2
00:00:10,432 --> 00:00:15,293
You've heard about the carbon cycle as
a series of reservoirs.

3
00:00:15,293 --> 00:00:19,781
All of which inhale and exhale carbon
dioxide to the atmosphere, which

4
00:00:19,781 --> 00:00:23,540
is  the center of the carbon cycle
in some ways.

5
00:00:23,540 --> 00:00:25,360
But there's another trick that the carbon
cycle

6
00:00:25,360 --> 00:00:28,940
has done on Earth, which involves this
chemical reaction.

7
00:00:30,840 --> 00:00:35,990
CO2 plus water combined with energy can be
put together,

8
00:00:35,990 --> 00:00:42,220
into a molecule of the family of
CH2O.

9
00:00:42,220 --> 00:00:43,610
Which is a very, very simple

10
00:00:43,610 --> 00:00:48,070
representation of what chemists call
organic carbon.

11
00:00:48,070 --> 00:00:51,670
Organic carbon is what plants and
people

12
00:00:51,670 --> 00:00:54,210
and things like that are made out of.

13
00:00:54,210 --> 00:00:56,990
And originally, organic carbon could only
be made

14
00:00:56,990 --> 00:01:00,350
by life, which is why it's called "organic"
carbon.

15
00:01:00,350 --> 00:01:01,170
Nowadays,

16
00:01:01,170 --> 00:01:04,760
if you go to the grocery store and you buy
some organic fruit,

17
00:01:04,760 --> 00:01:06,810
what it means is that

18
00:01:06,810 --> 00:01:11,390
they didn't use any industrial chemicals
to produce it.

19
00:01:11,390 --> 00:01:13,210
But actually a lot of those industrial

20
00:01:13,210 --> 00:01:17,470
chemicals, like pesticides, can actually be
made out

21
00:01:17,470 --> 00:01:19,890
of carbon in this  configuration

22
00:01:19,890 --> 00:01:22,420
that a chemist would actually call organic
carbon.

23
00:01:22,420 --> 00:01:26,910
There's different uses for the word
organic, that actually

24
00:01:26,910 --> 00:01:29,800
can be in conflict with each other, in
different situations.

25
00:01:31,310 --> 00:01:34,470
It takes energy to do this chemical reaction,

26
00:01:34,470 --> 00:01:37,450
to make organic carbon out of
CO2.

27
00:01:37,450 --> 00:01:41,770
And where the energy comes from is 
light,

28
00:01:41,770 --> 00:01:47,770
to power photosynthesis, so it takes
plants to do this chemical reaction.

29
00:01:47,770 --> 00:01:51,530
The other chemical on this side of the
equation is oxygen.

30
00:01:51,530 --> 00:01:55,035
This is O2, just like the oxygen in our
atmosphere.

33
00:01:56,210 --> 00:02:02,300
It turns out that when organic carbon
is

34
00:02:03,650 --> 00:02:09,210
buried in the earth, it leaves behind the
oxygen it produced.

35
00:02:09,210 --> 00:02:12,150
And this is where the oxygen in the
atmosphere comes from.

36
00:02:12,150 --> 00:02:16,615
Actually, most of the organic carbon
that's produced

38
00:02:17,090 --> 00:02:23,040
by photosynthesis is respired back to CO2
because somebody eats it.

39
00:02:23,040 --> 00:02:27,160
If we eat our lunch we are doing
respiration, or if bacteria degrade a

40
00:02:27,160 --> 00:02:31,820
leaf in the soil it
sends it back this way.

41
00:02:31,820 --> 00:02:37,820
But, in very special conditions this
organic carbon can be protected from

42
00:02:37,820 --> 00:02:42,310
being eaten by being buried in the mud in
the bottom of the ocean, or in a swamp.

44
00:02:43,460 --> 00:02:47,340
In which case, it leaves behind oxygen in
the atmosphere.

45
00:02:47,340 --> 00:02:52,500
What's happened is that the biosphere,
the life on earth,

46
00:02:52,500 --> 00:02:57,360
has charged up the surface of the earth
like a giant battery.

47
00:02:57,360 --> 00:03:02,590
And we are creatures of the battery, we
eat organic carbon and we breath oxygen in

48
00:03:02,590 --> 00:03:07,790
order to have everything we need on this
side of the reaction, to do respiration

49
00:03:07,790 --> 00:03:10,520
which is how we get energy back.

50
00:03:10,520 --> 00:03:16,555
There are really two reasons why why
life does photosynthesis.

51
00:03:16,555 --> 00:03:20,390
One is to store energy in this form of
organic carbon.

52
00:03:20,390 --> 00:03:24,740
And another is to put carbon in a

53
00:03:24,740 --> 00:03:27,760
chemical state where it has lots and lots
of

54
00:03:27,760 --> 00:03:30,860
versatility to do all the different things
that

55
00:03:30,860 --> 00:03:34,710
need to be done in the bio-molecular
machinery.

56
00:03:36,270 --> 00:03:38,600
The amazing thing about this battery
of

57
00:03:38,600 --> 00:03:43,070
the biosphere is that it's so remarkably
stable.

58
00:03:43,070 --> 00:03:48,494
There's no simple chemical reason why
we have

59
00:03:48,494 --> 00:03:55,060
20% O2 in the atmosphere instead of 10
times as much, or ten times less.

60
00:03:55,060 --> 00:03:58,390
It's kind of an accident of how much of
this organic carbon

61
00:03:58,390 --> 00:04:02,160
has gotten buried in the mud 
in the Earth.

62
00:04:02,160 --> 00:04:06,580
It'd be very easy for this system, it
seems like conceptually,

63
00:04:06,580 --> 00:04:10,010
to have built up to 10 times more oxygen
in the atmosphere.

64
00:04:10,010 --> 00:04:14,910
In which case a spark of lightning could
start a continent wide forest fire.

65
00:04:14,910 --> 00:04:18,990
No wet wood would burn.
Nothing could put it out.

66
00:04:18,990 --> 00:04:21,650
Or having 10 times less oxygen in the
atmosphere.

67
00:04:21,650 --> 00:04:26,790
in which case, all of us multi-cellular
types would would choke and die.

68
00:04:26,790 --> 00:04:29,540
And we know that for the last more than half a

70
00:04:29,540 --> 00:04:33,750
billion years there has been mutilcellular
life on earth.

71
00:04:33,750 --> 00:04:37,320
The oxygen concentration in the
atmosphere is

72
00:04:37,320 --> 00:04:41,270
somehow  regulated in a very
miraculous  way.

73
00:04:41,270 --> 00:04:46,620
It has to be something like, if oxygen in
the atmosphere gets too low, then you get

74
00:04:48,530 --> 00:04:54,170
anoxic conditions 
in the ocean, which

75
00:04:54,170 --> 00:04:57,840
make it easier to bury organic carbon,
which then

76
00:04:59,580 --> 00:05:02,850
acts as an extra source of oxygen to the
atmosphere.

77
00:05:02,850 --> 00:05:05,790
But scientists don't understand this very
well, this

78
00:05:05,790 --> 00:05:08,987
this uncanny, this eerie stability of
earth's biosphere.