would take over and drive the climate to
an extreme state, like has happened on Venus.
Unlikely for earth, but it's part of the conversation.
I've got to define okay, now there's another variable that
comes in, in how you define climate sensitivity.
And that has to do with, what feedbacks do you include in
the calculation?
So I'm going to define something called the Black
Body sensitivity, which has no feedbacks in it.
It's based on the Stefan-Boltzmann law only,
which you remember, from earlier in the course, is the
fact that the radiation emitted from an object's
surface goes like the fourth power of the temperature.
That was the Stefan-Boltzmann law.
There's something called the Charney sensitivity.
Charney was a scientist who chaired the first
investigative panel on global warming.
And he tried to base their final report on a particular
definition of climate sensitivity that included some
short-term feedbacks, but excluded some of
the longer term ones.
For example, he included water vapor feedbacks, snow/albedo
feedback, and cloud feedback.
But he excluded any feedback connected with the oceans, the
ice sheets, or carbon dioxide changes.
And that's become known, in the literature, as Charney
sensitivity.
It's probably valid on periods of time of, say, well 20 to 50
years, something like that.
When you go to longer terms, however, maybe 100, 200, 300
years, you have to realize that other slower adjustments
in the climate system are going to be happening, such as
changes to deep ocean temperature, changes in the
ice sheets.
Sea level will be rising.
Biomass on the continents may be changing.
And the carbon dioxide may be responding to changes in the
ocean and the land surface, the land biomass.
So that would be called long-term sensitivity.
Makes you realize this subject is not dirt simple.
I mean, it's got some complexity to it, depending on
the time frame that you're talking about and the
feedbacks that you're allowing in the calculation.
So here's the easy one, Black Body sensitivity.
If I've got an object, like a planet, receiving a certain
amount of radiation from the sun, you know that it'll
increase its temperature, or adjust its temperature, so
that it radiates to space just as much as it's
receiving from the sun.
So it'll adjust it's own temperature to come into that
kind of radiative equilibrium.
Well then, if I were to increase the amount received
from the sun by 1 watt per square meter, how much must
the temperature rise in order to establish a new
equilibrium?
Pretty simple idea, right?
So there's the Stefan-Boltzmann law.
I'm going to do a quick little calculus trick here that some
of you may not be familiar with.
But if I take the derivative of F with respect to
temperature, I get 4 times sigma T to the third times the
change in temperature.
And if I rearrange, bring the dF under and that down under
here, I get that formula.
The change in temperature for each change in flux is given
by 1 over 4 sigma T to the third.
I can plug numbers in.
That Stefan-Boltzmann constant is a fixed constant.
I can use the temperature of our planet, of 288 Kelvin,
plug it into that formula.
And I immediately get this rather interesting number.
For every watt per square meter that I add to the earth
in the form of radiative energy, it'll warm by 0.2
degrees Kelvin.
That is the simplest way to compute climate sensitivity.
That's a pretty small number.
And the reason why, as we will see the reason why it's fairly
small is we have not included any of
these positive feedbacks.
Now this is a calculation you can do for yourself.
Just try putting in different values of F or T and computing
it and you'll see that that is the rate at which the two
things are--the slope at which the two things are related to
each other.
Any questions on that?
So we use this value as kind of our foundation, right?
Now, if you look at the IPCC report let me get the lights
down this summarizes the radiative forcing, using the
definition that I gave a moment ago.
This is since the beginning of the industrial revolution.
It shows how much the radiation reaching the surface
of the earth has changed and for what reason.
Carbon dioxide is the biggest reason.
That increase in carbon dioxide, we've seen, caused
about 1.7 watts per square meter added greenhouse forcing
at the surface of the earth.
However, methane, N2O, and the halocarbons have added
something too.
Tropospheric ozone has added more.
And then on the negative side and I talked about this a
couple days ago aerosols, mostly sulfate aerosols from
coal burning, has had a negative effect.
Well when you add all these together, these
roughly cancel these.
And you get a value that's about what CO2 would
have been by itself.
That doesn't mean that CO2 is alone in importance.
The others are important too.
But generally these cancel those and you get a value of
about 1 and 1/2 meters per second for
the radiative forcing.
And so if you then just use the Black Body sensitivity for
that, that would give you about 0.3
degree Kelvin warming.
And we know, in fact, from our data, that the warming of our
planet since the industrial revolution began is about 0.8.
So it's quite a bit larger than this value.
We can formalize that.
So let's try to compute radiative forcing based on the
data that we have.
Over the last 100 years we've had about 0.8 degree warming.
From the last slide, we've got about 1.5 watts per square
meter radiative forcing.
So if I divide one by the other, I get a sensitivity
based on the radiation definition of 0.5 Kelvin.
That then is about 2 and 1/2 times the sensitivity there.
So the sensitivity that we're seeing in our planet is about
2 and 1/2 times what we compute from a simple Black
Body forcing law.
This then, is evidence that the feedbacks are working, the
positive feedbacks are working.
You could re-formula that as a warming per CO2 doubling.
And you get about 2 degrees Kelvin for every CO2 doubling.
I want to do this one other way.
I know I'm beating this to death, but you remember this
plot, from the Pleistocene, the advance and retreat of the
glaciers over the last 400,000 years.
Well, here we have a temperature signature on the
bottom and a CO2 signature on the top.
We can convert the CO2 signature
to a radiative forcing.
And here's how that argument goes.
So that's a warming of about 5 degrees between glacial and
interglacial times.
We estimate a change in the longwave forcing of about 7
watts per square meter.
Divide one by the other, I get about 0.7 degrees Kelvin per
watt per square meter.
Now that's slightly larger than the one we had on the
last slide from the last 100 years.
And most people believe that that number is larger than the
what did we have for that 0.5, 0.7, because you get these
extra long-term feedbacks coming in.
So there's still a lot of argument in the literature
about that.
But the general interpretation is that when you go to this
longer time scale of a couple hundred thousand years, you
get some longer--some other positive feedbacks that make
the climate even more sensitive
to radiative forcing.
Any questions on that?
Yeah, so I'll just wrap that up by saying, as expected, our
climate is about 3 times more sensitive to radiative forcing
than Black Body would suggest, due to positive feedbacks like
water vapor feedback and snow/albedo feedback.
And when you go to longer time scales, it becomes even
slightly more sensitive due to,
possibly, some other feedbacks.
Yes?
STUDENT: Was the 5K increase from like the lowest
temperature to the highest temperature on that graph?
PROFESSOR: On the glacial one?
STUDENT: Yeah.
PROFESSOR: Yeah, I tried to well, I didn't go to
the very max and the very min, but I tried to take an average
of the minimum and kind of a general value
for the higher ones.
I didn't push it to the limit.
You can't read that scale there.
But that would be slightly larger than 5 degrees if you
went from minimum to maximum.
So I've reduced it a little bit.
I didn't want to let the noise, you
know, drive my answer.
I wanted to use average values.
Yeah, okay.
So that's the summary.
So this is everything then.
We did all of these subjects.
And today we just finished up the concept of climate
sensitivity.
Are there any general questions
about global warming?
Okay, if not, we'll leave it there.
And I want to do one other thing today.
I think we can get it in.
Yeah, because population is a driver in global warming, I
wanted to just to spend the rest of the period I'm not an
expert in demography or population studies.
But there's a few things that are quite obvious, that even I
can understand.
And I wanted to lay those out for you.
Because it's intimately connected with all these
environmental issues, including the air pollution
which we'll be studying next time and on Friday.
So these are the terms you're going to have to know.
The fertility rate is the number of children that a
woman would bear in her lifetime.
The growth rate of a population would be the rate
of change of the population as a fraction of the existing
population.
In other words, a relative rate.
Not a number of people per year, but like a percent per
year, or something like that.
We'll talk about the special case of exponential growth,
where the population would follow a pure exponential
curving rising curve.
I'll talk about population density, urbanization, the
so-called demographic transition, and
the population pyramid.
So in the next few minutes, I'll introduce each of these
terms.
Population versus time, going back 50,000 years, human
population rose very slowly over most of the period of
time when humans were evolving, because, well, life
is difficult.
The death rate was high due to disease, due to accidents, due
to all kinds of things.
And it isn't really until you got in the very recent period
of time where you began to get this rapid, very rapid,
increase in human population.
Probably the biggest change in the rate of growth had to do
with the changes in medicine, I would say the germ theory of
disease and inoculations.
Both of which are rather recent.
We're talking about the 19th, in some cases the early part
of the 20th century.
1920, 1930, 1940 when mass inoculations began to be used.
Anyway it's a very curious curve, so slow and so flat for
so long, and then rising so rapidly now just in the last
100 years or so.
That is partly controlled by the fertility rate.
And here's a diagram from Wiki about that.
What you have for each country and this is by country is a
fertility rate from 0--or from 1 to 7.
So the United States would have a value
somewhere around 2.
Remember this is the number of births per woman in her life.
And a lot of other countries have that medium gray color,
which puts you around 2.
The Soviet Union, especially Ukraine, is even less than
that, however.
And most of Africa and the Middle East are dramatically
higher, generally in the range 4, 5, 6.
That doesn't necessarily mean that their population is
increasing rapidly, although that's true also.
But the fertility rate is one of the components that goes in
to determining the rate of population growth.
Everybody understand this definition of fertility rate?
It's controlled by a number of factors, but a
lot of it is cultural.
A lot of it has to do with expectations, having children
to take care of you in your old age, to help with the
farm, or because you don't have a way to control
population.
A number of--a wide variety of things come into
the fertility rate.
The role of women in society is a big one as well.
Here's the fertility rate plotted versus GDP, the Gross
Domestic Product, that makes it pretty clear there's a
relationship between how much money people have, their
standard of living, and the fertility rate.
USA is here.
Generally, for the developed world the fertility
rate is 2 or less.
Whereas for the underdeveloped world the fertility rate can
be very much higher.
And this magic value here, of 2.3, is the replacement rate.
You have to have a fertility rate of about 2.3 in order to
maintain a steady population.
There are a few outliers that have to do with the special
cultural conditions of those countries.
Stop me if there are questions on this.
So let's look at the fertility rate for the United States and
Canada since 1940.
There was a big bulge in fertility rate, reaching 3.7
for the USA around 1955.
And that is really what's responsible for what's called
the Baby Boom in the United States.
And I guess you are the children of the Baby Boom
generation or something approximate to that.
And then immediately after that came a movement called
ZPG, Zero Population Growth.
Maybe that movement caused this drop, or maybe it was
just changes in background cultural or economic
conditions.
But there was a dramatic drop in the fertility rate in the
United States from 3.7 down to less than 2.
And now it's come up a little bit for the United States to
just about the replacement rate.
Canada is roughly similar.
The details may be a little different, they haven't jumped
up at the end.
But they followed roughly the same curve.
Any questions on that?
So this is a wild swing, right?
This is a huge variation in just a few years in one
particular country.
Europe is famous for having a low fertility rate.
And here are some of the numbers.
Remember the replacement rate is about 2.3.
Italy has 1.3.
And some of the Eastern European countries are even
lower than that.
France is higher.
They've tried to keep their population, their
fertility rate up.
But they don't have it quite at replacement levels.
And Spain is down at 1.3 as well.
So Europe, as a whole, generally has a very low
fertility rate.
We saw that in the earlier map as well, here, as opposed to
some other developing parts of the world.
And China has gone through wild swings, as
has the United States.
For somewhat different reasons, perhaps, but the
curve is somewhat similar.
It climbed even higher than the United States, much higher
in fact, up to 6.5.
And that, I think, was part of the cultural revolution where
it was every woman's duty to have as many children as she
possibly could for the strength of the state.
And then talk about a complete turnabout they've now dropped
down to less than 2.
And, of course, that is largely due to China's one
child policy, kind of flipping the political pressure on
families and individuals.
So wild swings in fertility rate.
Now we'll move to this fractional growth rate then.
So, of course, this is now a combination of birth rate and
mortality rate.
How many children are born every year?
How many people die every year?
You have to know both of those to then get the total
fractional growth rate of the population.
And there again, these colors follow the same trend.
As the Soviet Union has the lowest growth rate, well 1--
well below 0.
They're losing population.
And all the other countries are gaining population, but at
a rate ranging from 3% per year to just a small fraction
of a percent per year.
So that's the total fractional growth rate.
Here is the way you would compute that.
Here are some numbers.
For example, China population in 2010 and 1990.
Well that's a 20 year span, so you can get an approximation
by just taking the percent change over that period of
time and dividing that by the number of years.
So divide that by 20 and you get a value that's just
slightly less than 1% per year.
And that would be that percent, or that fractional
growth rate for population in China.
That's how you do the calculation.
In some cases taking a 20 year estimate might be blurring
some details.
If the growth has been changing rapidly over time,
what you would be getting here is some kind of average over a
fluctuating quantity.
But nevertheless, you see how easy it is to do that
calculation, if you don't mind losing some of the details,
some of the rapid changes in time.
So globally then, this is what the fractional growth rate has
been doing and is projected to do.
But these projections are quite uncertain.
I wouldn't pay too much attention to the blue curve.
Globally we've had, over the last couple of decades, we've
had growth rates--
this is not fertility rate, this is growth rate--
of about 2% per year, which is quite a large growth rate.
It has been decreasing.
At the present time, it's more like 1% per year.
But that's still quite a large growth rate.
And then it's predicted that will continue to
drop as we go forward.
But that will depend a lot upon the future economy, the
future cultural development of the countries that have the
highest populations.
Now this concept of exponential growth, you've
probably heard it.
If the fractional growth rate is constant, that is, if this
quantity were constant in time, over some period of
time, then the population will grow exponentially.
Now what does that mean, to say the population grows
exponentially?
The exponential function defines a system whose annual
increase is proportional to the population itself.
In other words, its rate of increase is proportional to
the population that it has.
That is what is meant by an exponential increase.
Sometimes that word is misused.
Sometimes in the popular literature someone will be
saying, well, you know I'm rich, my bank account amount
is growing exponentially.
And by that he might just mean rapidly.
But that's not what exponential really means.
Exponential means it's growing at a rate proportional to how
much you have.
Now, if you're getting a fixed interest rate in your account,
then it would be exponential.
It'll grow at a rate that will increase as you get more and
more money in the bank.
So that might be an accurate representation.
Remember, exponential doesn't refer to some rate of
increase, it relates to the relationship between the rate
of increase and the amount of the standing population that
you have at any particular moment.
So in other words, births minus deaths, really,
proportional to population.
So we write it this way.
Population, as a function of time, is given by the
population at some reference time, call it 0, and then P to
the plus alpha t, where alpha is the growth rate and t is
the time that's elapsed since your
reference time has occurred.
So if the growth rate, for example, is 1% per year--
and that was a number that we have about
now for planet earth--
and if that were to remain constant.
in other words, if this curve was level at 1% per year, then
here's how you could do the calculation.
After 100 years, putting in 100 years into there and 0.01
year to the minus 1 in for alpha, the units will cancel.
And in this case, the product is equal to 1, in this special
simple example.
So it's going to be exp to the 1, which has a value-- you can
check it on your calculator--
of 2.7.
So a growth rate of 0.01 over 100 years would give a growth
rate factor of 2.7.
So however many people you had at the beginning of the 100
year period, put that there, multiply it times 2.7, and
you'll get--
it's roughly a tripling, roughly a tripling of the
population over 100 years if you have a growth rate of 1%
percent per year.
Remember it's compound interest. The rate increases
as the population increases.
Questions on that?
Now this is a bit of a fiction, because, as you've
just seen, population growth rates change
quite a bit over time.
And so, while you can do rough calculations with this, a real
population demographer would probably never use this.
He would go step by step and calculate it assuming a
variable growth rate rather than assuming a constant one.
So here are the current UN population
projections for our planet.
Data up to this point, and then uncertain projections,
depending on what you think the growth rate will
be around the world.
So we crossed 6 billion back in 2000, roughly 2000.
We crossed 7 billion just, what was that, 3
weeks ago, I think.
And in the future, well the projections vary
all over the place.
But by, let's say, 2080, this looks like 2080, some
calculations would have us at 12 billion.
Some would have us at 9.
And some would have us declining, population already
getting back towards 6 billion.
So great uncertainties in this.
But still great potential for environmental difficulties, if
either of those curves are realized.
Another issue that comes up is population density.
How many people per square kilometer do you have?
And this is a map of population density.
Units are people per square kilometer.
And there are some countries--
and this is by country so this can be quite misleading,
because for a given country they put a uniform color over
the whole country.
So it's the total population divided by the total area of
the country, then painted as a uniform color
over the whole country.
That's quite unrealistic.
But it will give us a place to start to talk
about population density.
India, as a country, has a population exceeding 1,000
people per square kilometer, which is enormous.
Whereas something like Australia is in the lightest
shade that they have, less than 10
people per square kilometer.
Wide variation in that quantity.
But again, this is quite misleading because it's done
by country.
United States, this is per square mile.
Remember, the United States unfortunately is still on an
old English system of measure.
So very often you'll find population per square mile
instead of per square kilometer.
You can do the conversion easily to go back and forth.
But once again, doing it by state is misleading, because
there are parts of California that are
nearly empty of people.
And there are very densely populated areas of
California and so on.
But even averaging over states, you'll find a big
difference between Montana, for example, and New Jersey
and Rhode Island.
Now, if you want to read about the role
of population density--
not all demographers would put a great emphasis on population
density, but Jared Diamond, in this book, Collapse--
how many have read that book?
I thought a few of you would have. He, in places in that
book, makes a pretty strong case for the importance of
population density.
He really thinks it matters that populations have enough
space to live.
And I partly agree with that.
But he maybe takes it--
some examples he gives wouldn't carry over to other
civilizations that he didn't talk about in his book.
But anyway, he makes a strong case for the importance of
population density.
And I party believe that too.
So I encourage you to take a look at that book if you get a
chance, because it makes the case very nicely for the
importance of population density.
When you think about what the limiting resources are for
living on our planet, water, food, energy, living space,
even these other ones, like water and food and energy, to
some extent they all depend on having enough area as well.
For example, energy from hydroelectric requires a large
catchment area of rain in order to get hydroelectric.
Farmland, obviously food from farms requires lots of area.
So just scanning this list and realizing that many of those
have a relationship to population density would, I
think, partly confirm Jared Diamond's idea that living
space is an important part of the success of a culture.
Okay, now to correct the misimpression from those
earlier country-based and
state-based population densities.
People don't live uniformly distributed over a
country or a state.
In fact, a higher and higher fraction of populations are
moving to cities.
They're abandoning the land and moving to cities.
So this is a plot, on the x-axis 1950 into the future,
2050, the percent of the population that's living in
urban versus rural areas.
Back in the 1950's, 70% was rural, 30% urban.
And we've crossed that boundary, we've crossed that
crossover point.
And in the future, we'll have more people living clustered
together in cities.
That doesn't mean they don't need the land around the
cities to grow the food and provide the energy and so on.
But at least their living density is much larger than
those country-averaged or those state-averaged values
that I gave you.
And these are some of the growing megacities, where now
more than half the people in the world are concentrated
into these large cities.
Yes?
STUDENT: [INAUDIBLE]?
PROFESSOR: I don't know.
When you do calculations--
I saw an article recently about how much energy people
used that live in New York city versus the rest of New
York state.
And the people who live in New York city use less energy per
capita then the people that live in upstate New York.
Probably because of the dense way that they live and the way
that transportation is so much easier when you live close.
So I think it could be argued that this might be a more
efficient way to live.
But that's only one measure.
And I haven't studied the issue in it's full breadth.
So that would be worth looking into.
To see whether this might actually be an advantage for
the environment, to have people clustered in cities.
Other comments on that?
We need to wrap up.
I do need to give you this.
So there's something called the demographic transition.
And it goes basically from a third world country converting
to a first world country.
What happens to its population?
In the beginning, you've got a high birth rate, a high death
rate, and a small population.
Standard of living begins to increase, health is improved.
The first thing that happens is the death rate drops.
The birth rate stays high.
Because the death rate has dropped and the birth rate
stays high, population begins to increase.
Then, at some point culturally, the birth rate
drops as well.
You end up with a lower birth rate and a lower death rate
and a larger total population.
We've seen this happen in country after country.
And if you look at that map of the world, clearly some
countries in the west, like the US and
Europe are in this state.
And a lot of the third world countries are
still in this state.
And a few are in the middle, doing the demographic
transition.
So this is an important concept of how you go from one
type of population situation to the other.
And the other one I want to leave you with--
I'm out of time-- but you have to know about
the population pyramid.
So a country that has a stable population, here's a plot of
how many people you have, male and female, in
different age groups.
A country that has a stable population usually has a
population pyramid like this.
A country that has a rapidly growing population has a
pyramid like this, a lot of young people.
And as they get older, of course, the overall population
will increase very rapidly.
Well, you can see this immediately, because if you
drive into a small town in the Middle East or in Africa, the
number of children that come running out of the house to
see who's visiting their village tells you you've got a
population curve like that.
And that will do it.
Okay, we're finished.