2
00:00:08,380 --> 00:00:11,596
So we constructed, at the very beginning
of the class, 

3
00:00:11,596 --> 00:00:14,611
a very simple model of the climate of the
earth 

4
00:00:14,611 --> 00:00:18,028
and since then we've been improving it and talking
about ways in which

5
00:00:18,028 --> 00:00:22,370
it should be more realistic in order to
approximate the real world.

6
00:00:22,370 --> 00:00:28,660
Here, we started with just
original bare rock model of energy

7
00:00:28,660 --> 00:00:32,400
coming into and out of a planet and that
determines the planet's temperature.

8
00:00:32,400 --> 00:00:33,510
Then we added the greenhouse effect,

9
00:00:33,510 --> 00:00:36,470
with this plate of glass that
absorbs

10
00:00:36,470 --> 00:00:39,010
all the infrared light coming from the
ground.

11
00:00:39,010 --> 00:00:42,370
And then we talked about how the
greenhouse gases

12
00:00:42,370 --> 00:00:45,390
in the atmosphere don't absorb all
of the light.

13
00:00:45,390 --> 00:00:49,230
They absorb only certain parts of 
the light energy.

14
00:00:49,230 --> 00:00:52,320
And those other, other frequencies, such
as the atmospheric

15
00:00:52,320 --> 00:00:55,272
window here, which greenhouse gases don't
absorb at all.

16
00:00:55,272 --> 00:00:59,150
And then in the last unit, we talked about
another mechanism

17
00:00:59,150 --> 00:01:03,480
of carrying heat around in the atmosphere,
in addition to

18
00:01:03,480 --> 00:01:09,010
the heat fluxes from
light, which is convection.

19
00:01:09,010 --> 00:01:12,390
And in particular, we found out that moist
convection

20
00:01:12,390 --> 00:01:16,480
is what drives the thermal structure of
our atmosphere.

21
00:01:16,480 --> 00:01:21,680
You have air rising and it cools and
condenses and that's where it rains,

22
00:01:21,680 --> 00:01:24,150
but it dumps latent heat up here and also
carries heat around.

23
00:01:24,150 --> 00:01:27,600
That's where we are.

24
00:01:27,600 --> 00:01:30,130
But there's one more thing that the
real world has

25
00:01:30,130 --> 00:01:33,970
that our conceptual models do
not have yet.

26
00:01:33,970 --> 00:01:37,880
And that is that the whole world is not a
single place.

27
00:01:37,880 --> 00:01:41,240
It's not all the same temperature, and the
sunlight intensity is

28
00:01:41,240 --> 00:01:47,535
not always in balance with the energy
going out by infrared locally.

30
00:01:48,300 --> 00:01:53,280
So the real geometry is that you have

31
00:01:53,280 --> 00:01:56,880
sunlight all coming from one direction
from the sun,

32
00:01:56,880 --> 00:02:00,480
and it's coming in most strongly at the
equator.

33
00:02:01,670 --> 00:02:05,570
Infrared light is leaving everywhere
around the earth,

34
00:02:05,570 --> 00:02:08,040
And the rate at which it leaves
depends on 

35
00:02:08,040 --> 00:02:12,290
the temperature of the ground according to
Epsilon's sigma T to the fourth.

36
00:02:12,290 -->  00:02:18,780
And it turns
out that if you look at a given

38
00:02:18,780 --> 00:02:24,720
location on the earth, the incoming and
outgoing energy budgets do not balance.

39
00:02:24,720 --> 00:02:30,070
They only balance for the earth as a whole
on a long enough time average.

40
00:02:30,070 --> 00:02:35,400
Here is plot of the sunlight intensity
in watts per

41
00:02:35,400 --> 00:02:37,780
square meter as a function of latitude and

42
00:02:37,780 --> 00:02:40,070
its very strong
at the equator.

43
00:02:40,070 --> 00:02:42,850
Here is a plot of the infrared energy

44
00:02:42,850 --> 00:02:45,370
leaving the earth as a function of
latitude.

45
00:02:45,370 --> 00:02:49,360
And its also stronger near the equator
because it is warmer at the equator than

46
00:02:49,360 --> 00:02:55,940
it is at the poles, but there is a
deficit.

47
00:02:55,940 --> 00:02:58,830
It's not hot enough at the Equator to
drive the

48
00:02:58,830 --> 00:03:01,920
energy out as quickly as the sun is
bringing it in.

49
00:03:01,920 --> 00:03:03,830
And it's not cold enough at the Equator,

50
00:03:03,830 --> 00:03:08,800
at the high latitudes, for the infrared to
be as low as the sunlight coming in.

51
00:03:08,800 --> 00:03:13,740
And the difference is what we call heat
transport.

52
00:03:13,740 --> 00:03:18,888
It's heat being carried by the atmosphere
and by the

53
00:03:18,888 --> 00:03:24,180
oceans from the low latitudes to the high
latitudes.